Common source

In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal enters and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous bipolar junction transistor circuit is the common-emitter amplifier.

The common-source (CS) amplifier may be viewed as a transconductance amplifier or as a voltage amplifier. (See classification of amplifiers). As a transconductance amplifier, the input voltage is seen as modulating the current going to the load. As a voltage amplifier, input voltage modulates the amount of current flowing through the FET, changing the voltage across the output resistance according to Ohm's law. However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier (ideally infinite), nor low enough for a decent voltage amplifier (ideally zero). Another major drawback is the amplifier's limited high-frequency response. Therefore, in practice the output often is routed through either a voltage follower (common-drain or CD stage), or a current follower (common-gate or CG stage), to obtain more favorable output and frequency characteristics. The CS–CG combination is called a cascode amplifier.


Figure 1: Basic N-channel JFET common-source circuit (neglecting biasing details).


Figure 2: Basic N-channel JFET common-source circuit with source degeneration.
Characteristics
At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can be derived.

Bandwidth
The bandwidth of the common-source amplifier tends to be low, due to high capacitance resulting from the Miller effect. The gate-drain capacitance is effectively multiplied by the factor , thus increasing the total input capacitance and lowering the overall bandwidth.

Figure 3 shows a MOSFET common-source amplifier with an active load. Figure 4 shows the corresponding small-signal circuit when a load resistor RL is added at the output node and a Thévenin driver of applied voltage VA and series resistance RA is added at the input node. The limitation on bandwidth in this circuit stems from the coupling of parasitic transistor capacitance Cgd between gate and drain and the series resistance of the source RA. (There are other parasitic capacitances, but they are neglected here as they have only a secondary effect on bandwidth.)

Using Miller's theorem, the circuit of Figure 4 is transformed to that of Figure 5, which shows the Miller capacitance CM on the input side of the circuit. The size of CM is decided by equating the current in the input circuit of Figure 5 through the Miller capacitance, say iM , which is:



to the current drawn from the input by capacitor Cgd in Figure 4, namely jωCgd vGD. These two currents are the same, making the two circuits have the same input behavior, provided the Miller capacitance is given by:


Usually the frequency dependence of the gain vD / vG is unimportant for frequencies even somewhat above the corner frequency of the amplifier, which means a low-frequency hybrid-pi model is accurate for determining vD / vG. This evaluation is Miller's approximation[1] and provides the estimate (just set the capacitances to zero in Figure 5):


so the Miller capacitance is


The gain gm (rO//RL) is large for large RL, so even a small parasitic capacitance Cgd can become a large influence in the frequency response of the amplifier, and many circuit tricks are used to counteract this effect. One trick is to add a common-gate (current-follower) stage to make a cascode circuit. The current-follower stage presents a load to the common-source stage that is very small, namely the input resistance of the current follower (RL ≈ 1 / gm ≈ Vov / (2ID) ; see common gate). Small RL reduces CM[2]. The article on the common-emitter amplifier discusses other solutions to this problem.

Returning to Figure 5, the gate voltage is related to the input signal by voltage division as:



The bandwidth (also called the 3dB frequency) is the frequency where the signal drops to 1/ √ 2 of its low-frequency value. (In decibels, dB(√ 2) = 3.01 dB). A reduction to 1/ √ 2 occurs when ωCM RA = 1, making the input signal at this value of ω (call this value ω3dB, say) vG = VA / (1+j). The magnitude of (1+j) = √ 2. As a result the 3dB frequency f3dB = ω3dB / (2π) is:


If the parasitic gate-to-source capacitance Cgs is included in the analysis, it simply is parallel with CM, so


Notice that f3dB becomes large if the source resistance RA is small, so the Miller amplification of the capacitance has little effect upon the bandwidth for small RA. This observation suggests another circuit trick to increase bandwidth: add a common-drain (voltage-follower) stage between the driver and the common-source stage so the Thévenin resistance of the combined driver plus voltage follower is less than the RA of the original driver.

Examination of the output side of the circuit in Figure 2 enables the frequency dependence of the gain vD / vG to be found, providing a check that the low-frequency evaluation of the Miller capacitance is adequate for frequencies f even larger than f3dB. (See article on pole splitting to see how the output side of the circuit is handled.)


Figure 3: Basic N-channel MOSFET common-source amplifier with active load ID.


Figure 4: Small-signal circuit for N-channel MOSFET common-source amplifier.


Figure 5: Small-signal circuit for N-channel MOSFET common-source amplifier using Miller's theorem to introduce Miller capacitance CM.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

JFET Amplifier

So far we have looked at the Bipolar type amplifiers and especially the Common Emitter amplifier, but small signal amplifiers can also be made using Field Effect Transistors or FET's. These devices have the advantage over bipolar devices of having an extremely high input impedance along with a low noise output making them very useful in amplifier circuits using very small signals. The design of an amplifier circuit based around a JFET (n-channel FET for this example) or even a MOSFET is exactly the same principle as that for a bipolar device and for a Class A amplifier as we looked at in the previous tutorial. A suitable Quiescent point still needs to be found for the correct biasing of the amplifier circuit with amplifier configurations of Common Source, Common Drain and Common Gate available for FET devices. In this tutorial we will look at the JFET Amplifier as a common source amplifier as this is the most widely used design. Consider the Common Source JFET Amplifier circuit below.

Common Source JFET Amplifier

The circuit consists of an N-channel JFET, but the device could also be an equivalent N-channel Depletion-mode MOSFET as the circuit diagram would be the same, just a change in the FET. The JFET Gate voltage Vg is biased through the potential divider network set up by resistors R1 and R2 and is biased to operate within its saturation region which is equivalent to the active region of the BJT. The Gate biasing voltage Vg is given as:


Note that this equation only determines the ratio of the resistors R1 and R2, but in order to take advantage of the very high input impedance of the JFET as well as reducing the power dissipation within the circuit, we need to make these resistor values as high as possible, with values in the order of 1 to 10MΩ being common.

The input signal, (Vin) is applied between the Gate terminal and 0v with the Drain circuit containing the load resistor, Rd. The output voltage, Vout is developed across this load resistance. There is also an additional resistor, Rs included in the Source lead and the same Drain current also flows through this resistor. When the JFET is switched fully "ON" a voltage drop equal to Rs x Id is developed across this resistor raising the potential of the Source terminal above 0v or ground level. This voltage drop across Rs due to the Drain current provides the necessary reverse biasing condition across the Gate resistor, R2. In order to keep the Gate-source junction reverse biased, the Source voltage, Vs needs to be higher than the gate voltage, Vg. This Source voltage is therefore given as:



Then the Drain current, Id is also equal to the Source current, Is as "No Current" enters the Gate terminal and this can be given as


This potential divider biasing circuit improves the stability of the common source JFET circuit when being fed from a single DC supply compared to that of a fixed voltage biasing circuit. Both Resistor, Rs and Capacitor, Cs serve basically the same function as the Emitter resistor and capacitor in the Common Emitter Bipolar Transistor amplifier circuit, namely to provide good stability and prevent a reduction in the signal gain. However, the price paid for a stabilized quiescent Gate voltage is that more of the supply voltage is dropped across Rs.

The basic circuit and characteristics of a common source JFET amplifier are very similar to that of the Common Emitter amplifier. A DC load line is constructed by joining the two points relating to the Drain current, Id and the supply voltage, Vdd intersecting the curves at the Q-point as follows.

JFET Amplifier Characteristics Curves


As with the Common Emitter circuit, the DC load line produces a straight line equation whose gradient is given as: -1/(Rd + Rs) and that it crosses the vertical Id axis at a point equal to Vdd/(Rd + Rs). The other end of the load line crosses the horizontal axis at a point equal to Vdd. The actual position of the Q-point on the DC load line is determined by the mean value of Vg which is biased negatively as the JFET as a depletion-mode device. Like the bipolar common emitter amplifier the output of the Common Source JFET Amplifier is 1800 out of phase with the input signal.

One of the main disadvantages of using Depletion-mode JFET is that they need to be negatively biased. Should this bias fail for any reason the Gate-source voltage may rise and become positive causing an increase in Drain current resulting in failure of the Drain voltage, Vd. Also the high channel resistance, Rds(on) of the JFET, coupled with high quiescent steady state Drain current makes these devices run hot so additional heatsink is required. However, most of the problems associated with using JFET's can be greatly reduced by using enhancement-mode MOSFET devices instead.

MOSFETs or Metal Oxide Semiconductor FET's have much higher input impedances and low channel resistances compared to the equivalent JFET. Also the biasing arrangements for MOSFETs are different and unless we bias them positively for N-channel devices and negatively for P-channel devices no Drain current will flow, then we have in effect a fail safe transistor.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

High Performance Silicon Nanowire Field Effect Transistors

Silicon nanowires can be prepared with single-crystal structures, diameters as small as several nanometers and controllable hole and electron doping, and thus represent powerful building blocks for nanoelectronics devices such as field effect transistors. To explore the potential limits of silicon nanowire transistors, we have examined the influence of source-drain contact thermal annealing and surface passivation on key transistor properties. Thermal annealing and passivation of oxide defects using chemical modification were found to increase the average transconductance from 45 to 800 nS and average mobility from 30 to 560 cm2/V·s with peak values of 2000 nS and 1350 cm2/V·s, respectively. The comparison of these results and other key parameters with state-of-the-art planar silicon devices shows substantial advantages for silicon nanowires. The uses of nanowires as building blocks for future nanoelectronics are discussed.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

Other MOSFET types

Dual gate MOSFET
The dual gate MOSFET has a tetrode configuration, where both gates control the current in the device. It is commonly used for small signal devices in radio frequency applications where the second gate is normally used for gain control or mixing and frequency conversion.

FinFET
The Finfet, see figure to right, is a double gate device, one of a number of geometries being introduced to mitigate the effects of short channels and reduce drain-induced barrier lowering.

A double-gate FinFET device

Depletion-mode MOSFETs
There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device. In essence, the depletion-mode device is equivalent to a normally closed (on) switch, while the enhancement-mode device is equivalent to a normally open (off) switch.[1]

Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in TV sets. Depletion-mode MOSFET families include BF 960 by Siemens and BF 980 by Philips (dated 1980s), whose derivatives are still used in AGC and RF mixer front-ends

NMOS logic
n-channel MOSFETs are smaller than p-channel MOSFETs and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place. With advances in technology, CMOS logic displaced NMOS logic in the 1980s to become the preferred process for digital chips.

Power MOSFET
Main article: Power MOSFET
Power MOSFETs have a different structure than the one presented above.[35] As with all power devices, the structure is vertical and not planar. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current. The voltage rating of the transistor is a function of the doping and thickness of the N-epitaxial layer (see cross section), while the current rating is a function of the channel width (the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the "silicon estate". With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.

It is worth noting that power MOSFETs with lateral structure are mainly used in high-end audio amplifiers. Their advantage is a better behaviour in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications.


Cross section of a Power MOSFET, with square cells. A typical transistor is constituted of several thousand cells

DMOS
DMOS stands for double-diffused metal–oxide–semiconductor. Most of the power MOSFETs are made using this technology

RHBD MOSFETs
Semiconductor sub-micron and nano-meter electronic circuits are the primary concern for operating within the normal tolerance in harsh radiation environments like space. One of the design approaches for making a radiation-hardened-by-design (RHBD) device is Enclosed-Layout-Transistor (ELT). Normally, the gate of the MOSFET surrounds the drain, which is placed in the center of the ELT. The source of the MOSFET surrounds the gate. Another RHBD MOSFET is called H-Gate. Both of these transistors have very low leakage current with respect to radiation. However, they are large in size and take more space on silicon than a standard MOSFET.

Newer technologies are emerging for smaller devices for cost saving, low power and increased operating speed. The standard MOSFET is also becoming extremely sensitive to radiation for the newer technologies. A lot more research works should be completed before space electronics can safely use RHBD MOSFET circuits of nanotechnology.

When radiation strikes near the silicon oxide region (STI) of the MOSFET, the channel inversion occurs at the corners of the standard MOSFET due to accumulation of radiation induced trapped charges. If the charges are large enough, the accumulated charges affect STI surface edges along the channel near the channel interface (gate) of the standard MOSFET. Thus the device channel inversion occurs along the channel edges and the device creates off-state leakage path, causing device to turn on. So the reliability of circuits degrades severely. The ELT offers many advantages. These advantages include improvement of reliability by reducing unwanted surface inversion at the gate edges that occurs in the standard MOSFET. Since the gate edges are enclosed in ELT, there is no gate oxide edge (STI at gate interface), and thus the transistor off-state leakage is reduced very much.

Low-power microelectronic circuits including computers, communication devices and monitoring systems in space shuttle and satellites are very different than what we use on earth. They are radiation (high-speed atomic particles like proton and neutron, solar flare magnetic energy dissipation in earth's space, energetic cosmic rays like X-ray, Gamma-ray etc.) tolerant circuits. These special electronics are designed by applying very different techniques using RHBD MOSFETs to ensure the safe space journey and also space-walk of astronauts.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

CMOS

Complementary metal–oxide–semiconductor (CMOS) (pronounced /ˈsiːmɒs/) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass successfully patented CMOS in 1967 (US patent 3,356,858).

CMOS is also sometimes referred to as complementary-symmetry metal–oxide–semiconductor (or COS-MOS). The words "complementary-symmetry" refer to the fact that the typical digital design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions.

Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Significant power is only drawn while the transistors in the CMOS device are switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example transistor-transistor logic (TTL) or NMOS logic, which uses all n-channel devices without p-channel devices. CMOS also allows a high density of logic functions on a chip. It was primarily this reason why CMOS won the race in the eighties and became the most used technology to be implemented in VLSI chips.

The phrase "metal–oxide–semiconductor" is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Aluminum was once used but now the material is polysilicon. Other metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process, as announced by IBM and Intel for the 45 nanometer node and beyond

Technical details
"CMOS" refers to both a particular style of digital circuitry design, and the family of processes used to implement that circuitry on integrated circuits (chips). CMOS circuitry dissipates less power than logic families with resistive loads. Since this advantage has increased and grown more important, CMOS processes and variants have come to dominate, thus the vast majority of modern integrated circuit manufacturing is on CMOS processes.[citation needed] As of 2010, the CPUs with the best performance per watt each year have been CMOS static logic since 1976.

CMOS circuits use a combination of p-type and n-type metal–oxide–semiconductor field-effect transistors (MOSFETs) to implement logic gates and other digital circuits found in computers, telecommunications equipment, and signal processing equipment. Although CMOS logic can be implemented with discrete devices (for instance, in an introductory circuits class), typical commercial CMOS products are integrated circuits composed of millions (or hundreds of millions) of transistors of both types on a rectangular piece of silicon of between 0.1 and 4 square centimeters.[citation needed] These devices are commonly called "chips", although within the industry they are also referred to as "die" (singular) or "dice", "dies", or "die" (plural).

Composition
The main principle behind CMOS circuits that allows them to implement logic gates is the use of p-type and n-type metal–oxide–semiconductor field-effect transistors to create paths to the output from either the voltage source or ground. When a path to output is created from the voltage source, the circuit is said to be pulled up. The other circuit state occurs when a path to output is created from ground and the output pulled down to the ground potential.
Output is inversion of input
CMOS circuits are constructed so that all PMOS transistors must have either an input from the voltage source or from another PMOS transistor. Similarly, all NMOS transistors must have either an input from ground or from another NMOS transistor. The composition of a PMOS transistor creates low resistance between its source and drain contacts when a low gate voltage is applied and high resistance when a high gate voltage is applied. On the other hand, the composition of an NMOS transistor creates high resistance between source and drain when a low gate voltage is applied and low resistance when a high gate voltage is applied.

The image on the right shows what happens when an input is connected to both a PMOS transistor (top of diagram) and an NMOS transistor (bottom of diagram). When the voltage of input A is low, the NMOS transistor's channel is in a high resistance state. This limits the current that can flow from Q to ground. The PMOS transistor's channel is in a low resistance state and much more current can flow from the supply to the output. Because the resistance between the supply voltage and Q is low, the voltage drop between the supply voltage and Q due to a current drawn from Q is small. The output therefore registers a high voltage.

On the other hand, when the voltage of input A is high, the PMOS transistor is in an off (high resistance) state so it would limit the current flowing from the positive supply to the output, while the NMOS transistor is in an on (low resistance) state, allowing the output to drain to ground. Because the resistance between Q and ground is low, the voltage drop due to a current drawn into Q placing Q above ground is small. This low drop results in the output registering a low voltage.

In short, the outputs of the PMOS and NMOS transistors are complementary such that when the input is low, the output is high, and when the input is high, the output is low. Because of this opposite behavior of input and output, the CMOS circuits' output is the inversion of the input.

Static CMOS Inverter

Duality
An important characteristic of a CMOS circuit is the duality that exists between its PMOS transistors and NMOS transistors. A CMOS circuit is created to allow a path always to exist from the output to either the power source or ground. To accomplish this, the set of all paths to the voltage source must be the complement of the set of all paths to ground. This can be easily accomplished by defining one in terms of the NOT of the other. Due to the De Morgan's laws based logic, the PMOS transistors in parallel have corresponding NMOS transistors in series while the PMOS transistors in series have corresponding NMOS transistors in parallel.

Logic
More complex logic functions such as those involving AND and OR gates require manipulating the paths between gates to represent the logic. When a path consists of two transistors in series, then both transistors must have low resistance to the corresponding supply voltage, modeling an AND. When a path consists of two transistors in parallel, then either one or both of the transistors must have low resistance to connect the supply voltage to the output, modeling an OR.

Shown on the right is a circuit diagram of a NAND gate in CMOS logic. If both of the A and B inputs are high, then both the NMOS transistors (bottom half of the diagram) will conduct, neither of the PMOS transistors (top half) will conduct, and a conductive path will be established between the output and Vss (ground), bringing the output low. If either of the A or B inputs is low, one of the NMOS transistors will not conduct, one of the PMOS transistors will, and a conductive path will be established between the output and Vdd (voltage source), bringing the output high.

An advantage of CMOS over NMOS is that both low-to-high and high-to-low output transitions are fast since the pull-up transistors have low resistance when switched on, unlike the load resistors in NMOS logic. In addition, the output signal swings the full voltage between the low and high rails. This strong, more nearly symmetric response also makes CMOS more resistant to noise.

See Logical effort for a method of calculating delay in a CMOS circuit.


NAND gate in CMOS logic

Example: NAND gate in physical layout
This example shows a NAND logic device drawn as a physical representation as it would be manufactured. The physical layout perspective is a "bird's eye view" of a stack of layers. The circuit is constructed on a P-type substrate. The polysilicon, diffusion, and n-well are referred to as "base layers" and are actually inserted into trenches of the P-type substrate. The contacts penetrate an insulating layer between the base layers and the first layer of metal (metal1) making a connection.

The inputs to the NAND (illustrated in green coloring) are in polysilicon. The CMOS transistors (devices) are formed by the intersection of the polysilicon and diffusion: N diffusion for the N device; P diffusion for the P device (illustrated in salmon and yellow coloring respectively). The output ("out") is connected together in metal (illustrated in cyan coloring). Connections between metal and polysilicon or diffusion are made through contacts (illustrated as black squares). The physical layout example matches the NAND logic circuit given in the previous example.

The N device is manufactured on a P-type substrate. The P device is manufactured in an N-type well (n-well). A P-type substrate "tap" is connected to VSS and an N-type n-well tap is connected to VDD to prevent latchup.


The physical layout of a NAND circuit. The larger regions of N-type diffusion and P-type diffusion are part of the transistors. The two smaller regions on the left are taps to prevent latchup.

Cross section of two transistor in a CMOS gate, in an N-well CMOS process‎

Power: switching and leakage
CMOS logic dissipates less power than NMOS logic circuits because CMOS dissipates power only when switching ("dynamic power"). On a typical ASIC in a modern 90 nanometer process, switching the output might take 120 picoseconds, and happen once every ten nanoseconds. NMOS logic dissipates power whenever the output is low ("static power"), because there is a current path from Vdd to Vss through the load resistor and the n-type network.

CMOS circuits dissipate power by charging the various load capacitances (mostly gate and wire capacitance, but also drain and some source capacitances) whenever they are switched. The charge moved is the capacitance multiplied by the voltage change. Multiply by the switching frequency on the load capacitances to get the current used, and multiply by voltage again to get the characteristic switching power dissipated by a CMOS device: P = CV2f.

An additional form of power consumption became significant in the 1990s as wires on chip became narrower and the long wires became more resistive. CMOS gates at the end of those resistive wires see slow input transitions. During the middle of these transitions, both the NMOS and PMOS networks are partially conductive, and current flows directly from Vdd to Vss. The power thus used is called crowbar power. Careful design which avoids weakly driven long skinny wires has ameliorated this effect, and crowbar power is nearly always substantially smaller than switching power.

Both NMOS and PMOS transistors have a gate–source threshold voltage, below which the current (called subthreshold current) through the device drops exponentially. Historically, CMOS designs operated at supply voltages much larger than their threshold voltages (Vdd might have been 5 V, and Vth for both NMOS and PMOS might have been 700 mV). A special type of the CMOS transistor with near zero threshold voltage is the native transistor.

To speed up designs, manufacturers have switched to constructions that have lower voltage thresholds;[citation needed] but because of this a modern NMOS transistor with a Vth of 200 mV has a significant subthreshold leakage current. Designs (e.g. desktop processors) which include vast numbers of circuits which are not actively switching still consume power because of this leakage current. Leakage power is a significant portion of the total power consumed by such designs. Further technology advances that use even thinner gate dielectrics have an additional leakage component because of current tunnelling through the extremely thin gate dielectric. Using high-k dielectrics instead of silicon dioxide that is the conventional gate dielectric allows similar device performance, but with a thicker gate insulator, thus avoiding this current. Leakage power reduction using new material and system design is critical to sustaining scaling of CMOS. A good overview of leakage and reduction methods are explained in the book Leakage in Nanometer CMOS Technologies ISBN 0-387-25737-3.

Analog CMOS
Besides digital applications, CMOS technology is also used in analog applications. For example, there are CMOS operational amplifier ICs available in the market. Transmission gates may be used instead of signal relays. CMOS technology is also widely used for RF circuits all the way to microwave frequencies, in mixed-signal (analog+digital) applications.

Temperature range
Conventional CMOS devices work over a range of −55 °C to +125 °C. There were theoretical indications as early as August 2008 that silicon CMOS will work down to 40 kelvins, or −233 °C. Functioning temperatures near 40 kelvins have since been achieved using overclocked AMD Phenom II processors with a combination of liquid nitrogen and liquid helium cooling

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

MOSFET scaling

MOSFET scaling
Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but modern integrated circuits are incorporating MOSFETs with channel lengths of tens of nanometers. Intel began production of a process featuring a 32 nm feature size (with the channel being even shorter) in late 2009. The semiconductor industry maintains a "roadmap", the ITRS, which sets the pace for MOSFET development. Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process, the need to use very low voltages, and with poorer electrical performance necessitating circuit redesign and innovation (small MOSFETs exhibit higher leakage currents, and lower output resistance, discussed below).

Reasons for MOSFET scaling
Smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area. This results in a chip with the same functionality in a smaller area, or chips with more functionality in the same area. Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer. Hence, smaller ICs allow more chips per wafer, reducing the price per chip. In fact, over the past 30 years the number of transistors per chip has been doubled every 2–3 years once a new technology node is introduced. For example the number of MOSFETs in a microprocessor fabricated in a 45 nm technology is twice as many as in a 65 nm chip. This doubling of the transistor count was first observed by Gordon Moore in 1965 and is commonly referred to as Moore's law

It is also expected that smaller transistors switch faster. For example, one approach to size reduction is a scaling of the MOSFET that requires all device dimensions to reduce proportionally. The main device dimensions are the transistor length, width, and the oxide thickness, each (used to) scale with a factor of 0.7 per node. This way, the transistor channel resistance does not change with scaling, while gate capacitance is cut by a factor of 0.7. Hence, the RC delay of the transistor scales with a factor of 0.7.

While this has been traditionally the case for the older technologies, for the state-of-the-art MOSFETs reduction of the transistor dimensions does not necessarily translate to higher chip speed because the delay due to interconnections is more significant.

Trend of Intel CPU transistor gate length

Difficulties arising due to MOSFET size reduction
Producing MOSFETs with channel lengths much smaller than a micrometer is a challenge, and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology. In recent years, the small size of the MOSFET, below a few tens of nanometers, has created operational

Higher subthreshold conduction
As MOSFET geometries shrink, the voltage that can be applied to the gate must be reduced to maintain reliability. To maintain performance, the threshold voltage of the MOSFET has to be reduced as well. As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available; the circuit design is a compromise between strong current in the "on" case and low current in the "off" case, and the application determines whether to favor one over the other. Subthreshold leakage (including subthreshold conduction, gate-oxide leakage and reverse-biased junction leakage), which was ignored in the past, now can consume upwards of half of the total power consumption of modern high-performance VLSI chips

Increased gate-oxide leakage
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 1.2 nm (which in silicon is ~5 atoms thick) the quantum mechanical phenomenon of electron tunneling occurs between the gate and channel, leading to increased power consumption.

Insulators (referred to as high-k dielectrics) that have a larger dielectric constant than silicon dioxide, such as group IVb metal silicates e.g. hafnium and zirconium silicates and oxides are being used to reduce the gate leakage from the 45 nanometer technology node onwards. Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance (capacitance is proportional to dielectric constant and inversely proportional to dielectric thickness). All else equal, a higher dielectric thickness reduces the quantum tunneling current through the dielectric between the gate and the channel. On the other hand, the barrier height of the new gate insulator is an important consideration; the difference in conduction band energy between the semiconductor and the dielectric (and the corresponding difference in valence band energy) also affects leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant.

Increased junction leakage
To make devices smaller, junction design has become more complex, leading to higher doping levels, shallower junctions, "halo" doping and so forth,[27][28] all to decrease drain-induced barrier lowering (see the section on junction design). To keep these complex junctions in place, the annealing steps formerly used to remove damage and electrically active defects must be curtailed[29] increasing junction leakage. Heavier doping is also associated with thinner depletion layers and more recombination centers that result in increased leakage current, even without lattice damage.

Lower output resistance
For analog operation, good gain requires a high MOSFET output impedance, which is to say, the MOSFET current should vary only slightly with the applied drain-to-source voltage. As devices are made smaller, the influence of the drain competes more successfully with that of the gate due to the growing proximity of these two electrodes, increasing the sensitivity of the MOSFET current to the drain voltage. To counteract the resulting decrease in output resistance, circuits are made more complex, either by requiring more devices, for example the cascode and cascade amplifiers, or by feedback circuitry using operational amplifiers, for example a circuit like that in the adjacent figure.


MOSFET version of gain-boosted current mirror; M1 and M2 are in active mode, while M3 and M4 are in Ohmic mode, and act like resistors. The operational amplifier provides feedback that maintains a high output resistance

Lower transconductance
The transconductance of the MOSFET decides its gain and is proportional to hole or electron mobility (depending on device type), at least for low drain voltages. As MOSFET size is reduced, the fields in the channel increase and the dopant impurity levels increase. Both changes reduce the carrier mobility, and hence the transconductance. As channel lengths are reduced without proportional reduction in drain voltage, raising the electric field in the channel, the result is velocity saturation of the carriers, limiting the current and the transconductance.

Interconnect capacitance
Traditionally, switching time was roughly proportional to the gate capacitance of gates. However, with transistors becoming smaller and more transistors being placed on the chip, interconnect capacitance (the capacitance of the wires connecting different parts of the chip) is becoming a large percentage of capacitance.[30] [31] Signals have to travel through the interconnect, which leads to increased delay and lower performance

Heat production

Large heatsinks to cool power transistors in a TRM-800 audio amplifierThe ever-increasing density of MOSFETs on an integrated circuit creates problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have reduced reliability and shorter lifetimes. Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.

Power MOSFETs are at risk of thermal runaway. As their on-state resistance rises with temperature, if the load is approximately a constant-current load then the power loss rises correspondingly, generating further heat. When the heatsink is not able to keep the temperature low enough, the junction temperature may rise quickly and uncontrollably, resulting in destruction of the device.



Large heatsinks to cool power transistors in a TRM-800 audio amplifier

Process variations
With MOSFETS becoming smaller, the number of atoms in the silicon that produce many of the transistor's properties is becoming fewer, with the result that control of dopant numbers and placement is more erratic. During chip manufacturing, random process variations affect all transistor dimensions: length, width, junction depths, oxide thickness etc., and become a greater percentage of overall transistor size as the transistor shrinks. The transistor characteristics become less certain, more statistical. The random nature of manufacture means we do not know which particular example MOSFETs actually will end up in a particular instance of the circuit. This uncertainty forces a less optimal design because the design must work for a great variety of possible component MOSFETs. See process variation, design for manufacturability, reliability engineering, and statistical process control

Modeling challenges
Modern ICs are computer-simulated with the goal of obtaining working circuits from the very first manufactured lot. As devices are miniaturized, the complexity of the processing makes it difficult to predict exactly what the final devices look like, and modeling of physical processes becomes more challenging as well. In addition, microscopic variations in structure due simply to the probabilistic nature of atomic processes require statistical (not just deterministic) predictions. These factors combine to make adequate simulation and "right the first time" manufacture difficult.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

MOSFET construction

Gate material
The primary criterion for the gate material is that it is a good conductor. Highly-doped polycrystalline silicon is an acceptable but certainly not ideal conductor, and also suffers from some more technical deficiencies in its role as the standard gate material. Nevertheless, there are several reasons favoring use of polysilicon:

1.The threshold voltage (and consequently the drain to source on-current) is modified by the work function difference between the gate material and channel material. Because polysilicon is a semiconductor, its work function can be modulated by adjusting the type and level of doping. Furthermore, because polysilicon has the same bandgap as the underlying silicon channel, it is quite straightforward to tune the work function to achieve low threshold voltages for both NMOS and PMOS devices. By contrast, the work functions of metals are not easily modulated, so tuning the work function to obtain low threshold voltages becomes a significant challenge. Additionally, obtaining low-threshold devices on both PMOS and NMOS devices would likely require the use of different metals for each device type, introducing additional complexity to the fabrication process.
2.The Silicon-SiO2 interface has been well studied and is known to have relatively few defects. By contrast many metal–insulator interfaces contain significant levels of defects which can lead to Fermi-level pinning, charging, or other phenomena that ultimately degrade device performance.
3.In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better-performing transistors. Such high temperature steps would melt some metals, limiting the types of metal that can be used in a metal-gate-based process.
While polysilicon gates have been the de facto standard for the last twenty years, they do have some disadvantages which have led to their likely future replacement by metal gates. These disadvantages include:

1.Polysilicon is not a great conductor (approximately 1000 times more resistive than metals) which reduces the signal propagation speed through the material. The resistivity can be lowered by increasing the level of doping, but even highly doped polysilicon is not as conductive as most metals. In order to improve conductivity further, sometimes a high-temperature metal such as tungsten, titanium, cobalt, and more recently nickel is alloyed with the top layers of the polysilicon. Such a blended material is called silicide. The silicide-polysilicon combination has better electrical properties than polysilicon alone and still does not melt in subsequent processing. Also the threshold voltage is not significantly higher than with polysilicon alone, because the silicide material is not near the channel. The process in which silicide is formed on both the gate electrode and the source and drain regions is sometimes called salicide, self-aligned silicide.
2.When the transistors are extremely scaled down, it is necessary to make the gate dielectric layer very thin, around 1 nm in state-of-the-art technologies. A phenomenon observed here is the so-called poly depletion, where a depletion layer is formed in the gate polysilicon layer next to the gate dielectric when the transistor is in the inversion. To avoid this problem, a metal gate is desired. A variety of metal gates such as tantalum, tungsten, tantalum nitride, and titanium nitride are used, usually in conjunction with high-k dielectrics. An alternative is to use fully-silicided polysilicon gates, a process known as FUSI.
Insulator
As devices are made smaller, insulating layers are made thinner, and at some point tunneling of carriers through the insulator from the channel to the gate electrode takes place. To reduce the resulting leakage current, the insulator can be made thicker by choosing a material with a higher dielectric constant. To see how thickness and dielectric constant are related, note that Gauss' law connects field to charge as:



with Q = charge density, κ = dielectric constant, ε0 = permittivity of empty space and E = electric field. From this law it appears the same charge can be maintained in the channel at a lower field provided κ is increased. The voltage on the gate is given by:


with VG = gate voltage, Vch = voltage at channel side of insulator, and tins = insulator thickness. This equation shows the gate voltage will not increase when the insulator thickness increases, provided κ increases to keep tins /κ = constant (see the article on high-κ dielectrics for more detail, and the section in this article on gate-oxide leakage).

The insulator in a MOSFET is a dielectric which can in any event be silicon oxide, but many other dielectric materials are employed. The generic term for the dielectric is gate dielectric since the dielectric lies directly below the gate electrode and above the channel of the MOSFET.

Junction design
The source-to-body and drain-to-body junctions are the object of much attention because of three major factors: their design affects the current-voltage (I-V) characteristics of the device, lowering output resistance, and also the speed of the device through the loading effect of the junction capacitances, and finally, the component of stand-by power dissipation due to junction leakage.

The drain induced barrier lowering of the threshold voltage and channel length modulation effects upon I-V curves are reduced by using shallow junction extensions. In addition, halo doping can be used, that is, the addition of very thin heavily doped regions of the same doping type as the body tight against the junction walls to limit the extent of depletion regions.[33]
The capacitive effects are limited by using raised source and drain geometries that make most of the contact area border thick dielectric instead of silicon. [34]
These various features of junction design are shown (with artistic license) in the figure.
Junction leakage is discussed further in the section increased junction leakage.




MOSFET showing shallow junction extensions, raised source and drain and halo implant. Raised source and drain separated from gate by oxide spacers.

PRESENTACION DE PASOS DE CONSTRUCION DE UN FET
http://cleanroom.byu.edu/virtual_cleanroom.parts/MOSFETProcess.html

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

MOSFET

El metal-óxido-semiconductor transistor de efecto de campo (MOSFET, MOS-FETO MOS FET) Es un dispositivo usado para amplificar o conmutación electrónica señales. El principio básico de que el dispositivo fue propuesta por primera vez por Edgar Julius Lilienfeld en el año 1925. En los MOSFET, una tensión en el electrodo de óxido de puerta aislada puede inducir un la realización de canal entre los dos contactos de otros llamados fuente y el drenaje. El canal puede ser de de tipo n o tipo p (Véase el artículo sobre dispositivos semiconductores), Y, por ello pidió una nMOSFET o un pMOSFET (también comúnmente nMOS, pMOS). Es de lejos el más común transistor en ambos digital y analógica circuitos, aunque el transistor de unión bipolar fue en un tiempo mucho más común.
El 'metal' en el nombre se encuentra a menudo un nombre equivocado porque el material de metal previamente puerta se encuentra a menudo una capa de polisilicio (Silicio policristalino). Aluminio había sido el material de puerta hasta mediados de la década de 1970, cuando llegó a ser dominante polisilicio, debido a su capacidad para formar puertas de auto-alineados. puertas metálicas están recuperando popularidad, ya que es difícil aumentar la velocidad de funcionamiento de los transistores, sin puertas metálicas.

IGFET es un término relacionado con el sentido de aislamiento-puerta del transistor de efecto de campo, y es casi sinónimo de MOSFET, aunque puede hacer referencia a FETs con un aislador de la puerta que no es el óxido. Otro sinónimo es MISFET FET de metal-aislante-semiconductor.



Composición
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Por lo general, la semiconductor de elección es la silicio, Pero algunos fabricantes de chips, sobre todo IBM, Recientemente comenzó a utilizar un compuesto químico (Bond, no una mezcla) de silicio y germanio (SiGe) En los canales MOSFET. Por desgracia, muchos semiconductores con mejores propiedades eléctricas del silicio, como arseniuro de galio, No forman una buena interfaz de semiconductores-a-aislante, por lo tanto no son adecuados para MOSFETs. La investigación continúa en la creación de aisladores con aceptables características eléctricas de material semiconductor otros.

A fin de superar aumentar el poder de consumo debido a puerta corriente de fuga, κ dieléctrico de alta reemplaza dióxido de silicio para el aislador de la puerta, aunque el regreso de puertas de metal mediante la sustitución de polisilicio (Véase el anuncio de Intel[1]).

La puerta está separada del canal por una delgada capa aislante, tradicionalmente de dióxido de silicio y luego de oxinitruro silicio. Algunas compañías han empezado a introducir una κ dieléctrico de alta + de compuerta metálica combinación en el De 45 nanómetros nodo.

Cuando se aplica un voltaje entre la puerta y terminales cuerpo, el campo eléctrico generado penetra a través del óxido y crea un supuesto "capa de inversión" o "canal" en la interfase semiconductor-aislante. El canal de inversión es del mismo tipo, tipo P o tipo N, como la fuente y el drenaje, por lo que proporciona un canal a través del cual puede pasar corriente. Variando el voltaje entre la puerta y el cuerpo modula la conductividad de esta capa y permite controlar el flujo de corriente entre el drenaje y la fuente

Símbolos
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Una variedad de símbolos se utilizan para el MOSFET. El diseño básico es generalmente una línea para el canal con la fuente y el drenaje dejando en ángulo recto y luego doblar hacia atrás en ángulo recto en la misma dirección que el canal. A veces, tres segmentos de línea se utilizan para en modo mejorado y una línea continua para el modo de agotamiento. Otra línea es paralela al canal de la puerta.

La conexión a granel, si aparece, se muestra conectado a la parte posterior del canal con una flecha que indica OGP o NMOS. Las flechas apuntan siempre de P a N, por lo que un NMOS (N-canal en la categoría P-bien o sustrato P-) tiene la flecha apuntando en (a partir de la mayor parte de la canal). Si la mayor parte se conecta a la fuente (como suele ser el caso de los dispositivos discretos) a veces es en ángulo para encontrarse con la fuente dejando el transistor. Si la mayor parte no se muestra (como ocurre a menudo en el diseño de IC, ya que generalmente son a granel común) un símbolo de la inversión se utiliza a veces para indicar PMOS, alternativamente, una flecha en la fuente puede ser utilizado en la misma forma que para los transistores bipolares ( a cabo para nMOS, en la pMOS).

Comparación de la mejora en modo de empobrecimiento y símbolos-MOSFET, junto con JFET símbolos (dibujado con fuente y el drenaje ordenó de tal manera que parecieran más altos voltajes más altos en la página de tensiones inferiores):



Para los símbolos en los que la mayor parte, o el cuerpo, la terminal se muestra, es que aquí se muestra conectado internamente a la fuente. Esta es una configuración típica, pero de ninguna manera la única configuración importante. En general, el MOSFET es un dispositivo de cuatro terminales, y, en muchos circuitos integrados de la cuota de MOSFETs una conexión entre el cuerpo, no necesariamente conectadas a los terminales de la fuente de todos los transistores

MOSFET operación

Metal-óxido-semiconductor estructura

Un metal tradicional-óxido-semiconductor (MOS) la estructura se obtiene por cultivo de una capa de dióxido de silicio (SiO2) En la parte superior de un sustrato de silicio y depositar una capa de metal o de silicio policristalino (Este último es de uso general). A medida que el dióxido de silicio es un dieléctrico material, su estructura es equivalente a un plano condensador, Con uno de los electrodos sustituye por un semiconductor.

Cuando se aplica un voltaje a través de una estructura MOS, que modifica la distribución de cargas en el semiconductor. Si consideramos un semiconductor tipo P (con NUn la densidad de aceptantes, p la densidad de agujeros; p = NUn a granel neutro), una tensión positiva, VGB, De puerta a cuerpo (véase la figura), se crea una agotamiento de la capa obligando a los agujeros de carga positiva de distancia desde la interfaz de gate-insulator/semiconductor, dejando al descubierto una región libre de operador inmóvil, iones con carga negativa aceptor (véase dopaje (semiconductores)). Si VGB es lo suficientemente alto, una alta concentración de portadores de carga negativa se forma en una capa de inversión situado en una capa delgada al lado de la interfaz entre el semiconductor y el aislador. A diferencia de los MOSFET, donde los electrones capa de inversión se suministran rápidamente de la fuente / electrodos de drenaje, en el capacitor MOS se producen mucho más lentamente por la generación térmica a través de compañía de generación y recombinación centros en la región de agotamiento. Convencionalmente, el voltaje de la puerta en la que la densidad de volumen de los electrones en la capa de inversión es la misma que la densidad de volumen de agujeros en el cuerpo se denomina umbral de tensión.

Esta estructura con cuerpo de tipo P es la base del MOSFET del N-tipo, que requiere la adición de una fuente de tipo N y las regiones de drenaje.

Ejemplo de circuito con un MOSFET del N-Canal. Cuando se activa el interruptor. El LED se activa a través del resistor limitador de corriente. La resistencia por debajo del interruptor garantiza la capacitancia de la puerta del MOSFET es deplated cuando el interruptor no está activado.




MOSFET estructura y formación de canales
Un transistor de metal-óxido-semiconductor de efecto de campo (MOSFET) se basa en la modulación de la concentración de carga por una capacidad MOS entre un cuerpo electrodo y un puerta electrodos situados por encima del cuerpo y de todas las regiones aisladas otro dispositivo por una puerta capa dieléctrica que en el caso de un MOSFET es un óxido, como el dióxido de silicio. Si dieléctricos que no sea un óxido como el dióxido de silicio (a menudo citado como el óxido) se emplean el dispositivo puede ser referido como un transistor FET de metal-aislante-semiconductor (MISFET). En comparación con el condensador MOS, el MOSFET incluye dos terminales adicionales (fuente y desagüe), Cada uno conectado a las distintas regiones altamente dopadas que están separadas por la región del cuerpo. Estas regiones pueden ser p o tipo n, pero ambos deben ser del mismo tipo, y de tipo opuesto a la región del cuerpo. La fuente y el drenaje (a diferencia del cuerpo) son altamente dopado según lo significado por un signo "+" después de que el tipo de dopaje.
Si el MOSFET es un n-FET de canal o nMOS, entonces la fuente y el drenaje son "regiones n + 'y el cuerpo es una' p 'región. Como se describió anteriormente, con suficiente voltaje de la puerta, por encima de un umbral de tensión valor, los electrones de la fuente (y posiblemente[cita requerida] como el drenaje) se introducen en la capa de inversión o N-Canal en la interfase entre la región P y el óxido. Este canal conductor se extiende entre la fuente y el desagüe, y la corriente se lleva a cabo a través de él, cuando se aplica un voltaje entre la fuente y el drenaje.

Para tensiones de puerta por debajo del valor umbral, el canal es poco poblada, y sólo una muy pequeña subliminales fugas corriente puede fluir entre la fuente y el drenador.

Si el MOSFET es un p-FET de canal o pMOS, entonces la fuente y el drenaje son "regiones + p 'y el cuerpo es una' n 'región. Cuando una puerta-fuente de voltaje negativo (fuente positiva-gate) se aplica, se crea una P-Canal en la superficie de la región n, de forma análoga al caso de canal n, pero con polaridades opuestas de las cargas y tensiones. Cuando una tensión menos negativo que el valor umbral (de una tensión negativa para el canal p) se aplica entre la puerta y la fuente, el canal desaparece y sólo una pequeña corriente subumbral puede fluir entre la fuente y el drenador.

La fuente se llama así porque es la fuente de los portadores de carga (electrones de canal n, los agujeros de canal p), que pasan a través del canal, de manera similar, la fuga es donde los portadores de carga deja el canal.

El dispositivo puede comprender un dispositivo de silicio sobre aislante (SOI) dispositivo en el que un Enterrado óxido (BOX) se forma debajo de una capa semiconductora delgada. Si la región del canal entre la puerta dieléctrica y una región Enterrado Óxido (BOX) es muy delgada, la región del canal muy delgado se conoce como un canal ultra delgado (UTC) región con la fuente y el drenaje de las regiones formadas a ambos lados del mismo y o encima de la capa semiconductora delgada. Por otra parte, el dispositivo podrá constar de un semiconductor sobre aislante (SEMOI) dispositivo en el que otros semiconductores de silicio que se emplean. Muchos materiales semicondutor alternativa puede ser empleado.

Cuando la fuente y las regiones de drenaje se forman por encima del canal en la Fuente de todo o en parte, se refieren a ellas como Criado / drenaje (RSD) regiones.






Modos de funcionamiento

El funcionamiento de un MOSFET se pueden separar en tres modos diferentes, dependiendo de las tensiones en los terminales. En la discusión siguiente, un modelo simplificado se utiliza algebraica que sólo es exacto para la tecnología antigua. Moderno características MOSFET requieren modelos de computadora que tienen un comportamiento bastante más compleja.

Para una de modo mejorado, n-MOSFET de canal, Los tres modos de funcionamiento son:

De corte, subliminales, o débil inversión en modo



con CD = capacidad de la agotamiento de la capa y COX = capacidad de la capa de óxido. En un dispositivo largo del canal, no hay dependencia de voltaje de drenaje de la actual, una vez VDS >> VT, Pero como la longitud del cauce se reduce fuga inducida por bajar la barrera introduce la dependencia de drenaje de voltaje que depende en una forma compleja de la geometría del dispositivo (por ejemplo, el dopaje canal, el dopaje de unión y así sucesivamente). Con frecuencia, voltaje de umbral Vº de este modo es definido como el voltaje de la puerta en la que un valor seleccionado de la corriente ID0 ocurre, por ejemplo,D0 = 1 μA, que no podrá ser el mismo Vº-Valor que se utiliza en las ecuaciones para los siguientes modos.

Algunos circuitos analógicos microcentrales están diseñados para aprovechar de la conducción subumbral. Al trabajar en la región débil inversión, los MOSFETs en estos circuitos de brindar la mayor proporción posible de transconductancia a corriente, a saber: gm / YoD = 1 / (nVT), Casi la de un transistor bipolar.

El subliminales I-V curva depende de manera exponencial en tensión de umbral, al presentar una fuerte dependencia de cualquier variación de fabricación que afecta a tensión de umbral, por ejemplo: variaciones en el espesor de óxido, la profundidad de unión, o el organismo antidopaje que cambiar el grado de fuga inducida por bajar la barrera. La sensibilidad a las variaciones resultantes fabricational complica la optimización de las fugas y el rendimiento





riodo modo o región lineal (también conocido como el modo óhmico)))
¿Cuándo VGS > Vº y VDS <(VGS - Vº ) El transistor se enciende, y un canal se ha creado lo que permite que la corriente fluya entre el drenaje y la fuente. El MOSFET funciona como una resistencia, controlada por el voltaje de la puerta respecto a la fuente y el drenaje de tensiones. La corriente del drenaje a la fuente se modela como:

donde μn es la movilidad de los portadores de carga efectiva, W es el ancho de puerta, L es la longitud de puerta y Cox es la capacidad de óxido de puerta por unidad de superficie. La transición de la región subumbral exponencial para la región triodo no es tan aguda como sugieren las ecuaciones.

Saturación o activa el modo
¿Cuándo VGS > Vº y VDS > (VGS - Vº )
El interruptor está encendido, y un canal se ha creado, lo que permite que la corriente fluya entre el drenaje y la fuente. Puesto que el voltaje de drenaje es mayor que el voltaje de la puerta, los electrones hacia fuera, y la conducción no es a través de un estrecho canal sino a través de una más amplia, de dos o distribución actual de tres dimensiones se extiende desde la interfaz y más profundo en el sustrato. El inicio de esta región es también conocida como pinch-off para indicar la falta de región de los canales cerca del desagüe. La corriente de drenaje es ahora débilmente dependiente de voltaje de drenaje y controlado principalmente por la tensión compuerta-fuente, y modelado muy aproximadamente como:

El factor adicional que implique una λ, la canal de longitud de modulación parámetro, la dependencia de los modelos actuales sobre la tensión de fuga debido a la Los primeros efectos, O el canal de modulación de longitud. De acuerdo a esta ecuación, un parámetro clave del diseño, la transconductancia MOSFET es:

donde la combinación Vov V =GS - Vo se llama el tensión de saturación. Otro parámetro clave de diseño es la resistencia de salida MOSFET rO dada por:

rout es la inversa de gds donde

VDS es la expresión en la región de saturación.
Si λ se toma como cero, una resistencia de salida infinita de los resultados del dispositivo que lleva a predicciones circuito de poco realista, sobre todo en circuitos analógicos.
Como la longitud del cauce se hace muy corto, estas ecuaciones llegar a ser bastante inexactos. Nuevos efectos físicos surgir. Por ejemplo, el transporte del portador en el modo activo puede llegar a ser limitada por velocidad de saturación. Cuando la velocidad de saturación domina, la fuga de corriente de saturación es más casi lineal de segundo grado en el VGS. En longitudes aún más corto, las compañías de transporte con cerca de cero de dispersión, conocida como cuasi-transporte balístico. Además, la corriente de salida se ve afectada por fuga inducida por bajar la barrera de la tensión de umbral.
consumo de corriente en función del drenaje-fuente de tensión y la puerta-fuente sesgo sobre la tensión umbral.

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2

JFET

El JFET (Junction Field-Effect Transistor, en español transistor de efecto de campo de unión) es un dispositivo electrónico, esto es, un circuito que, según unos valores eléctricos de entrada, reacciona dando unos valores de salida. En el caso de los JFET, al ser transistores de efecto de campo eléctrico, estos valores de entrada son las tensiones eléctricas, en concreto la tensión entre los terminales S (fuente) y G (puerta), VGS. Según este valor, la salida del transistor presentará una curva característica que se simplifica definiendo en ella tres zonas con ecuaciones definidas: corte, óhmica y saturación.
Físicamente, un JFET de los denominados "canal P" está formado por una pastilla de semiconductor tipo P en cuyos extremos se sitúan dos patillas de salida (drenador y fuente) flanqueada por dos regiones con dopaje de tipo N en las que se conectan dos terminales conectados entre sí (puerta). Al aplicar una tensión positiva (en inversa) VGS entre puerta y fuente, las zonas N crean a su alrededor sendas zonas en las que el paso de electrones (corriente ID) queda cortado, llamadas zonas de exclusión. Cuando esta VGS sobrepasa un valor determinado, las zonas de exclusión se extienden hasta tal punto que el paso de electrones ID entre fuente y drenador queda completamente cortado. A ese valor de VGS se le denomina Vp. Para un JFET "canal N" las zonas p y n se invierten, y las VGS y Vp son positivas, cortándose la corriente para tensiones mayores que Vp.
Así, según el valor de VGS se definen dos primeras zonas; una activa para tensiones negativas mayores que Vp (puesto que Vp es también negativa) y una zona de corte para tensiones menores que Vp. Los distintos valores de la ID en función de la VGS vienen dados por una gráfica o ecuación denominada ecuación de entrada.
En la zona activa, al permitirse el paso de corriente, el transistor dará una salida en el circuito que viene definida por la propia ID y la tensión entre el drenador y la fuente VDS। A la gráfica o ecuación que relaciona estás dos variables se le denomina ecuación de salida, y en ella es donde se distinguen las dos zonas de funcionamiento de activa: óhmica y saturación

Ecuaciones del transistor JFET
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Mediante la gráfica de entrada del transistor se pueden deducir las expresiones analíticas que permiten analizar matemáticamente el funcionamiento de este. Así, existen diferentes expresiones para las distintas zonas de funcionamiento.
Para VGS la IDSS la ID de saturación que atraviesa el transistor para VGS = 0, la cual viene dada por la expresión:Los puntos incluidos en esta curva representan las ID y VGS (punto de trabajo, Q) en zona de saturación, mientras que los puntos del área inferor a ésta representan la zona óhmica.
Para VGS > Vp (zona de corte): ID = 0En la gráfica de salida se pueden observar con más detalle los dos estados en los que el JFET permite el paso de corriente. En un primer momento, la ID va aumentando progresivamente según lo hace la tensión de salida VDS. Esta curva viene dada por la expresión:
que suele expresarse como siendo
Por tanto, en esta zona y a efectos de análisis, el transistor puede ser sustituido por una resistencia de valor Ron, con lo que se observa una relación entre la ID y la VDS definida por la Ley de Ohm. Esto hace que a esta zona de funcionamiento se le denomina zona óhmica.

A partir de una determinada VDS la corrient

Por tanto, en esta zona y a efectos de análisis, el transistor puede ser sustituido por una resistencia de valor Ron, con lo que se observa una relación entre la ID y la VDS definida por la Ley de Ohm. Esto hace que a esta zona de funcionamiento se le denomina zona óhmica.

A partir de una determinada VDS la corriente ID deja de aumentar, quedándose fija en un valor al que se denomina ID de saturación o IDSAT. El valor de VDS a partir del cual se entra en esta nueva zona de funcionamiento viene dado por la expresión: VDS = VGS − Vp. Esta IDSAT, característica de cada circuito, puede calcularse mediante la expresión:e ID deja de aumentar, quedándose fija en un valor al que

YOSEPH L. BUITRAGO L.

C.I. 18.257.871

EES. SECCION 2