FIELD EFFECT TRANSISTORS (FET)

CHAPTER 5 EE201-SEMICONDUCTOR DEVICES

BYPN. RUHIYAH NAZIHAH ZAHKAI

ELECTRICAL ENGINEERING DEPARTMENTPOLYTECHNIC SULTAN IDRIS SHAH

IntroductionThe field-effect transistor (FET) is a three-terminal

device used for a variety of applications that match, to a large extent, those of the BJT Although there are important differences between the two types of devices.

The primary difference between the two types of transistors is the fact that the◦ BJT transistor is a current-controlled device◦ JFET transistor is a voltage-controlled device

In each case the current of the output circuit is being controlled by a parameter of the input circuit—in one case a current level and in the other an applied voltage.

Two types of FETs will be introduced in this chapter :◦ Junction Field-Effect Transistor (JFET) ◦Metal-Oxide-Semiconductor Field-Effect

Transistor (MOSFET)

JFETJFET is a three-terminal device

with one terminal capable of controlling the current between the other two.

For the JFET transistor the n-channel device will appear as the prominent device than p-channel.

The basic construction of the n-channel JFET .

The major part of the structure is the n-type material that forms the channel between the embedded layers of p-type material.

The top of the n-type channel is connected through an ohmic contact to a terminal referred to as the drain (D).

The lower end of the same material is connected through an ohmic contact to a terminal referred to as the source (S).

The two p-type materials are connected together and to the gate (G) terminal.

Physical structure

Schematic Symbol

N-Channel P-Channel

I-V characteristics

IDSS = Max ID

VP = Pinch Voltage

Operating region

MOSFETThe metal–oxide–semiconductor field-effect

transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronic signals.

In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain.

The channel can be of n-type or p-type, called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS).

MOSFET is further broken down into depletion and enhancement types.

N-MOS & P-MOS

N-MOSn-channel MOSFETs are smaller than

p-channel MOSFETs and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler.

P-MOSP-type metal-oxide-semiconductor

logic uses p-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits.

PMOS transistors have four modes of operation: cut-off (or subthreshold), triode, saturation (sometimes called active), and velocity saturation.

Physical & Schematic diagram

D-MOSFET (Depletion mode)

Have similar characteristics to those of a JFET between cutoff and saturation at IDSS.

Additional feature of characteristics that extend into the region of opposite polarity for VGS.

A slab of p-type material is formed from a silicon base and is referred to as the substrate. (SS) It is the foundation upon which the device will be constructed.

The source and drain terminals are connected through metallic contacts to n-doped regions linked by an n-channel

The gate is also connected to a metal contact surface but remains insulated from the n-channel by a very thin silicon dioxide (SiO2) layer.

D-MOSFET OperationD-MOSFETs can operate in the

depletion and enhancement modes.

Zero bias: The gate is shorted to the source, so drain current equals the IDSS rating of the component.

Depletion mode: The negative gate-source voltage forces free electrons away from the gate, forming a depletion layer that cuts into the channel. As a result, ID < IDSS

Enhancement mode: The positive gate-source voltage attracts free electrons in the substrate toward the channel while driving valence-band holes (in the substrate) away from the channel. As a result, the material to the right of the channel effectively becomes n-type material. ID > IDSS.

E-MOSFETThere are some similarities in construction and

mode of operation between depletion-type and enhancement-type MOSFETs

The characteristics of the enhancement-type MOSFET are quite different from anything obtained thus far.

Current control in an n-channel device is now effected by a positive gate-to-source voltage rather than the range of negative voltages encountered in n-channel depletion-type MOSFETs.

The source and drain terminals are again connected through metallic contacts to n-doped regions.

The absence of a channel between the two n-doped regions.

This is the primary difference between the construction of depletion-type and enhancement-type MOSFETs

E-MOSFET OperationE-MOSFETs are restricted to

enhancement-mode operation.

When an E-MOSFET is zero biased, there is no channel between the source and drain materials, and ID =0A. When VGS exceeds the threshold voltage rating for the component VTH, a channel is formed.

This allows a current to pass through the component. The operation of the E-MOSFET is represented by the transconductance curve. Note that the IDSS rating for the component is, by definition, the value of drain current when VGS =VTH. Since the channel is just beginning to form when VGS= VTH , IDSS≈0A

NMOS I-V Characteristics

Cutoff Mode• Occurs when VGS ≤ VTH(N)

ID= 0

Triode/Linear Mode• Occurs when VGS > VTH(N) and

VDS < VGS-VTH(N)

Saturation Mode• Occurs when VGS > VTH(N) and

VDS ≥ VGS -VTH(N)

PMOS IV Characteristics

• ID, VGS, VDS, and VTH(P) are all negative for PMOS.

• Channel formed when VGS < VTH(P)

• Saturation occurs when VDS ≤ VGS – VTH(P)

P-MOS I-V Curve

Cutoff Mode• Occurs when VGS ≥ VTH(P)

ID= 0

Triode/Linear Mode• Occurs when VGS < VTH(P) and

VDS > VGS -VTH(P)

Saturation Mode• Occurs when VGS < VTH(P) and

VDS ≤ VGS- VTH(P)

JFET AmplifierJFET's can be used to make single

stage class A amplifier circuits with the JFET common source amplifier.

The main advantage JFET amplifiers have over BJT amplifiers is their high input impedance which is controlled by the Gate biasing resistive network formed by R1 and R2 as shown.

The control of the Drain current by a negative Gate potential makes the Junction Field Effect Transistor useful as a switch.

It is essential that the Gate voltage is never positive for an N-channel JFET as the channel current will flow to the Gate and not the Drain resulting in damage to the JFET.

The principals of operation for a P-channel JFET are the same as for the N-channel JFET, except that the polarity of the voltages need to be reversed.

The FET can be used as a linear amplifier or as a digital device in logic circuits.

The enhancement MOSFET is quite popular in digital circuitry, especially in CMOS circuits that require very low power consumption.

FET devices are also widely used in high-frequency applications and in buffering (interfacing) applications.

Common Source Amplifier

The common-source (CS) amplifier may be viewed as a transconductance amplifier or as a voltage amplifier.

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.

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).

Common Drain Amplifier

Also known as a source follower, typically used as a voltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source is the output, and the drain is common to both (input and output), hence its name.

In addition, this circuit is used to transform impedances. ◦For example, the Thévenin resistance of a

combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower, a small resistance. That resistance reduction makes the combination a more ideal voltage source.

Conversely, a voltage follower inserted between a driving stage and a high load (ie a low resistance) presents an infinite resistance (low load) to the driving stage, an advantage in coupling a voltage signal to a large load.

Common Gate Amplifier

Typically used as a current buffer or voltage amplifier. In this circuit the source terminal of the transistor serves as the input, the drain is the output and the gate is common to both.

This configuration is used less often than the common source or source follower.

Usually used in CMOS RF receivers for ease of impedance matching and potentially has lower noise.

MOSFET as SwitchesMOSFET switches use the MOSFET

channel as a low–on-resistance switch to pass analog signals when on, and as a high impedance when off.

In this application the drain and source of a MOSFET exchange places depending on the voltages of each electrode compared to that of the gate.

For a simple MOSFET without an integrated diode, the source is the more negative side for an N-MOS or the more positive side for a P-MOS. All of these switches are limited on what signals they can pass or stop by their gate-source, gate-drain and source-drain voltages, and source-to-drain currents; exceeding the voltage limits will potentially damage the switch.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

MOSFET CHARACTERISTIC CURVE

1. Cut-off RegionHere the operating conditions of the

transistor are zero input gate voltage ( VIN ), zero drain current IDand output voltage VDS = VDD Therefore the MOSFET is switched "Fully-OFF".

Saturation RegionHere the transistor will be biased so

that the maximum amount of gate voltage is applied to the device which results in the channel resistance RDS(on) being as small as possible with maximum drain current flowing through the MOSFET switch. Therefore the MOSFET is switched "Fully-ON".

•The input and Gate are grounded (0v)•Gate-source voltage less than threshold voltage VGS < VTH

•MOSFET is "fully-OFF" (Cut-off region)•No Drain current flows ( ID = 0 )•VOUT = VDS = VDD = "1"•MOSFET operates as an "open switch"

Then we can define the "cut-off region" or "OFF mode" of a MOSFET switch as being, gate voltage,VGS < VTH and ID = 0. For a P-channel MOSFET, the gate potential must be negative.

N-MOS as switch

•The input and Gate are connected to VDD

•Gate-source voltage is much greater than threshold voltage VGS > VTH

•MOSFET is "fully-ON" (saturation region)•Max Drain current flows ( ID = VDD / RL )•VDS = 0V (ideal saturation)•Min channel resistance RDS(on) < 0.1Ω•VOUT = VDS = 0.2V (RDS.ID)•MOSFET operates as a "closed switch"

Then we can define the "saturation region" or "ON mode" of a MOSFET switch as gate-source voltage,VGS > VTH and ID = Maximum. For a P-channel MOSFET, the gate potential must be positive.

P-channel MOSFET Switch

APPLICATION MOSFET AS SWITCH

In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch a simple lamp "ON" and "OFF" (could also be an LED). The gate input voltage VGS is taken to an appropriate positive voltage level to turn the device and therefore the lamp either fully "ON", ( VGS = +ve ) or at a zero voltage level that turns the device fully "OFF", ( VGS = 0).

If the resistive load of the lamp was to be replaced by an inductive load such as a coil, solenoid or relay a "flywheel diode" would be required in parallel with the load to protect the MOSFET from any self generated back-emf.

In a P-channel device the conventional flow of drain current is in the negative direction so a negative gate-source voltage is applied to switch the transistor "ON". This is achieved because the P-channel MOSFET is "upside down" with its source terminal tied to the positive supply +VDD. Then when the switch goes LOW, the MOSFET turns "ON" and when the switch goes HIGH the MOSFET turns "OFF".

This upside down connection of a P-channel enhancement mode MOSFET switch allows us to connect it in series with a N-channel enhancement mode MOSFET to produce a complementary or CMOS switching device as shown across a dual supply.