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The 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 transistor. Although there are important

differences between the two types of devices, there are also many similarities that will be

pointed out in the sections to follow. The primary difference between the two types of

transistors is the fact that the BJT transistor is a current-controlleddevice as depicted in Fig.

5.1a, while the JFET transistor is a voltage-controlleddevice.

The field-effect transistor (FET) is a transistor that relies on an electric field to control the

shape and hence the conductivity of a channel of one type ofcharge carrier in

a semiconductor material. FETs are sometimes called unipolar transistors to contrast their

single-carrier-type operation with the dual-carrier-type operation ofbipolar (junction)

transistors (BJT). The conceptof the FET predates the BJT, though it was not physically

implemented until afterBJTs due to the limitations of semiconductor materials and the relative

ease of manufacturing BJTs compared to FETs at the time.

FETs are majority-charge-carrier devices. The device consists of an active channel through

which majority charge carriers, electrons or holes, flow from the source to the drain. Source

and drain terminal conductors are connected to semiconductor through ohmic contacts. The

conductivity of the channel is a function of potential applied to the gate.

The FET's three terminals are:

Source (S), through which the majority carriers enter the channel. Conventional current

entering the channel at S is designated by IS.

Drain (D), through which the majority carriers leave the channel. Conventional current

entering the channel at D is designated by ID. Drain to Source voltage is VDS.

Gate (G), the terminal that modulates the channel conductivity. By applying voltage to G,

one can control ID.

Two types of FETs:

Junction field-effect transistor(JFET)

Metal-oxide-semiconductor field-effect transistor (MOSFET).
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The MOSFET category is further broken down into depletion and enhancement types,

which are both described. The MOSFET transistor has become one of the most important

devices used in the design and construction of integrated circuits for digital computers. Its

thermal stability and other general characteristics make it extremely popular in computer

circuit design. However, as a discrete element in a typical top-hat container, it must be handled

with care. Once the FET construction and characteristics have been introduced, the biasing

arrangements. The analysis performed using BJT transistors will prove helpful in the derivation

of the important equations and understanding the results obtained for FET circuits.

Junction gate field-effect transistor (JFET or JUGFET)

The junction gate field-effect transistor (JFET or JUGFET) is the simplest type offield-effecttransistor. It can be used as anelectronically-controlledswitchor as a voltage-controlled

resistance.Electric chargeflows through a semiconducting channel between "source" and "drain"

terminals. By applying a biasvoltageto a "gate" terminal, the channel is "pinched", so that theelectric

currentis impeded or switched off completely.

Construction Diagram

N-channel JFET P-channel JFET
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Schematic Diagram

N-channel JFET P-channel JFET

Characteristic Curve

JFET Transfer Characteristic Curve


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Operation

VDD provides a drain-source voltage, VDS, that causes a drain current. iD, from drain

to source.

The drain current, iD, which is identical to the source current, exists in the channel

surrounded by

p-type gate. The gate to source voltage, VGS, which is equal to -VGG, creates a depletion

region in the channel, which reduces the channel width and hence increases the

resistance between drain and source. Since the gate-source junction is reverse-biased, a

zero gate current results.

So basically, the more voltage is applied at the gate, the less current will flow from drain

to source. Think of this as a garden hose, the more you squeeze it, the less water will

flow through it.

When we increase VDS, the drain current, iD, also increases. As VDS further increases, a

point is reached where the drain current is in its saturation point. If we increase

VDS beyond this point, iD remains constant.

Metal

oxide

semiconductor field-effect transistor (

The metaloxidesemiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is

atransistorused for amplifying or switching electronicsignals. Like otherfield-effect transistors, a

MOSFET is usually a three-terminal device with source (S), gate (G), and drain (D) terminals; the

substrate of the MOSFET is sometimes connected to the source terminal, and is sometimes a separate

fourth terminal.
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In MOSFETs, a voltage on the oxide-insulated gate electrode can induce aconducting

channelbetween the two other contacts called source and drain. The channel can be ofn-typeorp-

type(see article onsemiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also

commonly nMOS, pMOS). It is by far the most commontransistorin bothdigitaland analog circuits,

though thebipolar junction transistorwas at one time much more common.

The 'metal' in the name is now often amisnomerbecause the previously metal gate material is now

often a layer ofpolysilicon(polycrystalline silicon).Aluminiumhad been the gate material until the mid

1970s, when polysilicon became dominant, due to its capability to formself-aligned gates. Metallic gates

are regaining popularity, since it is difficult to increase the speed of operation of transistors withoutmetal

gates.

Types of MOSFET:

Depletion Enhancement MOSFET

Enhancement type MOSFET

DEMOSFET-Depletion Enhancement MOSFET

We know that when the gate is biased negative with respect to the source in an N-channel JFET, the

depletion region widths are increased. The increase in the depletion regions reduces the channel

thickness, which increases its resistance. The net result is that drain current ID is reduced.

If the polarity of VGG were reversed so as to apply a positive bias to the gate with respect to source, the

P-N junctions between the gate and the channel would then be forward biased. Since a forward bias

reduces the width of a depletion region, the thickness of channel would increase with a corresponding

decrease in channel resistance. As a result, drain current IDwould increase beyond the JFETs IDSSvalue.

The normal operation of aJFETis in its depletion mode of operation. However, as discussed above, it is

also possible to enhance the conductivity of the JFET channel. However, the forward bias of the siliconP-

N junctionis usually restricted to a maximum of 0.5 V (more conservative limit is 0.2 V) so as to limit the

gate current.

As we have seen that, the greater the ID is compared to IDSS the greater the transconductance gmwill be.

We have seen before that the voltage gain is directly proportional to gm. So, in general, the higher

the gm, the better it is. This is one of the advantages of being able to enhance the channel.
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As its name suggests, the depletion-enhancementMOSFET(DE-MOSFET)-was developed to be used in

either or both the depletion and enhancement modes.


Schematic Symbols of DEMOSFET

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Operation

DE-MOSFET can be operated with either a positive or a negative gate. When gate is positivewith respect to the source it operates in the enhancementor E-mode and when the gate is

negative with respect to the source, as illustrated in figure, it operates in depletion-mode.

When the drain is made positive with respect to source, a drain current will flow, even with zero

gate potential and the MOSFET is said to be operating in E-mode. In this mode of operation gate

attracts the negative charge carriers from the P-substrate to the N-channel and thus reduces the


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channel resistance and increases the drain-current. The more positive the gate is made, the more

drain current flows.

On the other hand when the gate is made negative with respect to the substrate, the gate repels

some of the negative charge carriers out of the N-channel. This creates a depletion region in the

channel, as illustrated in figure, and, therefore, increases the channel resistance and reduces thedrain current. The more negative the gate, the less the drain current. In this mode of operation the

device is referred to as a depletion-mode MOSFET. Here too much negative gate voltage can

pinch-off the channel. Thus operation is similar to that of JFET.

Enhancement type MOSFET

Although there 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 differentfrom anything obtained thus far. The transfer curve is not defined by Shockleys equation, and the drain

current is now cut off until the gate-to-source voltage reaches a specific magnitude. In particular, 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 for n-channel JFETs and n-channel depletion-type MOSFETs.


Schematic Symbol


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Schematic symbols for an N-channel E-MOSFET are shown in figure. For zero value of VGS,

the E-MOSFET is OFF because there is no conducting channel between source and drain.

Each of schematic symbols shown in figures, has broken channel line to indicate this

normally OFF condition. As we know that for VGS exceeding the threshold voltage VGST, an

N-type inversion layer, connecting the source to drain, is created. In each of the schematic

symbols, the arrow points to this inversion layer, which acts like an N-channel when the

device is conducting. In each case, the fact that the device has an insulated gate is indicated

by the gate not making direct contact with the channel. The schematic symbol shown in

figure shows the source and substrate internally connected, while the other symbol shown in

figure shows the substrate connection brought out separately from the source.

The schematic symbols for a P-channel E-MOSFET are also shown. In these cases the arrow

points outwards.


Drain characteristics of an N-channel E-MOSFET are shown in figure. The lowest curve is

the VGSTcurve. When VGS is lesser than VGST, ID is approximately zero. When VGS is greater

than VGST, the device turns- on and the drain current ID is controlled by the gate voltage. The

characteristic curves have almost vertical and almost horizontal parts. The almost vertical


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components of the curves correspond to the ohmic region, and the horizontal components

correspond to the constant current region. Thus E-MOSFET can be operated in either of

these regions i.e. it can be used as a variable-voltage resistor (WR) or as a constant current

source.

Figure shows a typical transconductance curve. The current IDSS at VGS


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As its name indicates, this MOSFET operates only in the enhancement mode and has no

depletion mode. It operates with large positive gate voltage only. It does not conduct when

the gate-source voltage VGS= 0. This is the reason that it is called normally-off MOSFET. In

these MOSFETsdrain current ID flows only when VGS exceeds VGST [gate-to-

source threshold voltage]. When drain is applied with positive voltage with respect to source and no potential is

applied to the gate two N-regions and one P-substrate from two P-N junctions connected

back to back with a resistance of the P-substrate. So a very small drain current that is,

reverse leakage current flows. If the P-type substrate is now connected to the source

terminal, there is zero voltage across the source substrate junction, and the-drain-substrate

junction remains reverse biased.

When the gate is made positive with respect to the source and the substrate, negative (i.e.

minority) charge carriers within the substrate are attracted to the positive gate and

accumulate close to the-surface of the substrate. As the gate voltage is increased, more and

more electrons accumulate under the gate. Since these electrons can not flow across the

insulated layer of silicon dioxide to the gate, so they accumulate at the surface of the

substrate just below the gate. These accumulated minority charge carriers N -type channel

stretching from drain to source. When this occurs, a channel is induced by forming what is

termed an inversion layer(N-type). Now a drain current start flowing. The strength of

the drain current depends upon the channel resistance which, in turn, depends upon the

number of charge carriers attracted to the positive gate. Thus drain current is controlled by

the gate potential.

Since the conductivity of the channel is enhanced by the positive bias on the gate so this

device is also called the enhancement MOSFETor E- MOSFET.

The minimum value of gate-to-source voltage VGS that is required to form the inversion

layer (N-type) is termed the gate-to-source threshold voltageVGST. For VGS below VGST, the

drain current ID = 0. But for VGS exceeding VGST an N-type inversion layer connects the

source to drain and the drain current ID is large. Depending upon the device being used,

VGST may vary from less than 1 V to more than 5 V.

JFETs and DE-MOSFETs are classified as the depletion-mode devices because their

conductivity depends on the action of depletion layers. E-MOSFET is classified as an

enhancement-mode device because its conductivity depends on the action of the inversion

layer. Depletion-mode devices are normally ON when the gate-source voltage VGS = 0,whereas the enhancement-mode devices are normally OFF when VGS = 0.

Application


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