So far we have studied ordinary transistors which are also called Bipolar Junction Transistors (BJT). But a BJT is noisy in operation and has low input impedance. Due to these two factors BJTs are replaced by the new type of transistors called field effect transistors (FETS), which have been proved to be very efficient device especially in I.C. form.
A FET is a three-terminal, unipolar semiconductor device in which current conduction takes place due to an electric field applied at its input.
CLASSIFICATION OF FET
FETs may be put into two major categories.
1. Junction field effect transistors. JFET or simply FET)
The FETs may be an N-type or a P-type.
2. Insulated gate field effect transistors (IGFET)
The IGFET are also called 'metal oxide semiconductor field effect transistors' (MOSFET or simply MOS).
The MOSFETS are again of two types:
(a) Enhancement MOSFET
(b) Depletion MOSFET
The ordinary FET is sometimes also known as JFET (Junction Field Effect Transistor).
CONSTRUCTION OF A JFET/FET
The JFET (or FET) is a three terminal device, with one terminal capable of controlling the current between the other two. An FET is essentially N-type or P-type silicon bar containing two PN junction diodes on its sides.
Accordingly, the FET may be:
1. N-channel FET: When the bar is of N-type silicon material, such a FET is called N-channel FET.
2. P-channel FET: When the bar is of P-type silicon material, such a FET is called P-channel FET.
Terminals: The three terminals of an FET are
(a) Gate (G) :The two P-N junctions forming diodes are connected together internally and it forms a common terminal called "Gate" (G).
(b) Source (S) : The bottom of the channel is connected through an ohmic contact to a terminal called "Source" (S).
(c)Drain (D): The top of the channel is connected through an ohmic contact to a terminal called "Drain" (D).
BIASING OF FET
Usually a biasing voltage (VDS) is applied between Source (S) and Drain (D). Thus the source is at greater negative potential with respect to the drain so that electrons (electric current) move from S to D. However, if the drain is kept at higher negative potential with respect to source, electrons will move from drain to source because S and D are interchangeable.
A reverse bias (VGS) is applied at the gate (G) with respect to the source (S). (VGS) is the electric field which controls the amount of electrons (current from S to D or D to S).
OPERATION OF N-JFET
The basic working principle of an FET is the field effect'. Its operation depend upon the electric field applied at its input GATE terminal (G). It will be easy to understand this with an example:
We are familiar with ability of a magnet to pull (attract) metal fillings without any physical contact between magnet and the fillings. The "magnetic field of the magnet attracts them through magnetic lines of force as short as possible. Similarly, in case of a FET, an electric field is established by the charges present, that will control the conduction path of the output circuit without any direct contact between controlling and the controlled quantities.
The operation of the circuit can be explained in following steps.
(a)When a voltage VDS is applied between drain and source and no voltage is applied on the gate (switch is open), the P-N junctions on the sides will produce depletion layers and, as S is at higher negative potential with respect to D, the electrons will move from S to D. The number of electrons (i.e., current) from S to D will depend upon the width of passage between the two depletion layers. If the depletion layers are narrow, the passage between them will be more and hence the current will be more.
(b) If the switch of VGS is closed, a reverse bias is applied on the gate (G). (Recall that G is the common terminal of both P-N junctions). This reverse bias will increase width of depletion layers. This will reduce the passage between the layers and now less number of electrons (i.e., current) can pass through from S to D.
It may be noted that gate is the controlling terminal for the current flow from source to drain. In other words, the current can be controlled by the electric field (VGS) applied on the gate of a FET. This justifies name field effect transistor (FET) for the device.
OPERATION OF P-JFET
A P-FET works in the same manner as described for an N-FET. Moreover, the conduction will be through holes and not through electrons.
IMPORTANT POINTS ABOUT FET
1. A FET is a 'solid equivalent" of vaccum triode. Just as the grid carries very small current and grid voltage controls the operation of the tube, similarly the gate in an FET takes very small current and gate voltage (VGS) controls the FET operation. Thus, a vaccum triode and a FET both are voltage driven devices.
2. From the above discussion we can conclude that a FET has combination of properties or vaccum tube as well as of transistor.
3. In a bipolar transistor, the current is to pass through two junctions and, therefore, the operation becomes noisy. In a FET, there is no junction in the passage of the current from S to D, hence operation of FET is almost noise free.
4. Due to high input impedance an FET provides a high degree of isolation between input and output circuits. This property is useful for its use as buffer amplifier.
5. The FET also produces a phase reversal of 180° between input and output as a transistor.
6. The FET may be used in any of the following three configurations (as in the case of transistors):
(a) Common source configuration
(b) Common drain configuration
(c)Common gate configuration
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