Field Effect Transistor
It is a three terminal semiconductor device in which current conduction is by one type of charge carrier, i.e. either by holes or electrons and is controlled by the effect of electric field. The FET has high input impedance and low noise level.
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| Field effect transistor |
Construction of FET
A FET consists of a p-type and n-type silicon bar containing two PN junctions at the sides. The bar forms the conducting channel for the charge carriers. When the bar FET is of n-type, it is called n-channel FET and when the bar of FET is of p-type, it is called p-channel FET. The two PN junctions that make up the diode are connected internally and the common terminal is called a gate. Other terminals of FET are source and drain taken out from the bar. Thus a FET has essentially three terminals viz., gate (G), source (S) and drain (D).
Schematic symbol of FET
The schematic symbol of a FET is shown in figure. The vertical line in the symbol can be visualized as the channel to which the source and drain are connected. The third terminal gate is connected at the center of the vertical line. In a n-channel FET, the gate and the channel from a PN junction, the gate arrow points inwards and in p-channel FET, the gate and channel from an PN junction, the gate arrow points outward.
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| Schematic symbol of FET |
Working method
When a voltage (VDS) is applied between drain and source terminals and voltage on the gate is zero, the two PN junctions at the sides of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion layers. The size of these layers determines the width of channel and therefore the current conduct through the bar.
When a reverse voltage (VGS) is applied between the gate and source, the width of the depletion layers is increased. This reduces the width of conducting channel, thereby increasing the resistance of n-type bar. Constantly, the current from source to drain is decreased. On the other hand, if the reverse voltage on the gate is decreased, the width of depletion layers also decreases. This increases the width of conduction channel and hence source to drain current.
Thus, it is obvious that the current from source to drain can be controlled by the application of potential on the gate. That is why, the device is known as field effect transistor (FET). It may be noted that p-channel FET also operates in the similar fashion as an n-channel FET except that the channel current carriers will be holes instead of electrons and all the polarities will be reversed.
Output Characteristics of FET
The output characteristics of the FET is a curve drawn between drain current and drain-source voltage of a FET at constant gate-source voltage.
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| Characteristics of FET |
Advantages of FET
A FET is a voltage controlled, constant current device in which variations in input voltage control the output current. It combines the many advantages of both bipolar transistor and vacuum pentode. Some of the advantages of a FET are :
It has a very high input impedance. This permits high degree of isolation between input and output circuits.
The operation of a FET depends upon the bulk material current Carriers that do not cross junctions.
A FET has a negative temperature coefficient of resistance. This avoids the risk of thermal runway.
A FET has a very high power gain. This eliminates the necessity of using driver stages.
A FET has a smaller size, longer life and high efficiency.