The Field Effect Transistor is a device which enables us to use one electrical signal to control another. The name ‘transistor’ is a shortened version of the original term, transfer resistor, which indicates how the device works. Most transistors have three connections. The voltage on (or current into/out of) one wire has the effect of controlling the ease with which current can move between the other two terminals. The effect is to make a ‘resistance’ whose value can be altered by the input signal. We can use this behaviour to ‘transfer’ patterns of signal fluctuation from a small input signal to a larger output signal.
A wide variety of devices are called transistors. Here will just look at one example, called an N-channel Junction-FET (J-FET). Figure 7.1 illustrates what this looks like. This sort of transistor is made by forming a channel of N-type material in a substrate of P-type material. Three wires are then connected to the device. One at each end of the channel. One connected to the substrate. In a sense, the device is a bit like a PN-junction diode, except that we've connected two wires to the N-type side.
Electrons can move along the channel, so when we apply a voltage between the two end-wires a current will flow along the channel. We can maintain this by continually putting electrons in one end (the source) and removing them at the other (the drain). The effective resistance between the two ends will depend upon the size & shape of the channel and the properties of the N-type material. Note, however, that electrons moving in the channel will — just as with the diode — be repelled by the fixed charges in the P-type substrate. As a result, the current doesn't fill the whole channel. It avoids the depletion zones near the walls.
Consider now what happens when we use the third wire to apply a small -ve voltage to the substrate. This increases the forces which push channel electrons away from the walls. As a result, the depletion zones get bigger and the cross-sectional area the current has to squeeze through gets smaller. The effect is a bit like what happens when we put our foot on a garden hose. The -ve voltage applied to the third wire makes it harder for the current to get through. If we squeeze hard enough we can reduce the flow to zero and pinch off the current. The third connection is called the gate because it can be used to control the flow of electrons along the channel.
If we wish, we can apply a small +ve voltage to the gate, make the depletion zones smaller, and let the current through more easily. However, we can't take this too far. Remember that the substrate-channel boundary is a PN-junction. If we apply too large a +ve gate voltage we'll make it possible for channel electrons to cross the walls and move into the substrate. This won't help us get them moving from source to drain, so it isn't any use. What's worse, modern FET's have such tiny gates that even a small channel-gate current will blow up the transistor.
(One of the basic rules of practical electronics is that; “Transistors blow up faster than fuses!”)
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.