13.2 Active circuit elements  (Page 6/7)

The three terminals of the FET are called the source (S), drain (D) and gate (G), as shown in [link] . When the gate is not connected, a current of electrons can flow from source (S) to drain (D) easily along the channel. The source is, accordingly, the negative terminal of the transistor. The drain, where the electrons come out, is the positive terminal of the transistor. A few electrons will flow from the n-type channel into the p-type semiconductor of the gate when the device is manufactured. However, as these electrons are not removed (the gate is not connected), a depletion band is set up which prevents further flow into the gate.

In operation, the gate is connected to negative voltages relative to the source. This makes the p-n junction between gate and channel reverse-biased. Accordingly no current flows from the source into the gate. When the voltage of the gate is lowered (made more negative), the depletion band becomes wider. This enlarged depletion band takes up some of the space of the channel. So the lower the voltage of the gate (the more negative it is relative to the source), the larger the depletion band. The larger the depletion band, the narrower the channel. The narrower the channel, the harder it is for electrons to flow from source to drain.

The voltage of the gate is not the only factor affecting the current of electrons between the source and the drain. If the external circuit has a low resistance, electrons are able to leave the drain easily. If the external circuit has a high resistance, electrons leave the drain slowly. This creates a kind of `traffic jam' which slows the passage of further electrons. In this way, the voltage of the drain regulates itself, and is more or less independent of the current demanded from the drain.

Once these two factors have been taken into account, it is fair to say that the positive output voltage (the voltage of the drain relative to the source) is proportional to the negative input voltage (the voltage of the gate relative to the source).

For this reason, the field effect transistor is known as a voltage amplifier. This contrasts with the bipolar transistor which is a current amplifier.

Field effect transistors

1. What are the two types of bipolar transistor? How does their construction differ?
2. What are the three connections to a bipolar transistor called?
3. Why are very few electrons able to flow from emitter to collector in an NPN transistor if the base is not connected?
4. Why do you think a bipolar transistor would not work if the base layer were too thick?
5. “The bipolar transistor is a current amplifier.” What does this statement mean?
6. Describe the structure of a FET.
7. Define what is meant by the source, drain and gate. During normal operation, what will the voltages of drain and gate be with respect to the source?
8. Describe how a depletion layer forms when the gate voltage is made more negative. What controls the width of the depletion layer?
9. “The field effect transistor is a voltage amplifier.” What does this statement mean?
10. The amplifier in a cheap radio will probably contain bipolar transistors. A computer contains many field effect transistors. Bipolar transistors are more rugged and less sensitive to interference than field effect transistors, which makes them more suitable for a simple radio. Why are FETs preferred for the computer?