The field-effect transistor is probably the closest device to the Bell Labs team's original dream of a solid-state equivalent to the vacuum tube, or thermionic valve as we call it in the UK :-)

FET's come in two polarities, N-channel and P-channel, depending on the doping of the semiconductor. The 'channel' forms the bulk of the device, and has a connection at each end, one called the 'source', the other the 'drain'. On the sides of the channel, the semiconductor is doped with the opposite polarity (e.g. P-type in an N-channel FET) and a connection called the 'gate' is made.

On the sides of the channel, as with any PN junction, a depletion layer forms where the majority carriers migrate across the junction and combine with their opposite counterpart. Now if a voltage is applied between the source and the drain, and the gate is held at a constant voltage, a source-drain current flows and a voltage gradient develops between the two electrodes. As with a reverse-biased PN junction, the depth of the depletion layer depends upon the difference between the gate voltage and the voltage in the channel and therefore varies along the length of the channel because of the voltage gradient. Changing the gate voltage extends or reduces the depth of the depletion layer. This gives very sensitive control of the channel width and hence controls the current flowing between source and drain. Because of the lack of carriers in the depletion layer, no current flows in the gate circuit itself.

The above describes a junction FET. A more modern development is the MOSFET, in which a layer of oxide is used to insulate the gate from the channel. This device is capable of astonishing performance: varying the gate voltage by 12 volts can change the source-drain resistance from milliohms to Megohms. The device can handle tens of amperes of source-drain current, tens or hundreds of volts between source and drain, but requires microamperes of gate current. Truly this has the voltage control of the vacuum tube with the current handling of the bipolar transistor!

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