THE TRANSISTOR

Consider this structure: a piece of P semiconductor with a piece of N+ connected to each end. This is equivalent to two diodes back-to-back, so no current can flow in either direction.

This may not seem to be very useful, but we're not finished yet. Next we put a piece of metal over the P region, with insulation between them, and apply a positive voltage to it.

The electric field pulls a bunch of electrons up to the top, where they temporarily turn part of the P-type region into N-type. This forms a conducting channel between the two

N+ regions and allows current to flow.

So, we have a voltage-controlled switch. The metal electrode is called a gate because it controls whether the switch is on or off. With a positive voltage at the gate, the switch is on; with a zero or negative voltage, it's off.

The other two terminals are called the source and drain because one acts as a source of electrons and the other drains them away.

This kind of device is called a
field effect transistor, or

FET [1].
This particular type is called a MOSFET,
because of the three layers it's made up
of -- Metal, Oxide and Semiconductor.

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[1] There's another kind of transistor
called a "bipolar junction transistor" or
BJT, that works quite differently. We
won't be using them, because FETs make
much better logic circuits.