Field-effect transistor

The Field-Effect Transistor (FET) is a type of transistor that works by

modulating a microscopic electric field inside a semiconductor material.

There are two general type of FET's, the MOSFET and JFET.

The simplest FET is the JFET, or junction field-effect transistor. It

consists of a long channel of semiconductor material, either P or N doped,

with two contacts on each end, labeled Source and Drain. The third control

terminal (called the gate) is arranged to contact the edges of the channel,

and is doped to the opposite polarity from the channel. When a voltage is

applied between source and drain, current flows. The current flow can be

modulated by applying a voltage between the gate and source terminals.

When this occurs, the electric field applied effectively narrows the

channel, and the flow of current is restricted. In this way the current can

be modulated, creating an amplifier or switching circuit.

JFET's have several advantages over the usual BJT transistors. They do not

require any input current to function, which makes them useful for circuits

requiring a high input impedence. However, their gain is usually pretty low

in comparison. They are used in low-noise low-signal level analog

applications, and sometimes used in switching applications.

The MOSFET, or metal-oxide-semiconductor field-effect transistor, is similar

to the JFET; it contains a channel connected on each end to source and

drain terminals. But the gate terminal is merely a metalized layer of

aluminum covering the channel but separated from the channel by a thin layer

of silicon dioxide (glass). When a voltage is applied between the gate and

source pins, the electric field generated penetrates through the glass and

into the channel, causing it to become more or less conductive.

MOSFET's are used almost exlusively for switching, especially for digital

circuits. The glass layer between the gate and the channel prevents any

current from flowing, making design easier and reducing power consumption.

As switching speeds increase, however, large quantities of current are

consumed by the charging and discharging of the gate capacitance, erasing

any power savings from the high input resistance. MOSFET's also have a

problem with static discharge: the thin layer of glass is very fragile, and

can be penetrated by as little as 20 volts, depending on the design, while a

small static discharge is over 400 volts.

See Also: bipolar transistors