Field effect transistor

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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