A field-effect transistor (FET) controls the current through a channel with the electric field from a gate. Its three terminals are the source (where carriers enter), the drain (where they leave) and the gate. Because the gate is controlled by voltage and draws almost no steady current, a FET has a very high DC input impedance, far higher than a bipolar transistor.
How it works
Picture a strip of N-type semiconductor from source to drain: current flows through it like water through a hose. The gate squeezes or opens that hose.
JFET: the gate is a reverse-biased PN junction beside the channel. More reverse voltage widens the depletion region, which pinches the channel and cuts the current. A JFET conducts with zero gate voltage and is turned down by the gate.
MOSFET: the gate is a metal (or polysilicon) plate separated from the channel by a thin layer of insulating oxide, so almost no steady gate current flows. The gate acts like one plate of a capacitor.
MOSFET: the gate is insulated from the channel by a thin layer.
MOSFETs come in two modes:
Enhancement mode: no channel at zero gate voltage, so it is off. A gate voltage beyond a threshold pulls charge carriers in and forms the channel.
Depletion mode: a channel exists at zero gate voltage, so it conducts until the gate pushes the carriers out. This is the case where a FET passes current with no gate voltage applied.
Depletion mode: conducts with zero gate voltage; gate voltage squeezes the channel shut.
Drain current (relative)25 %
On the symbol, an arrow pointing toward the channel marks an N-channel device; pointing away marks P-channel. A dual-gate MOSFET has a second gate, handy for gain control and mixing.
FET or bipolar?
Bipolar
FET
Controlled by
base current
gate voltage
Input impedance
moderate
very high
Gain measured by
current gain (β)
transconductance
Strengths
simple, cheap, high gain
low noise, easy to drive, efficient switch
In practice
Switching: a fully-on MOSFET looks like a small resistor, so it wastes little power. At 10 A through 10 mΩ the loss is I²R = 10² × 0.01 = 1 W. That makes MOSFETs the standard choice for switching power supplies, motor drivers and transmit/receive switching.
RF: JFETs and GaAs FETs make low-noise preamplifiers, dual-gate MOSFETs serve in older receivers' mixers and gain-controlled stages, and RF power MOSFETs build the output stages of many modern transmitters and solid-state amplifiers.
Static damage: the oxide under the gate is extremely thin, and a spark of static electricity can puncture it permanently. Touch grounded metal (or wear a wrist strap) before handling MOSFETs, and keep them in conductive foam until installed.
Built-in protection: many devices include a Zener across gate and source to clamp spikes.
The zener clamps the spike below the oxide limit.
Never leave the gate floating. A gate with nothing attached can drift and turn the device partly on; a resistor to source gives it a defined state.