NO OXIDE NEEDED

Three other ways to make a field effect

JFET The gate is a reverse-biased p–n junction that squeezes the channel

MESFET The gate is a metal–semiconductor Schottky contact, usually on gallium arsenide

HEMT The channel is a sheet of electrons trapped at a heterojunction

Shared trait All are normally on: a channel exists at zero gate voltage, and a gate bias removes it

The oxide is a convenience of silicon. Other materials found other gates.

JUNCTION GATE

Pinching a channel

I_D = I_DSS(1 − V_GS/V_P)²

A reverse-biased p–n junction has a depletion region whose width grows with the voltage across it. In a JFET that depletion layer eats into the channel from both sides, and at the pinch-off voltage V_P the two meet and the current stops. The gate draws only a reverse-diode leakage, so the input resistance is still very high.

JFETs are quiet and simple, which is why they survive as the input stage of precision amplifiers.

THE HEMT

A sheet of electrons with no dopants

Grow a wide-bandgap AlGaN layer on GaN. Polarization at the interface pulls electrons into a thin triangular well on the GaN side, forming a two-dimensional electron gas at a density near 10¹³ cm⁻² without any intentional doping. With no dopant atoms to scatter them, the electrons are fast, and the gate depletes the sheet like any other FET.

High density plus high mobility is what makes a GaN HEMT a power and radio-frequency champion.

WHERE THEY LIVE

Radar, radio and sensors

Radio-frequency power GaN HEMTs power 5G base stations, radar and satellite amplifiers

Low-noise receivers Cooled indium-phosphide HEMTs amplify the faintest signals in radio telescopes

Fast chargers Enhancement-mode GaN devices switch at high frequency inside compact power adapters

Sensors The electron sheet lies only about 20 nm below the surface, so AlGaN/GaN HEMTs make very sensitive pH and biosensors

A surface-near electron sheet is a natural charge detector, the same idea as the ISFET (Part 7).

CHECK THE COUSINS

JFET, MESFET or HEMT?

Junction gate, Schottky gate, or heterojunction channel.

UNIT 21 STUDY COMPLETE

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Ready for the Fab Challenge?

You've met the field-effect transistors that have no oxide, and the HEMT's electron sheet that powers radar and fast chargers.