A field-effect transistor in which a heterojunction is used to separate the conducting channel from the doped supply region. The mobility of the electrons in the channel is therefore higher than in conventional FETs because of the absence of carrier scattering by the ionized donors. The following are typical material systems:

• AlGaAs (doped supply layer)/GaAs (channel)  single heterojunction FET;

• AlGaAs/InGaAs (channel)/GaAs;

• AlInAs/InGaAs/InP double heterojunction FET.

Only n-channel devices are used, since the hole mobility in these III–V compound semiconductors is too low to be useful practically. These structures are illustrated in the diagram.

HEMTs are used for low-noise amplifier and IC applications at microwave and millimetre-wave frequencies. The high-frequency performance is attributed to the improved control of the channel charge modulation by the gate voltage, due to the electron confinement in the narrow quantum well channel.

High electron mobility transistor: schematic of conduction band of heterojunction FETs