HEMTs

HEMT stands for High Electron Mobility Transistor, a type of field-effect transistor that uses a heterojunction to form a high-mobility conducting channel. The key feature is a two-dimensional electron gas (2DEG) created at the interface between two semiconductor materials with different bandgaps. This 2DEG is confined in a quantum well and exhibits very high electron mobility compared to bulk material, allowing faster switching and higher frequency operation.

Most classic HEMTs use a wide-bandgap barrier layer and a narrower-gap channel material, such as GaAs/AlGaAs or InP-based systems. In these structures, dopants are placed in the barrier layer (modulation doping) to supply electrons to the 2DEG while keeping impurities away from the channel. The gate, often a Schottky contact on the barrier, modulates the electron density in the 2DEG and thus the drain current. Devices can be designed in depletion or enhancement mode, depending on the gate bias required to form or modulate the channel.

Materials science has expanded HEMT variants to include GaN/AlGaN, which offer higher breakdown voltages and power-handling capabilities, making them well-suited for RF power and millimeter-wave applications. Other III-V material systems, including InP-based and AlGaAs-based heterostructures, are used for high-speed, low-noise applications.

Applications of HEMTs span RF and microwave amplifiers, oscillators, and receivers in communications, satellite downlinks, radar, and emerging high-frequency and power-electronics domains. Compared with MOSFETs and MESFETs, HEMTs provide higher transconductance and superior high-frequency performance due to the distinctive 2DEG channel, though device design must address issues such as threshold control, reliability, and surface passivation.