What Are HEMTs?

HEMTs, or high electron mobility transistors, are a class of field-effect transistors that exploit a heterojunction interface between two semiconductor materials with different bandgaps to create a conducting channel with exceptional electron mobility. By spatially separating the mobile electrons from the ionized donor atoms that supply them, HEMTs eliminate the impurity scattering that limits electron transport in conventional doped transistors. The result is a device with superior high-frequency gain and low noise characteristics, making HEMTs the dominant transistor technology in millimeter-wave amplifiers and low-noise receiver front ends. HEMTs are also referred to as modulation-doped field-effect transistors (MODFETs) and heterostructure FETs (HFETs), all three names describing the same fundamental device concept.

The key physics was identified independently by research groups at Bell Labs and in Japan around 1980, following theoretical work on modulation doping by Horst Störmer and collaborators. The device concept brought together advances in molecular beam epitaxy, which enabled atomic-scale control over heterojunction interfaces, with an understanding of two-dimensional electron gas (2DEG) physics derived from condensed-matter studies.

Device Structure and Operating Principle

A HEMT consists of a thin undoped semiconductor layer, typically GaAs or GaN, adjacent to a wider-bandgap layer such as AlGaAs or AlGaN that is selectively doped. Because the conduction band minimum in the narrow-gap layer sits below that of the wider-gap layer, electrons donated by the doped layer transfer into the undoped layer, forming a thin sheet of charge, the two-dimensional electron gas, confined at the heterointerface. This 2DEG layer is free from the ionized donor atoms that remain in the doped supply layer, so the electrons encounter very little impurity scattering. The high electron mobility transistors overview at Bulletin of Materials Science, Indian Academy of Sciences describes the quantitative relationship between 2DEG density, mobility, and the design of the heterostructure. A metal gate electrode on the surface controls the 2DEG density through the electric field it applies to the supply layer, enabling transistor action. Electron mobilities in GaAs/AlGaAs HEMTs reach 8,500 cm²/Vs at room temperature and exceed 1,000,000 cm²/Vs at cryogenic temperatures, values far above those achievable in bulk-doped devices.

Materials and Performance

The GaAs/AlGaAs system was the original HEMT material, and it remains in use for applications below approximately 30 GHz. InP-based HEMTs, using lattice-matched InGaAs channels with InAlAs barriers, offer even higher electron velocity and are used in low-noise amplifiers operating at millimeter-wave and submillimeter-wave frequencies, including radio astronomy receiver chains where noise temperatures below 10 K are achieved at cryogenic operation. GaN/AlGaN HEMTs tolerate much higher electric fields and power densities than GaAs-based devices, because GaN has a wide bandgap and a high breakdown voltage. The MIT paper on the HEMT at 30 years by Jesús del Alamo surveys the evolution of HEMT materials and benchmarks performance across material systems and device generations. GaN HEMTs now dominate solid-state power amplifiers in base station radios, radar transmitters, and satellite payloads above 10 W output power.

High-Frequency and Low-Noise Applications

HEMT technology enables practical amplifiers at frequencies from a few GHz to beyond 300 GHz, a range that encompasses satellite communications, point-to-point millimeter-wave links, automotive radar at 77 GHz, and imaging systems in the terahertz band. The low noise figure of InP HEMTs at cryogenic temperatures makes them the preferred preamplifier in radio telescopes and in readout circuits for superconducting quantum processors. The IntechOpen review of HEMT performance and research trends covers device scaling, thermal management, and reliability issues relevant to both commercial and defense applications.

Applications

HEMTs have applications in a wide range of fields, including:

• Low-noise amplifiers for satellite communications and radio astronomy
• Power amplifiers for 5G base stations and phased-array radar
• Automotive radar at 77 GHz and 79 GHz
• Readout electronics for quantum computing hardware at cryogenic temperatures
• Millimeter-wave imaging for security screening and medical diagnostics