What Are mHEMTs?

Metamorphic high electron mobility transistors (mHEMTs) are a class of compound semiconductor field-effect transistors that exploit a graded buffer layer to accommodate large lattice mismatches between the substrate and the active device layers, enabling high-indium-content channel materials to be grown on lower-cost GaAs substrates. The "metamorphic" designation refers to this intentional lattice engineering: the buffer layer undergoes a controlled, graded transition in lattice constant from the GaAs substrate to the InAlAs or InGaAs channel layers, relaxing the crystallographic mismatch through a managed network of dislocations that remain confined to the buffer and do not propagate into the active device region. This approach achieves performance approaching that of InP-substrate HEMTs while using the less expensive and more readily available GaAs wafer platform.

The HEMT device class as a whole exploits the two-dimensional electron gas (2DEG) that forms at the heterojunction between two semiconductors with different bandgap energies. Electrons from a wider-bandgap donor layer, typically InAlAs, accumulate in a quantum well formed by the narrower-bandgap InGaAs or InAs channel, where they experience very little ionized impurity scattering. The result is exceptionally high electron mobility and velocity, giving HEMTs their characteristic advantages in low-noise amplification and high-frequency switching. Increasing the indium content in the channel raises both electron velocity and mobility further, which is the fundamental motivation for the metamorphic approach on GaAs.

Device Structure and Materials

A typical mHEMT grown on a GaAs substrate consists of, from bottom to top, a graded InAlAs metamorphic buffer layer, an InAlAs barrier and electron supply layer, a strained InGaAs or pure InAs channel, a second InAlAs barrier, an InGaAs cap layer for ohmic contact formation, and a gate recess into which the Schottky gate metal is deposited. The indium content in the channel typically ranges from 30 to 80 percent, with higher indium concentrations delivering better high-frequency performance at the cost of increased lattice mismatch and tighter process control requirements. Devices at the 20-nm gate length scale with composite channels incorporating a thin InAs layer within an InGaAs matrix have demonstrated transconductance values exceeding 3,000 mS/mm and maximum oscillation frequencies above 1,000 GHz, according to research published on metamorphic HEMT technologies for terahertz integrated circuits.

Electrical Performance

The defining electrical figures of merit for mHEMTs are the current-gain cutoff frequency (fT) and the maximum oscillation frequency (fmax), which characterize how fast the device can amplify current and power, respectively. A closely related parameter is the minimum noise figure (NFmin), which quantifies the lowest noise the device adds to a signal at a given frequency. For low-noise amplifier applications in millimeter-wave and submillimeter-wave systems, mHEMTs with indium compositions of 42 to 80 percent achieve noise figures of fractions of a decibel at W-band (75 to 110 GHz) and below, making them among the quietest three-terminal devices available. Research on metamorphic HEMT technology for submillimeter-wave monolithic integrated circuits describes devices with gate lengths of 35 to 50 nm achieving operation at 300 GHz and beyond, covering the THz boundary that is increasingly relevant for imaging and spectroscopy applications.

Applications and Fabrication

mHEMTs are fabricated primarily by molecular beam epitaxy (MBE), which provides the atomic-layer precision needed to grow the graded metamorphic buffer and the thin channel layers. Electron-beam lithography defines the nanometer-scale gate lengths critical to high-frequency performance. GaAs metamorphic HEMTs are recognized as an attractive alternative to InP-substrate devices for millimeter-wave applications where substrate cost and wafer diameter matter, and they are commonly integrated into monolithic microwave integrated circuits (MMICs) for commercial and defense systems.

Applications

mHEMTs have applications in a range of fields, including:

• Low-noise amplifiers for millimeter-wave satellite and wireless communication receivers
• Radar front-ends operating at W-band and higher frequencies
• Terahertz imaging systems for security screening and scientific instrumentation
• Radio astronomy receivers requiring ultra-low noise at millimeter wavelengths
• High-speed logic for submillimeter-wave monolithic integrated circuits