What Are MOSHFETs?

MOSHFETs are metal-oxide-semiconductor heterostructure field-effect transistors, a class of compound semiconductor devices that combine an oxide gate insulator with a heterojunction channel formed at the interface of two dissimilar semiconductor layers. The acronym merges the MOS gate structure, which provides high input impedance and low gate leakage, with the HFET (heterostructure FET) channel, which exploits a two-dimensional electron gas confined by conduction band discontinuities. The resulting device inherits the electrostatic control and leakage suppression of MOS devices while preserving the high carrier mobility and current density achievable in III-V semiconductor heterostructures.

MOSHFETs are primarily realized in the AlGaN/GaN material system, where the spontaneous and piezoelectric polarization fields at the AlGaN/GaN interface generate a two-dimensional electron gas (2DEG) of high carrier density without intentional doping. Adding a gate oxide between the metal gate and the AlGaN surface significantly reduces gate leakage compared to conventional HFETs, enabling normally-off (enhancement-mode) operation that is preferred in power electronics for safe failure-mode behavior.

Structure and Gate Insulator Role

In a conventional HFET, the metal gate contacts the semiconductor surface directly, producing a Schottky barrier that limits the positive gate voltage swing before forward conduction begins. Replacing the Schottky contact with an oxide layer shifts the operating window to higher gate voltages, reduces gate leakage by more than six orders of magnitude, and improves threshold voltage stability. Atomic-layer-deposited silicon dioxide, aluminum oxide (Al2O3), and silicon nitride have all been used as gate dielectrics in AlGaN/GaN MOSHFETs. The IEEE publication on AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors established key performance benchmarks for this device class, reporting large-signal RF output power of 2.88 W/mm at 2 GHz alongside gate leakage suppressed far below conventional HFET values.

GaN/AlGaN Device Technology

The AlGaN/GaN heterostructure is grown by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on silicon carbide, sapphire, or silicon substrates. The 2DEG that forms at the lower AlGaN/GaN interface typically achieves sheet carrier densities near 10¹³ cm⁻² and room-temperature mobilities of 1500 to 2000 cm²/V·s in well-optimized material. Threshold voltage engineering in normally-off MOSHFETs is achieved by combining thin AlGaN barriers, gate recess etching, and the gate oxide stack to place the threshold voltage above zero volts. IEEE research on normally-off AlGaN/GaN-on-Si MOSHFETs with TaN floating gates and ALD SiO2 tunnel dielectrics demonstrates how gate dielectric engineering controls both threshold voltage and charge trapping, a critical reliability concern in wide-bandgap FETs.

High-Power and High-Frequency Performance

The combination of the wide bandgap of GaN (3.4 eV), the high electron velocity in the 2DEG, and the insulated gate of the MOSHFET structure produces devices suited to simultaneous high-voltage and high-frequency operation. Breakdown voltages exceeding 900 V have been demonstrated in AlGaN/GaN MOSHFETs on SiC, while cut-off frequencies above 100 GHz have been reported in scaled variants. IEEE studies of large-periphery AlGaN/GaN MOSHFETs on SiC with oxide bridging report continuous-wave microwave power at X-band, demonstrating the device's viability for radar and satellite communication power amplifiers where both power density and efficiency matter.

Applications

MOSHFETs have applications in a range of fields, including:

• High-power microwave amplifiers for radar and wireless base stations
• Power conversion in electric vehicles and renewable energy inverters
• Satellite communication transmit modules
• Normally-off power switches in safety-critical systems
• Radiation-tolerant electronics for aerospace and defense