High-electron-mobility transistors (HEMTs) represent a specialized class of field-effect transistors optimized for high-frequency and high-power operation. Their superior performance stems from the use of heterostructure materials and the formation of a two-dimensional electron gas (2DEG) at the interface between two semiconductors with different bandgaps. This article explores the underlying physics, material systems, and applications of HEMTs, with a focus on gallium nitride (GaN) and aluminum gallium nitride (AlGaN) heterostructures.

The operational principle of HEMTs relies on the formation of a 2DEG at the interface of two materials with differing bandgaps. In a GaN/AlGaN heterostructure, the spontaneous and piezoelectric polarization effects create a strong electric field, bending the conduction band and forming a triangular quantum well. Electrons from donor states in the wider-bandgap AlGaN layer spill into this well, creating a high-mobility electron channel. Since these electrons are spatially separated from ionized impurities in the donor layer, scattering is minimized, leading to significantly higher electron mobility compared to conventional FETs. Typical electron mobilities in GaN/AlGaN HEMTs exceed 1500 cm²/Vs, with sheet carrier densities reaching above 1×10¹³ cm⁻².

The high electron mobility and carrier density directly translate into superior electrical performance. HEMTs exhibit lower on-resistance, higher current density, and faster switching speeds than silicon-based MOSFETs or GaAs-based FETs. The wide bandgap of GaN (3.4 eV) also enables high breakdown voltages, often exceeding 1000 V, making these devices suitable for high-power applications. Additionally, the high saturation velocity of electrons in GaN (approximately 2.5×10⁷ cm/s) allows for excellent high-frequency performance, with cutoff frequencies (fₜ) and maximum oscillation frequencies (fₘₐₓ) exceeding 100 GHz in optimized devices.

Material selection and heterostructure design play a critical role in HEMT performance. The AlGaN/GaN system is particularly advantageous due to the large conduction band offset (approximately 2.1 eV for Al₀.₃Ga₀.₇N/GaN) and strong polarization effects. The Al composition in AlGaN is typically kept below 30% to balance strain management and carrier confinement. Strain engineering is crucial, as excessive lattice mismatch can lead to defect formation and degraded performance. Advanced heterostructures may incorporate additional layers, such as AlN interlayers or graded compositions, to further enhance electron confinement and reduce alloy scattering.

Device architecture also contributes to performance optimization. Most modern HEMTs employ a recessed gate structure to improve gate control and reduce short-channel effects. Gate lengths are scaled down to the sub-micron range for high-frequency operation, with 100 nm gates being common in RF devices. Field plates are often integrated to manage electric field distribution and improve breakdown characteristics. Passivation layers, typically silicon nitride, are used to suppress current collapse and improve reliability under high-power operation.

In radio frequency (RF) applications, HEMTs dominate high-power amplifier designs for wireless communication and radar systems. Their high power density allows for compact amplifier designs, with output power densities exceeding 10 W/mm at microwave frequencies. GaN HEMT-based amplifiers are widely deployed in cellular base stations, satellite communications, and military radar systems operating in the L-band through Ka-band. The combination of high efficiency and linearity makes them particularly suitable for modern modulation schemes used in 5G networks.

Power electronics represents another major application area for HEMTs. Their fast switching capability and high voltage handling make them ideal for power converters and inverters. In switched-mode power supplies, GaN HEMTs enable higher switching frequencies (into the MHz range), allowing for smaller passive components and increased power density. Electric vehicle power systems increasingly adopt HEMT-based designs for onboard chargers and traction inverters, benefiting from reduced losses and improved thermal performance. Industrial motor drives also utilize HEMTs to achieve higher efficiency and precise control.

Thermal management remains a critical consideration in HEMT design and operation. Despite the wide bandgap advantage, power densities can lead to significant junction temperatures. Advanced packaging techniques, including direct bond copper substrates and embedded cooling solutions, help mitigate thermal limitations. The negative temperature coefficient of threshold voltage in GaN HEMTs requires careful circuit design to ensure stable operation across temperature ranges.

Reliability challenges specific to HEMTs include dynamic on-resistance degradation and gate instability mechanisms. Trapping effects at surface states or buffer layers can cause temporary performance degradation under high-voltage switching. Extensive research focuses on material quality improvement, surface passivation techniques, and optimized processing to enhance device longevity. Accelerated lifetime testing indicates that modern GaN HEMTs can achieve mean time to failure values exceeding 1×10⁶ hours under proper operating conditions.

Future developments in HEMT technology continue to push performance boundaries. Research explores ultra-thin barrier designs to improve gate control while maintaining high 2DEG density. Vertical HEMT architectures are being investigated to further increase power density and simplify thermal management. Integration with other wide-bandgap materials and novel dielectric layers may enable new functionality and improved reliability. As manufacturing processes mature and costs decrease, HEMT adoption is expected to expand into broader commercial applications.

The evolution of HEMT technology demonstrates how heterostructure engineering can overcome fundamental limitations of traditional semiconductor devices. By leveraging quantum confinement effects and polarization engineering, HEMTs achieve unparalleled performance in both high-frequency and high-power domains. Their continued development plays a crucial role in advancing wireless communication, energy conversion, and power distribution systems worldwide.