gallium nitride

**Gallium Nitride (GaN) HEMT Epitaxy and Device Processing** is **the fabrication of high-electron-mobility transistors on GaN heterostructures that exploit a polarization-induced two-dimensional electron gas (2DEG) to achieve high frequency, high power density, and high efficiency** — GaN HEMTs dominate 5G base-station power amplifiers and are rapidly gaining ground in power conversion. - **Epitaxial Growth**: GaN HEMT epi structures are grown by metal-organic CVD (MOCVD) on SiC, sapphire, or silicon substrates. A typical stack starts with a nucleation layer, followed by a GaN buffer (2–5 µm), an AlGaN barrier (15–25 nm), and an optional GaN cap. The Al mole fraction and barrier thickness control 2DEG sheet charge (~1×10¹³ cm⁻²) and mobility (~2000 cm²/V·s). - **Buffer Engineering**: Iron, carbon, or carbon/iron co-doping of the GaN buffer creates a semi-insulating layer that suppresses leakage and supports high breakdown voltage. Strain-management interlayers (AlN or graded AlGaN) prevent cracking on large-diameter silicon substrates. - **Gate Formation**: Enhancement-mode (normally-off) HEMTs use a p-GaN gate, gate recess etch, or fluorine implant beneath the gate to shift threshold voltage positive. Depletion-mode devices use Schottky metal gates (Ni/Au). - **Ohmic Contacts**: Conventional Ti/Al/Ni/Au ohmic contacts require 800–850 °C anneal to alloy into the AlGaN barrier and reach the 2DEG. Low-resistance regrown n+ GaN ohmic regions are used in advanced RF processes. - **Passivation**: SiN passivation deposited by PECVD or in-situ MOCVD mitigates surface trapping that causes current collapse (DC-RF dispersion). Field plates engineered over the gate edge extend breakdown voltage and reduce peak electric fields. - **RF Performance**: GaN-on-SiC HEMTs achieve power densities exceeding 10 W/mm at X-band, with power-added efficiency above 60% in Doherty amplifier configurations for 5G massive MIMO. - **GaN-on-Si for Power Conversion**: 650 V GaN-on-Si HEMTs fabricated in 200 mm CMOS-compatible fabs deliver sub-50 mΩ on-resistance for server power supplies, EV on-board chargers, and data-center power. - **Reliability**: Hot-electron trapping, inverse piezoelectric stress, and gate degradation are key reliability mechanisms studied through high-temperature operating life (HTOL) and off-state step-stress tests. GaN HEMT technology combines unique material physics with sophisticated epitaxial engineering and device processing to deliver performance levels impossible with silicon, making it the semiconductor of choice for high-frequency and high-efficiency power applications.