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Fabrication of GaN devices presents a number of processing issues. Since the AlGaN/GaN material and sapphire substrates are transparent in the ultraviolet, light scattering can degrade the quality of the lithography. GaN and AlGaN are chemically inert materials, highly resistant to wet etching. Schottky contacts are fabricated with noble metals (Pt, Au) which present adhesion problems. The approaches to solving these difficulties and the routes to fabrication of high performance GaN-based modulation doped field effect transistors are presented. The light scattering has been addressed by the deposition of amorphous Si on the backside of the wafers. Mesa etching for device isolation has been performed with both chemically assisted ion beam etching and a newly developed wet etching technique. Finally, metal adhesion has been greatly improved by the initial deposition of a thin sputtered Pt layer, followed by a thick layer of Pt/Au by e-beam evaporation. A 100 μm wide device showed a maximum dc current density of 946 mA/mm, a peak transconductance of 160 mS/mm, a short circuit current gain cutoff frequency of 15.6 GHz, and a maximum oscillation frequency of 49.4 GHz.

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Optimization of the electrical characteristics of GaN/AlGaN HFETs grown on SiC substrates by ammonia‐MBE is reported. By optimizing the growth conditions, GaN/AlGaN HFETs with room temperature mobilities of ∼1000 cm2/Vs and electron sheet densities of up to 1.9 × 1013 cm—2 have been grown on SiC substrates with good reproducibility. These characteristics are comparable to the best characteristics of the GaN/AlGaN HFETs grown on sapphire. Devices fabricated from these optimized HFET layers showed excellent characteristics, e.g. maximum drain current density of 1.3 A/mm, fT of 70 GHz, fMAX of 130 GHz were measured for devices with 0.25 μm gate length and 100 μm gate width.