AlGaN/GaN HEMTS: material, processing, and characterization

The growth of AlGaN/GaN high electron mobility transistor (HEMT) structures on sapphire by metal organic vapor phase epitaxy (MOVPE) is described, with special emphasis on procedures to reduce dislocation density. All the processing steps involved in the fabrication of nitride-based HEMTs have been optimized, including dry etching by ion beam milling, evaporation of Pt/Ti/Au gate contacts, and SiN_x surface passivation. Devices with several gate lengths and different geometries have been fabricated by standard photo- and e-beam lithography. d.c. drain current and transconductance increase when gate length is reduced, up to 950 mA mm^–1 and 230 mS mm^–1, respectively, at V_GS=0 V, in HEMTs with a gate length L_G=0.2 m. A maximum output power higher than 5 W mm^–1 is estimated. Finally, small-signal measurements yield f_T=12 GHz and f_max=25 GHz for HEMTs with L_G=0.5 m, which increase up to 20 and 35 GHz for L_G=0.2 m, respectively. Limitation of high-frequency performance by parasitics is discussed.