Short-gate MODFET's of AlGaN/GaN on Sapphire have been fabricated and characterized with gate lengths in the .12 - .25 μm range. Values of ft = 50 GHz and fmax = 100 GHz have been obtained. Analyzing the performance, the average electron transit velocity is shown to be 1.25 × 107 cm/s and in some cases well under that value. This compares with theoretical predictions of ~ 2.0 × 107 cm/s. The electron scattering effects of dislocations, which are charged, are modeled to explain the lower mobility. Ion bombardment or dry etching is used for mesa isolation. Ti/Al/Ti/Au sintered for 100 seconds at 800 °C is used to yield ohmic contacts of .5 - 1.0 Ω-mm. Pt/Au Schottky gates are used. A high breakdown voltage, exceeding 100 V even for short gate MODFET's, shows that ten times higher load resistance values are possible, compared with GaAs MODFET's. Normalized output power levels well over 10 W/ mm are thus projected for GaN MODFET's on SiC substrates, where the thermal conductivity is about 5W/cm-°C. with future integrated traveling-wave, power-combining circuits, output power > 100 W at 10 GHz is predicted.

Initial results on 0.25 μm gate MODFET's have yielded ft=21.4 GHz and fmax=77.5 GHz. These devices have characteristics that agree with the gradual channel model dominated by the electron mobility. The AlGaN/GaN structure, grown on sapphire substrates, are polycrystalline, and thus yield low mobility (<100cm2/Vs) at low electron sheet density. Using a simple model, design optimization predicts electron sheet density values of 7.3 × 1012 cm−2 in thin, 3 nm quantum wells for single-sided doping with 5 nm spacer for use in future high frequency Al0.4Ga0.6N/In0.25Ga0.75N/GaN MODFET's with gate lengths of 0.10 μm. Double sided doping with 5 nm spacers would yield a sheet density of 1.4 × 1013cm−2 in such 3 nm quantum wells.