Saturation Velocity Measurement of Al0.7Ga0.3N-Channel High Electron Mobility Transistors

Gate length dependent (80 nm–5000 mm) radio frequency measurements to extract saturation velocity are reported for Al_0.85Ga_0.15N/Al_0.7Ga_0.3N high electron mobility transistors fabricated into radio frequency devices using electron beam lithography. Direct current characterization revealed the threshold voltage shifting positively with increasing gate length, with devices changing from depletion mode to enhancement mode when the gate length was greater than or equal to 450 nm. Transconductance varied from 10 mS/mm to 25 mS/mm, with the 450 nm device having the highest values. Maximum drain current density was 268 mA/mm at 10 V gate bias. Scattering-parameter characterization revealed a maximum unity gain bandwidth (f_T) of 28 GHz, achieved by the 80 nm gate length device. A saturation velocity value of 3.8 × 10⁶ cm/s, or 35% of the maximum saturation velocity reported for GaN, was extracted from the f_T measurements.