Abstract
We report on the novel normally-on/off AlN/GaN high electron mobility transistors (HEMTs) grown by plasma-assisted molecular beam epitaxy. With simple oxygen plasma exposure, the threshold voltage can be tuned from -2.76 V to +1.13 V depending on the treatment time. The gate current was reduced and gate current-voltage curve show metal-oxide semiconductor diode-like characteris-tics after oxygen plasma exposure. The extrinsic trans-conductance of HEMTs decreased with increasing oxy-gen plasma exposure time due to the thicker Al oxide formed on the gate area. The unity current gain cut-off frequency, fT, and the maximum frequency of oscillation, fmax, were 20.4 GHz and 36.5 GHz, respectively, for a enhancement-mode HEMT with the gate dimension of 0.4× 100 μm2.
