Role of the gate-to-drain distance in the performance of the normally-off InAlN/GaN HEMTs

We correlate dc maximal drain current I_DSmax, pulsed output characteristics, rf small-signal and breakdown performance of normally-off InAlN/GaN HEMTs with varied gate-to-drain distance d_GD. It is found that parasitic lag effects which are related to the possible surface states are not appearing at longer d_GD. On the other hand compromise need to be found between the improved gate performance and impaired I_DSmax and f_T as the d_GD is increased. The leakage current of the 0.5 μm-long gate may be reduced by up-to three orders of magnitude, down to μA/mm at -20 V bias if d_GD increases from 3 to 15 μm. On the other hand I_DSmax and f_T drop by about one third of the original value (from about 0.6 A/mm and 34 GHz down to 0.4 A/mm and 21 GHz, respectively) if d_GD changes.