Effect of Gate Insulator Thickness on RF Power Gain Degradation of Vertically Scaled GaN MIS-HEMTs at 40 GHz

We present an initial study of the RF reliability of SiN x/InAlN/AlN/GaN MIS-HEMTs with a molecular-beam epitaxy deposited SiN x gate insulator thickness varying from 0 (Schottky gate) to 6 nm. T-gate devices with 120 nm gate length were stressed under continuous-wave 40-GHz large-signal RF operation, biased in class AB with V_DS = 20 V. Degradation in large-signal output power gain was observed to various degrees for all devices. For structures with a 3- or 6-nm gate insulator thickness, output power degraded by approximately 1 dB or less after 250 h of operation. A rapid decrease in output power was observed for structures with a 1-nm gate insulator or a Schottky gate, with a 1-dB decrease in output power within the first 10 h of operation. Degradation in output power was associated with a reduction in drain current, likely caused by hot-electron-related trapping as the drain current was fully recoverable after exposing the devices to UV light. Simulations show that as the gate insulator thickness is reduced, the peak lateral electric field in the channel increases, which would be consistent with an increase in hot-electron-related degradation.