Current collapse scaling in GaN/AlGaN/SiC high electron mobility transistors

This study reports the scaling of current collapse in GaN/AlGaN HEMTs with respect to the un-passivated gate drain distance on the gate edge. The source drain current reduction increased from 4 mA to 28 mA, when un-passivated gap increased from 200 nm to 600 nm respectively mainly due to virtual gate formation at gate edge as a result of applied large reverse bias between the gate/drain electrodes. The length of virtual gate is a function of un-passivated gap that modifies the lateral electric field between gate-drain region and results in variable current reduction due to variation in available traps with gap. The simulated E-field distribution is found to vary strongly with the un-passivated gap up to 200 nm and weakly thereafter. The HEMT knee voltage shifted from 0.5 V to 1.2 V when gap is increased from 200 nm to 600 nm respectively due to electric field distribution modification and hence electron trapping in the un-passivated gap resulting in increased device on-resistance (R_on). The current collapse finally resulted in reduction of device saturated RF power to 1.2 W/mm at 2.2 GHz for HEMT with an un-passivated gap of 600 nm.