The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMT's

Surface passivation of undoped AlGaN/GaN HEMT's reduces or eliminates the surface effects responsible For limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2 x 125 x 0.5 mu m AlGaN/GaN sapphire-based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 mu m gate length HEMT's on the same wafer, Finally, 4 GHz power sweep data for a 2 x 75 x 0.4 mu m AlGaN/GaN HEMT on sapphire processed using the Si3N4 passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMT's.