Effect of Passivation on AlGaN/GaN HEMT device performance

AlGaN/GaN HEMTs are suitable for high power applications because of their high breakdown voltages and high currents. However, the large signal performance of these devices is not as good as one would expect from the DC performance of the device. The large signal performance of the device is improved by depositing a thin layer of SiN by PECVD[1]. In this work silicon nitride (SiN) was deposited in a variety of conditions on AlGaN/GaN HEMT structures on sapphire substrates to study the impact of the quality of SiN on the DC, breakdown, small signal and large signal performance of the device. A maximum increase in the output power from 1.3 W/mm to 2.3 W/mm at 4 GHz, 20V bias was observed with a SiN passivation film, grown in NH3 rich conditions and which gave the least etch rate of 25 nm/min in buffered oxide etch (6:1). This increase in power is mainly due to the increase in DC current. This is clearly shown by plotting the RF dynamic loadline for an unpassivated and passivated device.