Degradation mechanism of PHEMT under large signal operation

We have studied the degradation mechanism of AlGaAs/InGaAs pseudomorphic HEMTs (PHEMTs) under large signal operation. The output power of the PHEMT is degraded with increasing drain voltage (Vd), temperature, and humidity. The deteriorated devices show a decrease of the maximum drain current (Imax) around the knee voltage (Vk) and an increase of the drain resistance (Rd). Cross-sectional transmission electron microscopy (TEM) images from the deteriorated devices reveal the existence of a damaged recess surface region at the drain side of the device. In this damaged region, a. significant amount of oxygen is detected by energy dispersive X-ray spectroscopy (EDX), analysis. The damaged recess region leads to a reduced carrier density that results in decreased Imax and increased Rd. We suggest that the damaged recess region is caused by an electrochemical reaction that correlates with electric field, temperature and humidity. We have developed a special surface treatment that is applied prior to the deposition of the passivation film on the recess surface. This treatment successfully suppresses output power degradation in these devices. We demonstrate highly reliable RF operation with less than 0.2 dB reduction in output power during 1000 hr at Vd=5 V and Tch=175/spl deg/C.