Mechanism for recoverable power drift in PHEMTs

A new degradation mechanism is proposed for output power drift under rf overdrive of pseudomorphic high electron mobility transistors (PHEMT's). Similar to the previously reported power-slump mechanism, the gradual reduction in output power is caused by a decrease in the peak drain current. However, unlike power slump that is usually associated with permanent damage, in the present case the output power recovers readily once the rf input is interrupted. A possible mechanism for power drift involves charge trapping in the surface passivation layer. The passivation charge relaxes the electrical field in the semiconductor, which in turn increases the occupation of the interface states between the passivation and semiconductor. Measurements of dc and pulsed current-voltage (I-V) characteristics, rf large signal waveforms, deep level transient spectroscopy, light and temperature effects, as well as metal-insulator-metal leakage support this supposition.