Mechanisms for output power expansion and degradation of PHEMT's during high-efficiency operation

PHEMT stability in high-efficiency power amplifiers (where the PHEMT is typ ically driven into reverse gate-drain breakdown) has been examined under ac celerated de stresses. Such stresses initially caused a slight increase in drain current and decrease in threshold voltage due to hot-carrier injectio n into the buffer which was not seen in similarly stressed MESFET's, On the other hand, continued stressing resulted in significant decrease in drain current and increase in drain resistance due to hot-carrier injection into the surface passivation which is consistent with the typical MESFET degrada tion mode. The degradation rate of PHEMT's is approximately two orders of m agnitude faster than that of MESFET's, Two-dimensional physical device simu lation confirms that the faster degradation of PHEMT's is due to their high er sensitivity to surface conditions.