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 typically driven into reverse gate-drain breakdown) has been examined under accelerated dc stresses. Such stresses initially caused a slight increase in drain current and decrease in threshold voltage due to hot-carrier injection 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 degradation mode. The degradation rate of PHEMT´s is approximately two orders of magnitude faster than that of MESFET´s. Two-dimensional physical device simulation confirms that the faster degradation of PHEMT´s is due to their higher sensitivity to surface conditions