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Both the degradations of dc and RF characteristics of nMOS transistors due to hot-carrier effect and instantaneous high RF power stresses are presented in this paper. The drain current, threshold voltage, output power, and power-added efficient were degraded after the output power exceeded the power capacity. At this condition, the voltage between drain and gate became large and made the oxide soft breakdown happen. The load-pull system is used to set up the measurement for optimized input and output power matching networks. The shift of the optimum load impedance and constant power-gain circles for power match indicated that the parameters of the stressed cells were changed by the damage in the gate oxide. The signal distortion, gate voltage swing, and thermal effect were all considered in this paper. The cells were fabricated by a 0.18-μm CMOS process. All of the characteristics of the devices were measured at 5.2 GHz.