Light emission from power transistors at a compression level in the range of 2-3 dB has been imaged using a microscope-mounted camera. Results show that the emitted light intensity distribution across the transistor is highly nonuniform and depends on the load impedance, direct current, and RF conditions. The light intensity correlates with a negative gate current, which is a result of the RF-induced impact ionization in the transistors. The nonuniformity in the light intensity is attributed to the RF-induced voltage overstress in the transistors. The observed light emission may be used as a direct and contactless monitor of the RF-induced overstress in transistors and power amplifiers.Index Terms-Breakdown, high-power amplifier (HPA), impact ionization, light emission, parasitic oscillations, power transistor.

a b s t r a c tThe power performance of GaAs/AlGaAs pseudomorphic high electron mobility transistors (PHEMTs) has been modeled by using the statistical Design of Experiment approach. Empirical models for the small signal gain, output power and power added efficiency have been developed. The ''walk-out/in" phenomenon has been observed in the devices as a result of power measurements. The evolution of surface photovoltage spectra after RF power stress indicates accumulation of positive electrical charge in the buffer and the surface layer of the devices.

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