RF burn-in analysis of 100V P-band aerospace GaN radar transistors

Recent advances in solid-state amplifiers for aerospace applications have yielded GaN radar transistors operating at 75V and 100V in P-band with drain efficiency in the 80% range and output power of 250W and 500W. This study reports RF burn-in results at 100V and 125V bias with a signal at 430MHz, 100μs pulse width and 10% duty cycle to assess long term stability and reliability. With the test fixture kept at 26°C the output power drops by ~2% at 100V and ~5% at 125 V in a 1-week RF burn-in test. Measurements have been taken on GaN devices built on three different GaN on SiC epi configurations to determine the dependence of RF output power and gain degradation on properties such as iron (Fe) content in the epi buffer layer design. Results indicate that more stable operation with a bias of 100V and 125 V is achieved when the Fe concentration in the buffer layer of the GaN device exceeds 1×1018 cm-3. Modeling of the thermal resistance under pulse conditions predicts a peak junction temperature of 84°C at 125V bias when the test fixture is at 26°C temperature, and it is 151°C when the case temperature is increased to 80°C. The result is significant because 28V and 50V commercially available GaN technology has already been proved to be tolerant to radiation suggesting that higher voltage GaN technology might be too. High voltage operation has the advantage of not only achieving higher power density and therefore smaller and lighter amplifiers but it also facilitates the implementation of harmonic tuning to obtain both very high efficiency and high output power, while reducing the requirements of the thermal cooling subsystem design. To the author's knowledge this is the first time that an RF radar amplifier has been tested for reliability at 125 V with very promising results.