Capacitance controlled n-GaN SAW UV sensor

We report on the investigation of the response of the GaN SAW UV sensor operating at 307 MHz to the illumination from 375 nm LED. We demonstrate, for the first time, the impact of the UV-induced change in interdigital transducer (IDT) capacitance on the phase change of the SAW signal. The light was focused onto the sample surface, and the variation in signal phase (both in reflection and transmission modes) was measured as a function of the UV spot position. The maximum SAW phase change occurred when the IDT regions were illuminated. We attribute this effect to the decrease in depletion width under the IDT and the associated capacitance variation. From S-parameter measurements, the dependence of the IDT capacitance on UV illumination power was extracted. At UV power density of 1.7 μW/mm², we estimated 4 % increase in capacitance corresponding to the phase change of 10° in the transmitted SAW signal. Our results reveal the importance of photocapacitance for the operation of GaN based SAW UV sensors.