Direct observation of reach-through behavior in back-illuminated algan avalanche photodiode with separate absorption and multiplication structure

Back-Illuminated heterostructure AlGaN avalanche photodiode (APD) was fabricated with separate absorption and multiplication (SAM) structure. The SAM structure is able to modulate the electric field distribution of the photodetector and induce single carrier to trigger avalanche event. The fabricated p-i-n-i-n APD exhibited superior maximum gain of 1.8×105 at 70 V and distinct solar-blind responsivity peak at 280 nm. However, before the avalanche occurs, SAM APD needs to be reached through for facilitating carrier transport. By investigating the capacitance characteristics of the SAM APD, the reach-through behavior was directly observed. The capacitance of the detector demonstrated evident decline trends with the applied voltage in the range of 18 to 24 V at different frequencies. Simulated electric field and energy band also revealed that the absorption region was initially reached through at 18 V reverse bias, corresponding to the depletion of n-type interlayer into absorption region. Meanwhile, there were large differences in capacitances between low and high frequencies, which indicated the effects of impurity and defect states. These findings explicitly reveal the reach-through mechanism of the SAM APD, which is beneficial to the better understanding of device physics and further APD design.