InAlAs Digital Alloy Avalanche Photodiode with Partially Depleted Absorber

Digital alloy (DA) InAlAs has been employed as the multiplication layer in separate absorption, grading, charge, and multiplication (SAGCM) avalanche photodiodes (APDs) to suppress excess noise in our previous work. In this work, a partially depleted absorber (PDA) is introduced to enhance bandwidth, while a back-illumination structure with flip-chip bonding is utilized to improve quantum efficiency and further boost response speed through inductive peaking. The APD exhibits a dark current density of approximately 8.5 mA/cm² at 90% breakdown voltage, a multiplication gain of 50 before avalanche breakdown, and a responsivity of 0.49 A/W at unit gain. Frequency response measurements indicate a significant improvement after flip-chip bonding, with the bandwidth increasing from 15.6 GHz to 22 GHz. Eye diagrams were obtained at data rates up to 40 Gbps, demonstrating the device's high-speed performance.