Comparative analysis of dark current between SiN_x and polyimide surface passivation of an avalanche photodiode based on GaAs

In this paper, we present the effects of different surface passivation types, one with SiN_x and the other with polyimide (PI), on the dark (leakage) current of a GaAs-based avalanche photodiode. We identified that the reverse dark current originates from the surface, and not from the bulk, showing the nearly linear dependence on perimeters of active-mesa (A-M) up to 90% of breakdown voltage (V_br). From the theoretical results, total dark current consists of generation-recombination (G-R), shunt and tunneling components from a surface and the avalanche gain component from a bulk for both passivation types. Although the bulk component of avalanche gainbulk current generates the breakdown process, it appears only near V_br (12.7 V) because of a very small bulk current of a few fA in theory. For a surface current, SiN_x passivation has values two to eight times lower than PI passivation. The different behaviors of surface current between passivation types could be theoretically explained by quantitative description of the current components.