Performance of Thin Separate Absorption, Charge, and Multiplication Avalanche Photodiodes

Previously, it has been demonstrated that resonantcavity-enhanced separate-absorption-and-multiplication (SAM) avalanche photodiodes (APD’s) can achieve high bandwidths and high gain–bandwidth products while maintaining good quantum efficiency. In this paper, we describe a GaAs-based resonant-cavity-enhanced SAM APD that utilizes a thin charge layer for improved control of the electric field profile. These devices have shown RC-limited bandwidths above 30 GHz at low gains and gain–bandwidth products as high as 290 GHz. In order to gain insight into the performance of these APD’s, homojunction APD’s with thin multiplication regions were studied. It was found that the gain and noise have a dependence on the width of the multiplication region that is not predicted by conventional models. Calculations using width-dependent ionization coefficients provide good fits to the measured results. These calculations indicate that the gain–bandwidth product depends strongly on the charge layer doping and on the multiplication layer thickness and, further, that even higher gain–bandwidth products can be achieved with optimized structures.