Avalanche photodiodes (APDs) capable of providing useful gain at microwave frequencies are vital components for future multi-gigabit/s lightwave transmission systems. To date, the APD structure that has shown the most promise for high frequency operation consists of a wide-bandgap multiplication region and a narrow-bandgap absorbing layer separated by a transition region to reduce charge accumulation at the heterojunction interfaces (SAGM-APD). InP/InGaAsP/InGaAs APDs of this type have exhibited bandwidths as high as 8 GHz and gain-bandwidth products of 70 GHz. In this paper we discus the physical effects that determine their speed and project performance limits.
J. C. Campbell,
"Avalanche Photodiodes For High-Bit-Rate Lightwave Systems", Proc. SPIE 0839, Components for Fiber Optic Applications II, (4 February 1988); doi: 10.1117/12.942557; https://doi.org/10.1117/12.942557