A Compact Model for Si-Ge Avalanche Photodiodes Over a Wide Range of Multiplication Gain

Silicon-germanium (Si-Ge) avalanche photodiodes (APDs) have superior gain bandwidth product due to the low impact ionization ratio of silicon. To enable efficient optical transceiver systems, cosimulation environments are essential for optimization of optical devices and transceiver circuitry. A compact Si-Ge APD circuit model, which captures both electrical and optical dynamics over a wide range of multiplication gain, is presented. This model includes effects of carrier transit time, avalanche buildup time and electrical parasitics. Model parameters are extracted by small-signal measurement and impulse response measurement where three gain regimes are discussed corresponding to the different mechanisms of carrier transit time and electrical parasitics. Simulated and measured bandwidth versus gain have a good agreement for APD multiplication gains from M = 1 to M = 17. Excellent matching between simulated and measured 25 Gb/s eye diagrams at M = 2 and M = 5.9 is achieved.