The modeling and optimization of several photodetectors by semiconductor simulation tool Silvaco Atlas are reported. First is the simulations of p-i-n InP/In0.53Ga0.47As/InP photodetector at low bias. How the dark current, photoresponse and the transient response are influenced by the doping concentration and thickness of the absorption layer are reported. Second is a two-terminal p-n-p heterojunction phototransistors (2T-HPTs) based on In0.53Ga0.47As/InP. To optimize the device performance, the adjustment of the doping level, width, and compositional grading of base, the effects of high-low doping in collector region and the insertion of a thin undoped InGaAs layer in the base region have been investigated. The last is a simulation for InGaAs/InAlAs separate absorption, grading, charge, and multiplication avalanche photodetectors (SAGCM APDs), study the effect of multiplication layer parameters on the operating voltage ranges of APD.