A modified physical model on the optical response of InP-based In0.53Ga0.47As PIN photodetectors was presented. By introducing a collecting factor, the optical response and quantum efficiency were simulated. The influences of device parameters on quantum efficiency of typical In0.53Ga0.47As/InP PIN photodetectors under both front and backside illuminated conditions were investigated by using our model. Furthermore, two modified InGaAs/InP PD structures for back-illumination were proposed, and the optimal structural parameters were discussed.