A 1-D drift-diffusion model is developed for reach-through avalanche photodiodes. Based on this model an equivalent circuit is suggested for these devices. The equivalent circuit is suitable for simulating the device in circuit simulation packages such as PSpice. The model is validated by simulating an actual reach-through silicon avalanche photodiode in the steady state and the transient conditions. Some of the simulation results are compared to the manufacturer data by estimating the device parameters. This model is also used to investigate the effect of the device microstructure on its performance. For a certain operating bias voltage and wavelength, it is found that the multiplication width has a critical value for defining the device stability. The absorption width defines the wavelength for peak responsivity, which suggests a structure enhancement for IR detection. Also the effect of varying the doping concentration is studied.