We have studied the avalanche dynamics in shallow junction avalanche diodes designed for detection and timing of single photons. Space charge effects have been correctly taken into account, and both electron and hole transport were considered. Based on the results of the detailed computer simulation we have developed a simplified model of the detector which can be used for studying the avalanche transient in these devices and for optimizing the detector structure for the best timing performance. The results are in good agreement with the experiments. ©1995 American Institute of Physics.