Charge spreading from a point source in a depleted diode

This report describes simulations that have been made of the charge spreading that occurs in a semiconductor from a point generation event such as an X-ray strike. Simulations have been made using two- and three-dimensional blocks of silicon which are biased so as to produce very large depletion regions. Any charge generated in these regions will be transported to collecting contacts. The device modelling software package EVEREST has been modified to allow measurement of the charge spreading that occurs from the impact up to the time the charge is collected at a contact. Numerical results are compared to a simple diffusion model for 2D and 3D cases. The model is in reasonable agreement with the results at low charge density in 2D. Larger differences between the 3D measurements and the theory may be due to limitations of the simulation mesh size. A functional form for the spreading from any depth and reasonable size of charge event can be derived which may be used in analysis of CCD and pixel detector performance. (author)