Simulation of neutron radiation effects in silicon avalanche photodiodes
A new one-dimensional device simulation package developed for the simulation of neutron radiatiol! effects in silicon avalanche photodiodes is described. The software uses a finite difference technique to solve the time-independent semiconductor equations across a user specified structure. Impact ionisation and illumination are included, allowing accurate simulation with minimal assumptions about the device under investigation. The effect of neutron radiation damage is incorporated via the introduction of deep acceptor levels subject to Shockley-Read-Hall statistics. Two models are presented. A reverse reach through model, based on the EG&G C30626E reverse reach through avalanche photo diode originally proposed for use in the CMS electromagnetic calorimeter, and a reach through model, based on widely available commerical devices. A short experimental study on two commercial silicon avalanche photodiodes, a C30719F reverse reach through APD and a C30916E reach through APD, is presented for comparison with the simulation data. To allow full comparison with the simulated predictions, the commercial devices were irradiated at the Rutherford Appleton Laboratory's ISIS facility. The simulated data shows good qualitative agreement with the measurements performed on the commercial devices, quantitative predictions would require exact information about the doping profile. The characteristic behaviour of the devices is predicted over a wide range of conditions both before and after neutron irradiation. The effect of ionised deep acceptors in the bulk of the devices is investigated. The simulation package provides a useful tool for the analysis of semiconductor devices, particularly in areas where a non-ionising radiation damage is prevelent e.g. high energy physics, and provides a good basis for further development.