Modelling results of avalanche multiplication in AlGaAs soft X-ray APDs

First results from a new simple simulation of X-ray spectroscopy with an Al0.8Ga0.2As p+-p--n+ spectroscopic X-ray photon counting avalanche photodiode are presented and compared with experimentally obtained data. Particular attention is paid to the experimentally observed phenomenon of an additional peak being present in the spectrum from a radioisotope source that, whilst previously reported, had not been fully explained. The additional peak is shown to result from the unequal distributions of multiplications undergone by the charge carriers created by photons absorbed in the p+ and p- layers. The consequences of this effect for future AlGaAs X-ray photon counting avalanche photodiode designs are discussed.