  The dead space model of non-local impact ionization has been developed
independently by three different research groups in order to include a 'soft'
dead space. This paper seeks to compare these models for the first time. The
models are fitted to Monte Carlo simulation data and it is found that there are
no significant differences between the models except that the Liverpool model
cannot be used to fit one data set which has a very soft dead space. Likewise
the models give very similar results when used to calculate the excess noise
factor for low noise avalanche photodiodes.
