Comparison of signal rise-times in totally depleted P-type and N-type silicon surface barrier detectors

The rise-times of charge collection signals from the incidence of protons and alpha-particles of energies up to 25 MeV have been measured for totally depleted silicon surface barrier type detectors made from P-type and N-type silicon. The detectors were of similar thickness and the resistivities of the bulk materials such that their depletion voltages were comparable. The measurements show that P-type detectors can give at least as good particle identification by signal rise-time as can N-type detectors. Analyses of the experimental data show that the previous simple theory of charge collection rise-time identification has to be modified to take account of the finite internal space charge fields in the plasma columns along the particle tracks. The theory must also include contributions from the movement of holes. The velocities of electrons and holes deduced via this theory from the experimental data show a ratio which does not agree for mobility dominated velocities.