Effect of a surface channel on the performance of a P-type Point Contact HPGe detector

The observation of anomalous charge collection in a P-type Point Contact (PPC) High Purity Germanium (HPGe) detector is reported. By studying preamplifier charge signals from the detector, these anomalous events are shown to be consistent with charge carrier drift along the surface of the detector, prior to collection at the contact. It is believed that these events arise from the formation of a surface channel in the detector. By coupling analysis of these experimental signals with field calculations, an estimate of the radial drift velocity of holes at the surface has been made. This value is found to be around 40 times slower than the accepted value for saturated hole drift in the bulk, and is in good agreement with that calculated using a newly developed technique for modelling carrier transport.