Gamma photon absorbed in a semiconductor detector generates holes and electrons that flow towards the opposite contacts. Poisson and the continuity equations require that additional charge will come out of ohmic contacts and flow towards the gamma generated charge. The fast electron flow, from the negative contact towards the holes, overcomes holes trapping and transforms the detector into a practically single carrier device. The detector operation is optimized by trade off between the leakage current and the lifetime of the gamma generated electrons, that is, proper positioning of the Fermi level within the semiconductor forbidden band. Crystals that grow by standard methods, and contain too may traps for conventional detector operation, are applied to fabrication of uniform ohmic detector arrays.
"Ohmic contact gamma radiation detectors", Proc. SPIE 3768, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics, (19 October 1999); doi: 10.1117/12.366602; https://doi.org/10.1117/12.366602