Diffusion effects in semiconductor X-ray detectors with inhomogeneous distribution of electric fields

We derive an expression for the charge output of an X-ray semiconductor detector taking account of carrier drift and diffusion. It is found that diffusion effects may strongly influence charge collection patterns at the edges of the collection zones. We show that, in multi-electrode detectors, carrier diffusion across the boundaries between collection and non-collection zones depends on the nature of the boundary surfaces and the biasing conditions. Diffusion effects for a ring-drift detector are illustrated, and the conditions derived under which diffusion effects for collection around the outer edge of the collection zone are fully suppressed by the drift.