Improvement of excitation efficiency of excess mobile electron-hole pairs in semiconductor body during absorption of electro-magnetic radiation is the task under continuous consideration of everybody who does develop and/or manufacture ultimate performance radiation detectors based on semiconductors. We present results of excitation efficiency calculations performed on model structure consists of homogenous absorber layer with fixed energy gap and two graded-gap layers blocking absorber layer from front side and back side. Following assumptions were applied to consideration: interfaces are electrically neutral and density of recombination centers is negligible; value of graded-gap field Eggl is suddenly changed to zero at interfaces. It was shown that excitation efficiency of excess mobile electron-hole pairs in semiconductor body could be sufficiently high even under condition of infinite surface recombination speed on outer surfaces of the structure; it is important to note that big gradient (high intensity field) in graded-gap layers is not required in this case. Specific distribution of excess holes p(z) assists suppression the influx of excess holes from absorber layer to the outer surfaces. Could say that diffusion barrier is built in region of graded-gap layer sided with absorber that prevents excess holes from moving to surface. It is significant to note that strength of surface recombination suppression is almost completely independent on band-gap gradient value in blocking graded-gap layers and could be realized even when standard condition - drift velocity of holes in graded-gap field should prevail over diffusion velocity and surface recombination speed - is not satisfied. Estimation of excitation efficiency is given for different structures used for gamma and X-ray detectors.
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