Electron capture creating thermal electrons

Here X is a surface state that is occupied by an electron and is therefore able to capture a hole. Xf is the vacant state created that is now able to accept an electron from the conduction band. The surface concentration of X will depend on the total surface density of surface states and their electron occupancy. The rate constants for hole and electron capture, (3p and f can be defined as the products of the thermal velocities, vp and v and the capture cross sections ap and tr of X and X+. 

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