MODELING OF ELECTRON-HOLE SCATTERING IN SEMICONDUCTOR-DEVICES

It is generally assumed in device modeling that the effects of electron-hole scattering can be fully accounted for by a suitable reduction in the electron and hole mobilities with injection level, without modifying the semiconductor device equations themselves. Physical considerations indicate that this is not the case, and that electron-hole collisions introduce a direct coupling between the electron and hole currents. This is analyzed from first principles, and the results of a Boltzmann calculation are described. The key result is that the impact of an electron-hole scattering event depends upon the relative drift velocity between electrons and holes. In low injection, the effective minority-carrier diffusion mobility cannot be assumed to be identical to majority-carrier mobilities or to minority-carrier drift mobilities. In high injection, a reduction in the conductivity mobility does not imply a reduction in the ambipolar diffusion constant. The complete treatment in high-injection differs significantly from the conventional treatment in any power device where ambipolar diffusion length is important. This is analyzed for p-i-n diodes.