Asymptotical deduction of the ambipolar diffusion equation and boundary conditions in semiconductor physics

By means of boundary layer functions method the asymptotics is constructed for the solution of the singularly perturbed system of differential equations describing distribution of nonbalanced charge carriers in thin semiconductor plate with conductivity close to a natural one. It is shown that for main term of expansion well-known ambipolar diffusion equation in semiconductor physics arose. The boundary conditions found are different from those which are used in some physical papers.