Photoinduced charge dynamics in photorefractive semiconductors is studied from both, theoretical and experimental points of view in the case of an external field screening. This dynamics shows two different regimes depending on the parameter τ_M/τ_i where τ_M is the Maxwellian time and τ_i is the impurity ionization time. For τ_M ≪τ_i, the stratification effect arises and numerous space charge layers of alternating sign occur in the crystal. Another regime occurs for τ_i ≪ τ_M, the broadening of the single layer leads to the screening. In BSO, the first regime takes place for room crystal temperatures T, and the second one for 130 K < T < 200 K. Such charge dynamics determines optical nonlinearity of photorefractive semiconductors.