Charge storage in a semi-insulating layer with a gradient of conductivity

If a semi-insulating layer with a conductivity gradient is submitted to an external electric field, it stores a net electric charge. The stored charge density is calculated for a polarized layer with a conductivity of the form sigma (E,T)= sigma 0 (1+AE) exp k(T-T 0 ). Various aspects of this charge storage are briefly described.