Both model and experimental results of electron injection in amorphous silicon n+-i-n+ devices with heavily doped n+ layers are presented using a realistic model of such a structure, developed by the author. Spatial profiles of transport parameters were calculated changing the undoped layer (i layer) thickness and the energy and spatial distributions of the density of localized states in undoped film. Simulated current–voltage (I–V) characteristics were compared with space-charge-limited current (SCLC) dependences given by the drift currents. The effect of diffusion currents on the I–V characteristics was studied to determine the criteria for a correct application of the SCLC technique. As follows from numerical simulations, only the characteristics measured on a device with a sufficiently “thick” i layer can be used to apply the SCLC technique. The effect of contacts on the I–V dependence can be checked by the scaling law. © 2000 American Institute of Physics.