Diffusion effects are usually neglected in the consideration of the negative-resistance problem associated with double injection in insulators. In this paper, these effects are taken into account and are shown to be very important in determining the current-voltage characteristic. It is shown that if the length of the sample is greater than a critical value Lcr, the post-negative-resistance regime of the characteristic is one in which current density increases as the square of the applied voltage; and if the sample length is less than Lcr, the post-negative-resistance characteristic is one in which current increases at constant voltage. The existence of Lcr is shown to be related to the absence of presence of a particular conduction region. The analytic procedure used is the regional approximation method.