The carriers mobility is assumed to be field-independent. Neglecting every kind of diffusion process for the de-trapped carriers, the pre-exponential factor in the Poole–Frenkel formula is thus proportional to E . This depiction would be suitable for the description of the conduction either in dielectrics or semiconductors. However, Poole–Frenkel effect is likely to be observed only in materials characterized by low mobility values, for, in high mobility solids, the re-trapping of carriers would be gradually inhibited by carrier depletion. Still, different dependences of the pre-exponential factor from the field can be found: assuming that the carriers could be re-trapped, proportionality to E − − 1 / 2 or E 1 / 2 is obtained, depending on the electron retrapping occurring by the nearest neighbouring trap or after a drift. Moreover, a pre-exponential factor proportional to E 1 / 2 is found to be the result of random diffusion processes, while dependencies on E − − 3 / 2 and E − − 3 / 4 are found to be the result of hopping and diffusion transport processes respectively.