This thesis describes the forming process under constant current conditions of Cr/p⁺/V thin film devices (a-Si:H denotes hydrogenated amorphous silicon). In the initial stages of electro-forming by constant current stressing, with increasing injection of charge via either increasing bias or time, the J - V characteristics of devices exhibit an instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate, as measured by the voltage shift verse current in the J - V curves, is found to follow a square-root time-dependent law. Eventually, with further increasing current bias, the local current density reaches a critical value J_F, and a rapid 'runaway' processes occurs, which results in an irreversible change of the initial high-resistance state into a permanent 'formed' state of lower resistance. It has also been observed that formed samples sometimes show a metal-non metal (MNM) transition at low temperatures (60 - 100K). The electrical properties of these devices have been analysed in detail. The AC characteristic can be modelled using multi-component RC and RL equivalent circuits below and above the MNM transition region. An anomalous frequency dependence of the capacitance is explained in terms of a percolation-like critical behaviour of the dielectric constant _eff, which diverges at the percolation threshold p_c. A general approach for activated tunnelling in granular thin films is used to explain electron transport in formed devices. Analysis shows that the structure of formed Cr/p⁺a-Si:H}/V devices structure can be modelled as a heterogeneous filamentary medium composed of metallic particles and an insulator host (e.g.a-Si:H). The metallic particles originate from the diffusion of the top vanadium electrode during the forming process. This model is consistent with memory characteristics, such as the change in activation energy with device resistance.