Experimental results on the electronic properties of conditioned Cr/hydrogenated amorphous silicon (a-Si:H)/V thin film devices are presented. The devices under test were electro-formed, and had resistances in the range from several hundred Ohms to several kiloOhms. The current of conditioned devices varied non-linearly with bias at low voltages, but exhibited 'jumps' at a threshold voltage (V-th) (typically 2-3 V), leading to a resistance change of one to three orders of magnitude. Above V-th the current increased almost linearly with bias, and the carrier transport changed from a semiconducting behaviour to a more conducting (metallic) state. This was confirmed by a.c. characteristics of the conditioned devices, which showed a transition from a capacitive to an inductive behaviour around V-th. The threshold voltage V-th, was found to decrease with increasing temperature and disappeared at 340-350 K, but recovered when the temperature was reduced. The transition at V-th has been analysed in terms of an electrothermal mechanism. The calculated turnover temperature is approximately 346 K, close to that for the disappearance of Vh. We suggest that the observed transition could involve vanadium oxides such as VO2. The phase transition could also facilitate the underlying 'quantisation' effect.