If an isolator gets charged (e.g. by rubbing it to an other isolator), it can get charged. If it is negatively charged, I would expect the additional electrons to be in the conduction band, so that they can move freely through the insulator? If this is true, why can we not just inject an electron at the one side of the insulator and then extract it at the other side, so that it is in fact conductive? Here I think, that the energy needed to insert an electron into an insulator might be too high. However, is this effect in general possible, at which voltages this effect occurs and how is it called?