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Current injection in insulators provides a valuable technique for obtaining such information about defect states in insulators as their density, their energetic location in the forbidden gap, and their cross sections for capture of free electrons and holes. I t also enables determination of free carrier drift mobilities. The physical principles underlying one-carrier space-charge-limited current injection, both steady-state and transient, and steady-state, two-carrier, largely neutralized current injection are discussed. Experimental verifications of the different types of current injection are also discussed briefly.

Measurements have been made of the current flow in amorphous WO3 films containing electrons and mobile cations. In a configuration in which electrons are extracted at one contact and cations at the other, the current decays as t−3/4 over many decades of time. By using space-charge current flow ideas, we develop a theory that gives the correct time dependence and magnitude of the current for this double-extraction phenomenon.