The role of bulk traps in metal-insulator contact charging

Metal-insulator contact charging models are presented for insulators having (a) traps uniformly distributed in the energy gap and (b) discrete traps in the energy gap. It is shown that in the former case the relationship between the magnitude of the charge transferred and the workfunction difference is linear, while in the latter case it ranges from an exponential to a quadratic depending on the band bending and the energetic position of the traps. Screening lengths are calculated for the two cases, and it is shown that the thickness of the insulator is usually not the limiting factor in contact charging, even for thin-film insulators. Finally, it is shown that the times required for the charge transfer can be short.