Field effect in very thin semiconducting films

A field effect measurement is considered for a semiconducting film whose thickness is comparable to, or less than, its screening length. On the basis of a purely electrostatic argument a formula is derived for the charge induced on the contacts as a result of the field emerging from the rear face of the semiconductor. Some simple upper bounds for this effect are presented which justify neglecting it in many cases. A brief discussion is included of a different and larger phenomenon, namely the reduction in the capacity between a very thin film and the gate.