We have developed a computer program to calculate the field effect conductance for an amorphous semiconductor including the effects of surface states and fixed charge atbothsurfaces of the thin semiconductor film. For undoped films with a bulk density of states of less than 1017cm−3eV−1, the space‐charge region extends to a depth of 0.5 &mgr;m. A complete description of the potential distribution in the semiconductor must include the contribution of surface charge from the surfaceoppositethe gate electrode. This is of practical importance in thin film transistors, for example, where different transistor structures and processing of devices can affect the charge density of this surface.