An approximate theory of the thin film transistor is presented. It is proposed that the large observed transconductances of thin film transistors made from polycrystalline CdS films are the result of changes of carrier mobility as well as carrier concentration. It is proposed that mobility changes occur because of changes in the amount of ionized impurity scattering or because of changes in the heights of intercrystalline barriers. The frequency dependence of the transconductance is analyzed by considering the effect of traps. It is shown that the magnitude and phase of the small signal a.c. transconductance allows one to infer which physical mechanism is responsible for the conductivity modulation. © 1964.