An investigation of the electrical properties of CdSe thin‐film triodes has demonstrated the existence of shallow trapping levels distributed from 0.02 to 0.06 eV below the edge of the conduction band. These values have been experimentally determined by a combination of optical and field effect techniques. The investigation also indicates that field excitation of electrons from such levels is a fundamental mechanism of operation in the thin‐film field‐effect triode. Close correlation has been obtained between the dependence of device current on applied gate potential and the probability of field excitation of carriers with the field strengths achieved with these values of potential.