The electrical characteristics of the amorphous silicon (a‐Si:H) field‐effect transistors have been analyzed theoretically. It is found that if no interface states exist at the SiO2anda‐Si:H interface, the transistor with undoped channel gives the best subthreshold swing and on/off current ratio. However, the existence of the interface states shifts the threshold voltage and reduces the transconductance of the transistor. It is found from numerical analysis that the problem caused by the interface states could be compensated for by doping the channel slightly with phosphorus. The physical reason for the improved performance is discussed in detail.