A new theory ofa‐Si thin‐film transistor (TFT) operation is presented. In addition to the below‐ and above‐threshold regimes described previously, it predicts two new regimes of operation which occur at very high densities of the induced charge in thea‐Si TFT channel. In a crystallinelike regime the free‐electron concentration exceeds the localized charge concentration at thea‐Si‐insulator interface. In a transitional regime (at lower densities of the induced charge) almost all localized states in the energy gap of amorphous silicon near the interface are filled. In the crystallinelike regime, the field‐effect mobility is close to the band mobility and the operation of ana‐Si TFT is truly similar to the operation of a crystalline field‐effect transistor. Our estimates show that the gate voltage necessary to achieve the crystallinelike regime is about 50 V for ana‐Si TFT with an insulator 1000 A˚ thick and a relative permittivity of approximately 3.9.