We have investigated theoretically the electronic subband structures and the scattering probability of electrons by ionized impurities in &dgr;‐doped field‐effect transistors (&dgr;‐FETs). The self‐consistent calculation of the Poisson and Schro¨dinger equations shows that the electrons are quite extended, even though the impurities are very much confined (&dgr; doped). It is shown that the impurity scattering probability decreases if the carriers and impurities are well separated spatially, or if the kinetic energies of the carriers are large. For both the singly and doubly &dgr;‐FET, the averaged kinetic energies of the carriers are increased when the sample temperature is increased; the carriers are pushed away from the &dgr;‐doped impurity layers when the gate bias is increased. The combination of the two effects result in an enhanced electron mobility, as demonstrated by experiments. Our detailed numerical calculation thus provides us with some basic guidelines to improve the performance of &dgr;‐FETs: Multiple &dgr;‐doped layers and a high sheet density are desirable. ©1995 American Institute of Physics.