We present the results of our calculation of the effects of dynamical coupling of a charge carrier to the electronic polarization and the field-induced lattice displacements at the gate interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge carrier in the quasi-two-dimensional channel of a pentacene transistor by a factor of 2 from its bulk value when the gate is a high-permittivity dielectric such as (Ta<sub>2</sub>O<sub>5</sub>), while this reduction essentially vanishes using a polymer gate insulator. These results point to carrier effective bandwidth as a possible trigger of the dielectric effects on the mobility reported recently in OFETs. © 2006 American Institute of Physics.