Full-band Monte Carlo simulations were carried out to investigate hot carrier effects associated with impact ionization under the lateral electric field profiles typical of submicrometer Si-MOSFETs. It is shown that the temperature dependence of the band-gap energy of Si plays an important role for hot carrier suppression at low temperature in submicrometer devices. On the other hand, as the device size shrinks into the sub-0.1 regime, in which the high-field region is comparable in size to or smaller than the energy relaxation length, the number of electrons with energy below the supply drain voltage becomes less sensitive to temperature. As a result, the suppression of impact ionization at low temperature in sub-0.1 /spl mu/m devices could be ascribed to both quasi-ballistic transport characteristics and temperature-dependent band-gap energy.