The one- and two-electron ground-state energies of singly and doubly charged silicon (Si) dots modelled by a sphere covered with a silicon dioxide layer embedded in various dielectric media can be calculated as functions of the sphere size and the thickness of the oxide by extending the work of Allan et al and Babic et al. The electron-self-polarization, electron - electron and electron-polarization energies are treated by first-order perturbation theory, taking the confined free-electron state as the unperturbed state. By changing the thickness of the oxides or the surrounding dielectric medium, the applied voltage required for the tunnelling of one electron when one electron already exists inside the dot is greatly reduced. We discuss the possible consequence of electron tunnelling in a Si dot and the electroluminescence of porous Si.