Field-effect induced luminescence has been achieved from Si nanocrystals under alternate polarization. The emitting devices have a typical metal-oxide-semiconductor structure with a semitransparent polycrystalline Si top contact ~250 nm thick. The active layers have a thickness of ~45 nm and have been fabricated either by Si+ ion implantation into thermally grown silicon oxide or by plasma enhanced chemical vapour deposition (PECVD). The performances of both kinds of test structures have been analyzed and compared. In implanted devices, the application of a pulsed negative excitation gives rise to a luminescence combining a continuous and an alternate component. In particular, the continuous one proportionally increases with the Si amount, meanwhile the alternate one appears to be more sensible to quantum confinement effects in the Si nanocrystals. The threshold voltage of electroluminescence is about 3-4 V. PECVD devices exhibit a higher threshold (~18 V) and, in contrast with implanted samples, no continuous component is observed. Energy filtered transmission electron microscopy (EFTEM) results suggest that these differences are due to the partial depletion of silicon nanocrystals in PECVD devices in a tiny region close to the substrate.