Tunable electroluminescence properties of size-controlled Si nanocrystals embedded in silicon rich oxide films are demonstrated at room temperature, using an active light emitting layer in the metal oxide semiconductor device structure. Plasma enhanced chemical vapor deposited Si-rich oxide films were annealed at elevated temperatures to form Si nanocrystals of varying diameters. A typical redshift in the photoluminescence peak is observed with increasing annealing temperature, confirming the formation of quantum confined Si nanocrystals. The carrier transport and light emission mechanism have been studied in detail through current–voltage characteristics and ultrafast transient spectroscopy, respectively. The origin of electroluminescence and the size-tunable emission peak have been analyzed and attributed to the radiative recombination of carriers within Si nanocrystal quantum wells. The fabricated Si nanocrystal-based metal oxide semiconductor light emitting diode and the resultant size-dependent tunable electroluminescence are very attractive as a potential CMOS compatible optical source for future photonic integrated chips.