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The transient ultraviolet (UV) and visible luminescent dynamics of the metal-oxide-semiconductor light-emitting diodes (MOSLEDs) made on Si-rich SiOx with its O/Si composition ratio detuning from 0.75 to 1.62 are investigated. The size and luminescent wavelength of the buried Si quantum dots (Si-QDs) are controlled by adjusting the O/Si composition ratio of the Si-rich SiOx. Time-resolved photoluminescence shows a lifetime decaying from 11.5 μs to 67 ns with the Si-QD size reducing from 4.5 to >; 1.7 nm. The shorter lifetime for smaller Si-QDs is due to the increased nonphonon-assisted carrier recombination rate in smaller Si-QDs. The Si-QD size shrinkage is obtained by enlarging the O/Si composition ratio via the increase in the N2O/SiH4 fluence ratio during synthesis, which makes the SiOx matrix approaching a standard dioxide with a higher turn-on threshold field under Fowler-Nordheim tunneling. By increasing the O/Si composition ratio from 1.15 to 1.54, the obtained EL pattern changes its color from red to blue, which is associated with the turn-on voltage increasing from 40 to 175 V. Decreasing the Si-QD size to 1.7 nm inevitably attenuates the EL power to 100 nW and reduces the P/I slope to 0.63 mW/A. The UV EL patterns of MOSLEDs made on the SiO1.62 film are demonstrated with an EL power of 40 nW, and the decay of ITO transmittance to <; 30% at an EL wavelength of <; 375 nm also contributes to the power attenuation.