Modern semiconductor sensors are based on an MOS (metal–oxide–semiconductor) structure and their further development is tending to combine them together with UV irradiation. In this paper, a system is presented that can pave the way towards gas analyzers that rely on visible spectrum light. The article presents a semiconductor gas sensor induced with safe optical irradiation delivered from the LED matrix to the semiconductor surface. Different irradiation wavelengths (440, 530, 600, 710, nm) under constant flux were used separately to find the condition for the best sensor response on gas sorption. An output signal was recorded in zero-level emission and in the presence of ethanol. Changes in sensor response and signal rise/relaxation time constant in the presence of saturated ethanol vapor were observed. Sensor response to the ethanol vapor was detected for each used irradiation wavelength and it is approximated by the non-linear falling regression curve with the highest sensor response at 440, nm (R = 104%). In addition, the different rise and relaxation time constant of the signal trace was detected depending on the irradiation wavelength. The time constant ratio is approximated by the non-linear rising regression curve with a maximum value at 710 nm for both signal rise (τ = 0.61) and relaxation (τ = 2.08) parts.