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The SiGe quantum well (QW) tunneling diodes with different Si cap thickness are designed in this work to investigate the detailed mechanism of light emission for the metal-insulator-semiconductor light-emitting diode (MIS LED). At the accumulation (negative) gate bias, the electrons tunnel from the metal gate to the SiGe QW semiconductor substrate (p-type). Meanwhile, the negative gate bias also attracts holes in the interface of insulator and SiGe QW semiconductor and then the tunneling electrons can recombine with holes to emit the infrared. Light-emitting diodes with different Si/SiGe/Si structure are used to investigate the light emission mechanism and prove that the region of infrared emission is mainly from the interface of the insulator and semiconductor, not from the neutral region in the semiconductor substrate. The accurate energy band and carrier concentration in the SiGe QW structure with operated bias are also simulated and compared it with the experimental data. It shows that the concentrations of the tunneling electrons, which will be drifted by the gate bias, plays the main role on the light-emission and limits the light emission region near the interface of insulator and semiconductor. This work helps us understand clearly the light emission mechanism of the MIS LED, developed continuously by our group in the decade.