A stimulated emission under high optical pumping was investigated on InGaN/GaN multiple quantum well (MQW) structures as a function of well thickness at room temperature. With increasing quantum well thickness, the threshold of optical pumping decreases monotonically, similar to that conventionally observed in the AlGaAs/GaAs system. The intensity of the spontaneous emission under low excitation first increases, but then dramatically decreases as the well thickness increases from 1.4 to 3.9 nm. This is generally accepted to be due to the piezoelectric field-induced quantum-confined Stark effect (QCSE). In contrast, the emission mechanism in the process of stimulated emission is not dominated by the QCSE, since the stimulated emission is normally observed under high excitation power where the piezoelectric field is completely screened. Consequently, InGaN/GaN MQW behaves in the same manner as the classical AlGaAs/GaAs system. In this case, increasing well thickness can improve the confinement of wave function of carriers inside the quantum well, resulting in the decrease in threshold with increasing well thickness. In reality, an InGaN/GaN-based laser diode (LD) operates under high injection current that is similar to high optical pumping, while a light emitting diode (LED) works under low injection current that is similar to low optical pumping. This difference must be especially taken into account in designing InGaN/GaN LD and LED. (C) 2004 Elsevier B.V. All rights reserved.