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Theoretical analysis for different active layer structures is performed to minimize the laser threshold current of the ultraviolet GaN/AlGaN multiple-quantum-well laser diodes by using advanced device simulation. The simulation results show that the lower threshold current can be obtained when the number of quantum wells is two or three and the aluminum composition in the barrier layer is about 10%-12%. This result is attributed to several different effects including electron leakage current, nonuniform carrier distribution, interface charge density induced by spontaneous and piezoelectric polarization, and optical confinement factor. These internal physical mechanisms are investigated by theoretical calculation to analyze the effects of quantum-well number and different aluminum compositions in barrier layer on laser threshold properties. Furthermore, the effect of quantum-well thickness is discussed as well. It is found that the optimal quantum-well thickness is about 3 nm due to the balance of the advantages of a large confinement factor against the disadvantages of significant quantum-confined Stark effect (QCSE).