Why does a double hetero junction laser have a wide band gap?

A double heterojunction laser is designed with layers of semiconductor materials that have differing energy band gaps. The wide band gap in the active region serves primarily to enhance the optical confinement of the generated excitons (bound states of electrons and holes). This confinement is crucial because it helps to keep the carriers (electrons and holes) localized in the active region, ensuring that recombination occurs in a restricted volume where the light generation can be maximized.

By preventing the easy movement of electrons and holes outside of the active area, the device becomes more efficient at producing light. The carriers are thus forced to interact within the region where they can effectively recombine, leading to a greater intensity of light generated. This optical confinement is a key element in improving the performance and efficiency of the laser.

In summary, the wide band gap in a double heterojunction laser plays a fundamental role in creating conditions that favor efficient light emission by ensuring carriers are retained in the active region, thereby enhancing the overall operation of the laser device.