Such devices, called heterojunction lasers, can be made to operate continuously at room temperature to satisfy the requirements of optical communications.
- An example of a heterojunction laser is shown in Fig. In this structure the injected carriers are confined to a narrow region so that population inversion can be built up at lower current levels.
- The result is a lowering of the threshold current at which laser action begins. Carrier confinement is obtained in this single-heterojunction laser by the layer of AlGaAs grown epitaxially on the GaAs.
- In GaAs the laser action occurs primarily on the p side of the junction due to a higher efficiency for electron injection than for hole injection.
- In a normal p-n junction the injected electrons diffuse into the p material such that population inversion occurs for only part of the electron distribution near the junction.
- However, if the p material is narrow and terminated in a barrier, the injected electrons can be confined near the junction. In Fig.(a), an epitaxial layer of p-type AlGaAs (Eg- 2 eV) is grown on top of the thin p-type GaAs region.
- The wider band gap of AlGaAs effectively terminates the p-type GaAs layer, since injected electrons do not surmount the barrier at the GaAs-AlGaAs heterojunction (Fig.b).
- As a result of the confinement of injected electrons, laser action begins at a substantially lower current than for simple p-n junctions.
- In addition to the effects of carrier confinement, the change of refractive index at the heterojunction provides a waveguide effect for optical confinement of the photons.