Understanding Lasers

Chapter 9.4.3: Heterojunction Lasers

9.4.3 Heterojunction Lasers

Improvements came from replacing the homojunction with a heterojunction between layers of semiconductors with different composition. The idea and its implementation in diode lasers earned Herbert Kroemer and Zhores Alferov the 2000 Nobel Prize in physics.

The appeal of a heterojunction comes from differences between the electronic and optical properties of the two materials. Those differences can control the flow of electronic carriers or light between the two sides of the junction. Figure 9-9 shows the

effects of differences in the band-gap energy on conduction electrons. The electrons in the high-band-gap material have enough energy to enter the lower-band-gap material, but the conduction electrons in the low-gap material do not have enough energy to reach the conduction band in the high-gap material.

Similarly, differences in the refractive index can block light in a high-index material from entering a low-index material, if the light strikes the boundary between the two materials at a steep enough angle. That produces total internal reflection, the same phenomenon behind optical fibers, which effectively guides light through the high-index material.

A single heterojunction between two materials improves the confinement of light and electrons enough for a diode laser to operate in pulsed mode at room temperature. This confinement is illustrated schematically by the curves at the right for three types of lasers in Figure 9-10. Note how the light and electrons occupy a smaller area in the single-heterojunction laser than in the homojunction laser.

If one heterojunction is good, two should better confine light and electrons, as you can see at the bottom of Figure 9-10. In this case, the active layer is GaAs, which has a lower bandgap energy than the GaAlAs layers above and below it. This design is called a double-heterojunction or double-heterostructure laser, and it was the first diode laser capable of continuous-wave operation at room temperature. The double-heterostructure design is the basis of today's semiconductor laser industry.

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