9.2.4 Indirect and Direct Band Gaps

Silicon is the best-known semiconductor. It is the standard for most electronic applications, for light detection at visible and near-infrared wavelengths, and for solar cells. However, silicon is a very poor light emitter because it can only make an indirect transition from the conduction band to the lower-energy valence band. This condition, called an indirect bandgap, means that an electron in the conduction band must interact with something else in order to drop down to the valence band. Normally, that interaction takes a while, so it is milliseconds before the silicon can emit light, and by then other interactions with much shorter lifetimes have drained away the energy that could have been emitted as light. Germanium and some other semiconductors such as gallium phosphide also have indirect bandgaps that make them poor light emitters.

Silicon LEDs have been demonstrated in the laboratory, but they require special tricks to make the material behave differently. One example is fabricating nanostructures called quantum dots, which confine electrons and holes on the scale of a few nanometers. That quantum confinement changes the momentums of the electrons and holes to make it more likely that a conduction electron can drop directly into a hole in the valence band, making the silicon act more like a direct-bandgap material.

Direct-bandgap semiconductors can emit light efficiently because electrons can drop directly from the conduction band to the valence band without changing their momentum, which requires interactions that can drain away energy. The most important direct- bandgap semiconductors are compounds of elements from groups III and V in the periodic table, such as gallium arsenide and indium phosphide, known as III V compounds. They are the type used for LEDs and diode lasers.

The distinctions between direct and indirect bandgap compounds are not always sharp or obvious. GaAs has a direct bandgap and GaP has an indirect bandgap. Mix a little phosphorous with GaAs, and it remains a direct-bandgap semiconductor until the phosphorous level crosses a threshold.

Because this book is about lasers, we will focus on the family of compound semiconductors with direct bandgaps that allow light emission, making them useful in LEDs and semiconductor lasers.