Fundamentals of Optoelectronics

Optoelectronic devices exploit the interaction between light and semiconductors in both directions. In emission, LEDs and laser diodes convert electrical energy into photons: electrons from the conduction band recombine with holes in the valence band, releasing the energy as a photon. Detection runs the process in reverse – an absorbed photon lifts an electron into the conduction band, creating an electron-hole pair that shows up as photocurrent.

Together, these three device classes – LED, laser diode, photodiode – form the basis of virtually all optical data transmission, much of sensor technology, and modern display technology. Unlike the devices covered so far (MOSFET, bipolar transistor), the focus here isn't switching currents but the controlled conversion between electrical and optical energy.

Whether a semiconductor can emit light efficiently is decided by its band structure in k-space – energy plotted against the crystal momentum of the electrons. In a direct semiconductor like GaAs, the conduction band minimum and valence band maximum occur at the same k-value. An electron can recombine into the valence band without needing to change its momentum – momentum conservation is handled entirely by the emitted photon, whose momentum is negligible compared to that of the electron.

In an indirect semiconductor like silicon, the conduction band minimum and valence band maximum sit at different k-values. Recombination then additionally requires a phonon to absorb or supply the momentum difference. Since a three-particle process (electron, hole, phonon) is far less probable than a two-particle process, the radiative recombination rate in silicon is several orders of magnitude lower than in GaAs – internal quantum efficiency is typically around 10⁻⁶ for Si, versus close to 1 for GaAs. This is the physical core of why Si LEDs essentially don't exist and III-V compound semiconductors are indispensable for light emission.