Excess Carrier Phenomenon in Semiconductors

Abstract

The generation of excess carriers in a semiconductor may be accomplished by either electrical or optical means. For example, electron-hole pairs are created in a semiconductor when photons with energies exceeding the bandgap energy of the semiconductor are absorbed. Similarly, minority carrier injection can be achieved by applying a forward bias voltage across a p-n junction diode or a bipolar junction transistor. The inverse process to the generation of excess carriers in a semiconductor is recombination. The annihilation of excess carriers generated by optical or electrical means in a semiconductor may take place via different recombination mechanisms. Depending on the ways in which the energy of an excess carrier is removed during a recombination process, there are three basic recombination mechanisms which are responsible for carrier annihilation in a semiconductor. These include: (1) nonradiative recombination (i.e., the multiphonon process), (2) band-to-band radiative recombination, and (3) Auger band-to-band recombination. The first recombination mechanism, known as the nonradiative or multiphonon recombination process, is usually the predominant recombination process for indirect bandgap semiconductors such as silicon and germanium. In this process, recombination is accomplished via a deep-level recombination center in the forbidden gap, and the energy of the excess carriers is released via phonon emission. The second recombination mechanism, band-to-band radiative recombination, is usually the predominant process occurring in direct bandgap semiconductors such as GaAs and InP. In this case, band-to-band recombination of electron-hole pairs is accompanied by the emission of a photon.