ABSTRACT
In a semiconductor, light is produced by electrons making radiative transitions across the direct energy gap (Eg), with the energy lost by each electron being given up by the creation of a photon. The wavelength of the emitted light is determined by the magnitude of the direct energy gap and for short-wavelength emission we require a wide-gap semiconductor: 650 nm corresponds to a photon energy of 1.91 eV. However, as illustrated in figure B2.5.1.1, other energy gaps exist which are termed indirect because the electron has to change its momentum as well as its energy in making a transition. The ‘momentum’ associated with a photon is small and cannot compensate for the momentum difference between initial and final states of a transition across the indirect gap. The radiative transition rate across the indirect gap is therefore very small because another ‘particle’ such as a phonon must also be involved in the process to conserve momentum, consequently the dominant indirect recombination processes are non-radiative, producing heat rather than light. If the conduction band minimum of the indirect gap is lower than that of the direct gap then most of the electrons will reside in the indirect minimum and will not be available to make direct radiative transitions. Thus, the direct energy gap can only be exploited for light emission where it lies below the indirect gap. The indirect gap has a major influence on the design of red-emitting devices.
