recombination 8888° Recombination Carrier Holes FIGURE 83 1 Light emission due to radiative recombination of injected carriers in a forward-biased pn junction. Energy Indirect Recombination hala p -h≠/{a FIGURE 83.2 (a)Interband recombination in a direct-bandgap semiconductor;( b) recombination in an indirect-gap semiconductor also involves a momentum chan Equation(83. 1) implies that the radiation emitted will be monochromatic, but in practice AE> E, and there is a spectral distribution corresponding to the energy distributions of the carriers in the conduction and valence bands Semiconductor materials Silicon is the most common material used in current semiconductor technologies, but it is not at all suitable for an LED. The reason is that silicon has an indirect bandgap, and a direct bandgap is required for process efficiency. Direct and indirect bandgaps are compared in Fig. 83. 2, where carrier energy is plotted versus momentum for both cases. The photon momentum P=h=hu/c (83.2) e 2000 by CRC Press LLC© 2000 by CRC Press LLC Equation (83.1) implies that the radiation emitted will be monochromatic, but in practice DE > Eg, and there is a spectral distribution corresponding to the energy distributions of the carriers in the conduction and valence bands. Semiconductor Materials Silicon is the most common material used in current semiconductor technologies, but it is not at all suitable for an LED. The reason is that silicon has an indirect bandgap, and a direct bandgap is required for process efficiency. Direct and indirect bandgaps are compared in Fig. 83.2, where carrier energy is plotted versus momentum for both cases. The photon momentum p = hl = hu/c (83.2) FIGURE 83.1 Light emission due to radiative recombination of injected carriers in a forward-biased pn junction. FIGURE 83.2 (a) Interband recombination in a direct-bandgap semiconductor; (b) recombination in an indirect-gap semiconductor also involves a momentum change. VB CB E Eg eV np pn x p n npO pnO EFn EFp Hole Injection Electron Injection Light Out Holes Depletion layer Minority￾Carrier Density Light Out recombination e (V–VO) pnoe pnoe -x/Le -x/Lh recombination Light Output Indirect Transition Energy Change Momentum Change Injected Electron Direct Recombination with Hole VB CB E E Hole -h¹/a 0 h¹/a (a) p VB Hole -h¹/a 0 h¹/a (a) p