he valence to the conduction band of a semiconductor via current injection. In order to achieve photon-stimulated emission, the transition between valence and con- duction bands corresponding to the desired laser frequency has to be a "direct transition." This means that pseudo-electrons in the valence and conduction bands must have the same crystal momentum SO that decay can happen with photon emission without the need of higher-order processes to compensate the momentum difference. In Figure 5.1 [2] the band structure of silicon, characterized by an indirect transition between the valence and the conduction band, is shown, while the band structure of an InGaAs alloy [2], characterized by a direct transition in the near-infrared region, is shown in Figure 5.2. The requirement of direct transitions practically selects the materials that are suited to build semiconductor lasers: they are essentially the alloys of III-V semiconductors, start- ing from GaAs and GaAsP, used for pump lasers, to InGaAsP, which is generally the alloy in the active region of lasers used for transmission. Once the material is selected, a structure suitable for electron pumping has to be real- ized. The best possible structure is an inversely polarized p-n junction. In a p-n junction Technology for Telecommunications: Integrated Optics and Microelectronics Energy Indirect transition <100> <111> Crystal momentum Holes Heavy Light FIGURE 5.1 Schematic representation of the band structure of silicon. Energy Direct transition <100> <111> Crystal momentum Holes Heavy Light FIGURE 5.2 Schematic representation of the band structure of a II-V alloy having direct optical transition at the band gap. under inverse polarization, the field in the junction zone is opposite to the electron flux; thus if a current is pumped in the junction, electrons pass through being promoted to higher-energy levels to overcome the junction potential barrier. In this condition the aver- age population of the conduction band can become greater than the population of the 