Abstract
The second microchip age that is developing includes optoelectronic devices, in contrast to electronic devices, such as the transistor, the basis of the first microchip revolution. For optoelectronic devices the electronic carriers must be confined in at least one dimension, giving thin layers, which are then stacked or alternated for greater reliability and efficiency. The material of choice for electronic devices has been the elemental semiconductor silicon, whereas efficient optoelectronic conversion requires compound semiconductors such as GaAs. Confinement is achieved by alternating large energy gaps (AlAs) with small gaps (GaAs). For communications purposes the operating optical wavelength must be matched to the pass band of kilometric optical fibres, which in turn has led to the design and production of quaternary semiconductor alloy sandwiches. Although the economics of compound semiconductor materials is much less favourable than that of Si, junction lasers made from these materials with less stringent specifications are already appearing in popular consumer items such as compact-disc players. New research indicates that Si-Ge alloys fabricated on Si may replace the AlAs—GaAs layer structures in some applications, with large savings for high-quality optoelectronic devices.