SEMICONDUCTORS are the materials at the heart of many electronic devices. The elemental semiconductors are few, the only ones of any practical use being germanium (Ge) and silicon (Si) and only silicon is widely used nowadays. However, there are some compound semiconductors which find applications in light-emitting diodes, semiconductor lasers, microwave devices and other specialised areas. Most of these are based on elements from groups III and V of the periodic table (see Table 6.1) and occasionally from groups II and VI. These are known respectively as III–V and II–VI compounds, examples being gallium arsenide (GaAs), gallium phosphide (GaP) and cadmium sulphide (CdS).
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