The chapter discusses the hetero-junction, which is an important development that enhances the performance of the bipolar junction transistor. It explains the behavior of various types of semiconductor diodes, based on the physical principles enunciated. Metal–semiconductor contacts are not only unavoidable in semiconductor devices, but are also the main structure in some special-purpose diodes and transistors. The conductivity of the semiconductor increases with the rate of carrier generation, and hence with light intensity. For a given applied voltage, the current through the semiconductor therefore increases with the light intensity. Semiconductor photoelectric devices are examples of transducers that convert radiation energy to electric energy. In the semiconductor, the space charge arises from a reduced concentration of conduction electrons leaving an excess of positively charged donor ions. The density of these surface states is very much influenced by the crystal orientation, by the metal used, and by the treatment of the semiconductor surface.