Silicon (Si) is the best known semiconductor. Boron, germanium zinc sulfide, and mercury selenide are just a few semiconductors among hundreds. Semiconductors such as zinc selenide (ZnS), cuprite (CU2O) and galendum (PbS) can also be found in nature. Semiconductors are classified in numerous ways. They exist as pure elements such as Si, B and compounds such as gallium arsenide. Semiconductors can also be classified as elemental, binary, and ternary. • Elementary semiconductors: these are single elementary semiconductors belonging to subgroups B III, IV, V and VI of the periodic table. Carbon (diamond), silicon, boron, gray tin, phosphorus, germanium, selenium and tellurium are some of the elementary semiconductors. • Compound semiconductors: have a chemical composition of two or more elements mainly from groups III and V o Binary compound: As the name suggests, binary compounds are made up of two elements. For example SiC, GaAs and CdS. Binary semiconductors could be further divided into IV-VI, III-V, II-VI, and I-VII semiconductors. o Ternary compounds: AlGaAs, CdHgTe are some of the ternary compounds consisting of three elements. You also need to understand the meaning of Intrinsic and Extrinsic. Intrinsic materials have a pure chemical composition without impurities and therefore have lower electrical conductivity. For example, pure Si acts as an insulator since there are no free electrons in the structure of pure Si. Extrinsic crystals are formed when small amounts of impurities are added to the intrinsic crystal structure, so extrinsic crystals have better electrical conductivity than intrinsic crystals. The process of introducing impurities into the intrinsic crystal structure is known as doping and will be explained below....... middle of paper ....... voltage could alter the height of the potential barrier. When the positive terminal of a battery is connected to the n-type and the negative terminal of the battery is connected to the p-type, the electrons in the n-type will be attracted to the positive terminal while the holes in the p-type will be attracted to the negative terminal. Therefore the depletion layer will increase preventing current flow through the pn junctions. This is called reverse bias. A current flow is generated only when the positive terminal of the battery is connected to the P type and the negative terminal of the battery is connected to the P type. The negative terminal will repel electrons in the n type while the positive terminal will repel holes in the p type of As a result the depletion layer will be reduced and this is called forward polarization. Therefore the diodes will only work when they are forward biased.