Solar PV Theory.pdf

Doping however the behavior of silicon can be changed

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Doping However, the behavior of silicon can be changed to turn it into a conductor by mixing a small amount of impurity into the silicon crystal. This process is named doping. There are two types of impurities: N-type and P-type. In the N-type doping, phosphorus or arsenic is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they are out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it is free to move around. It takes only a very small quantity of impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, hence the name N-type. See Figure 11. Figure 11. Silicon doped with N-type impurities. In the P-type doping, boron or gallium is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor. See Figure 12. Phosphorus or arsenic atom
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The Diode Discussion Solar Power 7 Figure 12. Silicon doped with P-type impurities. The P-N junction A semiconductor crystal manufactured with P-type material at one end and N-type material at the other end has unique properties. The P-type material has a majority of positive charge carriers, holes, which are free to move about the crystal lattice. The N-type material has a majority of mobile negative carriers, electrons. Near the junction, the N-type material electrons diffuse across the junction, combining with holes in P-type material. The region of the P-type material near the junction takes on a net negative charge because of the electrons attracted. Since electrons departed the N-type region, it takes on a localized positive charge. The thin layer of the crystal lattice between these charges has been depleted of majority carriers, thus, it is known as the depletion region. It becomes nonconductive intrinsic semiconductor material. In effect, we have nearly an insulator separating the conductive P- and N-doped regions. See Figure 13. Figure 13. Semiconductor built with P-type material at one end and N-type at the other end (P-N junction). The separation of charges at the P-N junction constitutes a potential barrier. This potential barrier must be overcome by an external voltage source to make the junction conduct. The formation of the junction and potential barrier happens during the manufacturing process. The magnitude of the potential barrier is a function of the materials used in manufacturing. Silicon P-N junctions have a higher potential barrier than germanium P-N junctions.
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