Semi-Conductors

#Electronics Basically, conductors are materials that are full of free charges that are capable of moving around. When an electric field is applied to a conductor, a current is formed and the free charges move along (or in the opposite direction of) the field. Similarly, insulators are materials that do not have (or have very few) free charges. No current is induced in an insulator when an electric field is applied to it. Semi-conductors are what lies in between. They are materials whose number of free charges we can change using a procedure called doping. In this procedure, the semi-conductor is heated and impurities are inserted into the material. We can control the level of conductivity of the material by changing the heat and the doping of impurities. The most commonly used semi-conductor nowadays is Silicon. Imagine single-crystal Silicon atoms sitting in a pattern in order. Each Silicon atom is connected to four neighboring atoms. Therefore, in its natural form, Silicon does not have free charges and has a very high resistance. Now assume we were to add Phosphor to this pattern using the doping procedure described above. Some Phosphor atoms replace the Silicon atoms in the structure, sharing four electrons with their neighbors in covalent connections. As a result, they are left with an extra electron, which is separated from the Phosphor atom and can act as a free charge. The Phosphor atom is itself now positive, because it has lost an electron. This is exactly what we call N-type semi-conductor. N-type semi-conductors are a mixture of a bunch of Silicon atoms (which we don’t really care about), a bunch of fixed-in-place positive charges (the Phosphor atoms), and some free negative charges (electrons). We could do the same procedure with Boron as our impurity. The Boron atoms replace some of the Silicon atoms in the orderly array. They need to borrow an electron to be able to complete the covalent connection. In this way, the Boron atoms themselves become negatively charged and some holes are created which we show symbolically with positive charges. In summary, this structure, known as a P-type semi-conductor contains some Silicon atoms (which we don’t care about), some fixed-in-place negative charges (the Boron atoms), and some free positive charges (holes).

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