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Explain n-type and p-type semiconductors. |
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Answer» Solution : (i) n-type semiconductors : This type of semi conductors are obtained by addition of electron rich impurities in silicon or germanium. Silicon and germanium belong to group-14 and have four valance electrons in each. In their crystals each atom forms four covalent bonds with its neighbours.  When doped with a group-15 elements like P or As, which contains five valance electrons, they occupy some of the lattice sites in silicon and germanium crystal. Four out of five electrons are used in formation of four covalent bonds with four neighbouring silicon atoms. The fifth electron is extra and is delocalized. The delocalized electrons increase the conductivities of doped silicon or germanium and the increase in conductivity is due to negatively charged electron. Hence silicon doped with .TRON rich impurity is called n-type of semiconductor. (ii) p-type semiconductors: This type of conductors are obtained by addition of electron deficient impurities in silicon and germanium. When silicon or germanium is doped with a group-13 element such as B, Al or Ga which contains only three VALENCE electrons, the electron hole is formed where fourth electron is missing. This is also known as electron vacancy or hole.  An electron from neighbouring atom can come and fill the electron hole leaving behind electron hole at original position. If it happens, it would appear as if the electron hole has moved in the direction opposite to that of the electron that FILLED it. In presence of electric field, electrons would move towards the positively charged plate through electronic HOLES, but it would appear as if electron holes are positively charged and are MOVING towards negatively charged plate. This type of semiconductors are called p-type semiconductors. |
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