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| Intrinsic Semiconductors |
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| The atoms in a crystal are strongly held by covalent bonds in space at tetrahedral angles. On receiving energy, a covalent bond breaks and an electron is free to move in crystal lattice. This electron leaves an empty space (shown as an open circle) called a hole. |
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| An electron from a neighbouring atom can break and an electron can fill this hole, thereby creating a hole elsewhere. This indicates holes and electrons travel in opposite direction to applied electric field and so, there are two streams of current inside a semiconductor i.e., electron current and hole current i.e., I = Ie + In. |
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| In the above example, there are no impurities in the silicon crystal and their conductivity is solely governed by number of electrons excited from valency band to conduction band. Such semiconductors are called intrinsic semiconductors. |
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| Also in an intrinsic semiconductor ne = nn = ni, where ne, nn are the number densities of electrons, holes and ni is that of intrinsic carriers i.e., (electrons or holes). |
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| The deliberate addition of a desirable impurity atom to a pure semiconductor to modify its properties in a controlled manner is doping. |
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| The impurity atoms added are called dopants. It is worth to note that the electrical conductivity of semiconductors can be increased to a great extent. A doped semiconductor is called an extrinsic semiconductor. Depending on the impurity atom, semiconductors are of two types: |
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| 1. P type and |
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| 2. N type semiconductors. |
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