CRYSTAL STRUCTURE OF INTRINSIC SEMICONDUCTOR
- An outermost shell of an atom is capable of holding eight electrons. It is said to be completely filled and stable, if it contains eight electrons.
- But the outermost shell of intrinsic semiconductor like silicon has only four electrons.
- Each of these four electrons forms a bond with another valence electron of the neighboring atoms. This is nothing but sharing of electrons. Such bonds are called covalent bonds.
- The atoms align themselves to form a three dimensional uniform pattern called a crystal.
- The crystal structure of germanium and silicon materials consists of repetitive occurrence in three dimensions of a unit cell. This unit cell is in the form of a tetrahedron with an atom at each vertex.
- The covalent bonds are represented by a pair of dotted lines encircling the two electrons forming the covalent bond.
- Both the electrons are shared by the two atoms. Hence the outermost shell of all the atoms is completely filled, and the valence electrons are tightly bound to the parent atoms.
- No free electrons are available at absolute zero temperature. Hence such an intrinsic semiconductor behaves as a perfect insulator at absolute zero temperature.
- At room temperature, the number of valence electrons absorbs the thermal energy, due to which they break the covalent bond and drift to the conduction band. Such electrons become free to move in the crystal
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