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