is due to the electrons in the conduction band and holes in the valence band. The corresponding currents are represented as I e and I h respectively. E V Free electron Hole E g E C (b) Conduction Band Valence Band Figure . (a) The presence of free electron, hole and broken covalent bond in the intrinsic silicon crystal (b) Presence of electrons in the conduction band and holes in the valence band at room temperature presence of any other foreign atom in the crystal lattice.
The silicon lattice is shown in Figure . (a). Each silicon atom has four electrons in the outermost orbit and is covalently bonded with four neighbouring atoms to form the lattice. The band diagram for this case is shown in Figure .
(b). A small increase in temperature is sufficient enough to break some of the covalent bonds and release the electrons free from the lattice ( . (a)). As a result, some states in the valence band become empty and the same number of states in the conduction band will be occupied by (b) E V E C E g Conduction Band Valence Band Figure .
(a) Two dimensional crystal lattice of silicon (b) Valence band and conduction band of intrinsic semiconductor Covalent bond Valence electron Si Si Si Si Si Si Si Si Si (a) Si Si Si Free electron Broken covalent bond Hole Si Si Si Si Si Si (a) - - - - - - - - Unit electronics and Communication i) n -type semiconductor ii) p -type semiconductor i) n -type semiconductor A n -type semiconductor is obtained by doping a pure silicon (or germanium) crystal with pentavalent impurity atoms (from V group of periodic table) such as phosphorus, arsenic and antimony as shown in Figure . (a). The dopant has five valence electrons while the silicon atom has four valence electrons. During the process of doping, a few of the silicon atoms are replaced by pentavalent Definition of a hole: When an electron is excited, covalent bond is broken.
Now octet rule will not be satisfied. Thus each excited