Semiconductor: Covalent bond Theory
Keywords:Covalent, recombination, electron, equilibrium
This article is about semiconductor crystal and the concept of hole.
Si and Ge are available in the form of crystalline solids. They have diamond lattice structures, i.e, each atom is surrounded by four equidistant nearest neighbors which lie at the corner of a tetrahedron. Both of them belong to the group lV of the periodic table and have four valance electrons. Each atom forms four covalent bonds with four nearest neighbouring atoms by sharing of valence electrons with opposite spin. Because of this formation of covalent bonds the valence electrons are not available for the conduction of electricity. So, at 0K the no free carrier is available and the crystal behaves as a perfect insulator. However at room temperature a few of the electrons acquire sufficient kinetic energy from thermal agitation and break their covalent bonds and conduction is made possible. The dislodged electrons can wander freely in a random fashion throughout the crystal. The minimum energy required to break such a covalent bond is about 0.72 eV for Ge and 1.1 eV for Si. When an electron escapes from covalent bond an electron vacancy is created in the bond and such and incomplete bond is called a hole. The hole maybe imagined to behave like a positively charged particle and can take part in the conduction of electricity. Under the action of an external electric field an electron from a nearby filled covalent bond, having almost the same energy as the hole, may come and feel the hole. This electron in turn leaves a new hole or vacancy behind. Thus the hole appears to move in a direction opposite to the direction of movement of the electron. Both of them contribute to the current in the same direction. So far as electrical conduction is concerned the hole behaves like a particle having a charge equal and opposite to that an electron.
Fundamental principles of electronics—B. Ghosh.