Ionization and Energy-Band of Semiconductor Materials

SEMICONDUCTOR MATERIALS

The semiconductor materials such as Ge and Si have four electrons in their valence sheil that is outer most shell. The fig.3.6 shows atomic structure of the semiconductor materials germanium and silicon.

The germanium has a nucles with 32 protons. The electrons are distributed as 2 electrons in the first orbit, 8 in the second orbit and 18 in the third orbit. The remaining four electrons are in the outer orbit (or) valence orbit.

The silicon has nucles with 14 proton. In this atomic structure of the silicon also 4 electrons present in the outermost orbit (or) valence orbit. It is shown in fig 3.6 

Ionization:

If an electron is extracted from the outermost shell of an atom then the overall negative charge of that atom decreases as it looses negative charge in the form of an electron. But protons have same positive charge so atom as a looses its electrical neutral nature and becomes positively charged. Such an atom is called positive ion. Similarly by any means if an electrically neutral atom gains an additional electron then it becomes negatively charged and called negative ion. Thus by loosing (or) gaining an electron, which converts electrically neutral atom to a charged ion is called ionization.

Energy-Band

In every shell is associated with an energy level. An electron orbiting very close to the nucleus in the first shell is very much tightly bound to the nucleus and possesses only a small amount of energy.

Hence first shell has lowest energy level. Greater the distance of an electron from the nucleus, the greater is its energy. Hence the energy level of outer most shell has highest energy. Due to such high energy, the valence electrons in the outermost shell can be easily extracted out and hence such electrons take part in chemical reactions and in bonding the atoms together.

The valence electrons are shared by forming a bond with the valence electrons of an adjacent atom. Such bonds are called covalent bonds. The valence electrons possess highest energy level. When such electrons from the covalent bonds, due to the coupling between the valence electrons, the energy levels associated with the valence electrons merge into each other. This merging forms an energy band.

Similarly the energy level of various electrons present in the first orbit, second orbit etc, also merge to form the various energy bands. There are three energy bands are most important to understand the behaviour of solids. These bans are

(i) Valence band

(ii) Conduction band

(iii) Forbidden band

(i) Valence band:

The energy band formed due to merging of energy levels associated with the valence electrons. That is electrons in the last shell, is called valence band.

(ii) Conduction band:

The energy band formed due to merging of energy levels associated with the free electrons is called conduction band.

(iii) Forbidden band:

The energy gap which is present separating the conduction band and valence band is called forbidden band.

Under normal condition, the conduction band is empty and once energy is imparted, the valence electrons jump from valence band to conduction band and becomes free. While jumping from valence band to conduction band, the electrons have to cross an energy gap.

The energy imparted to the electrons must be greater than the energy associated with the forbidden gap, to extract the electrons form valence band and transfer them to conduction band. The energy associated to forbidden band is denoted as 'EG'. The graphical representation of the energy bands in a solid is called energy band diagram. The fig 3.7 shows the energy band diagram for a silicon.

The electrons in the various orbits revolving around the nucleus occupy the various bands including fully or party occupied valence band. The conduction band which is normally empty carries the electrons which get drifted from the valence band.

For any type of material the forbidden energy gap may be large, small (or) nonexistent. The classification of materials as insulators, conductors and semiconductors is mainly dependent on the width of the forbidden energy gap. 3.3