(i) Intrinsic Semiconductor

INTRINSIC SEMICONDUCTOR

A semiconductor in a extremely pure form is called as intrinsic semiconductors.Examples: Germanium and Silicon.

We know Germanium and Silicon are the two important pure (or) intrinsic semiconductors used in the manufacture of transistor and other semiconductor electronic devices. Both these materials are crystalline in nature.

We know the atomic number of Germanium is 32 and for silicon it is 14.

Electronic configuration of

Ge→ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p²

Si→ 1s² 2s² 2p⁶ 3s² 3p².

In both these cases the number of valence electrons = 4. The other electrons

are bound to the central positive core of the atom.

Let us consider two atoms of Germanium brought closer to each other. Now the positive core of one atom interacts with one of the electrons from the other atom and the two electrons are shared by two atoms. These electrons are called Electron Pairs.

When the attractive force is balanced, by the repulsive force between the two positive cores, a covalent bond is formed.

Explanation

In the case of Germanium or Silicon we have 4 valence electrons. It is the tendency of each germanium atom to have 8 electrons in the outer most shell. To do so, each germanium atom arranges itself between four other germanium atoms as shown in Fig. 2.1.

Here, neighboring atoms shares one valence electron with the central (main atom considered) atom. So, by this sharing, the central atom completes its last orbit by having 8 electrons. In this way, the central atom sets up covalent bonds (or) electron pairs, represented by curved dotted lines connecting the atoms.

Hence all the four valence electrons of the central atom gets tightly bound to the nucleus. Similar such bonding occurs among all the other atoms giving rise to extremely stable structure and hence has low conductivity (in pure form).

A three dimensional view of Ge, crystal lattice shows that each atom is surrounded symmetrically by four other atoms forming a Tetrahedral crystal as shown in Fig. 2.2.

Both Si and Ge are referred to tetravalent atoms: tetrahedral crystal. So, both Ge and Si are called covalent solids.

Note: Similar diagram and Explanations holds good for silicon also.

In these semiconductors the electrons and holes can be created only by thermal agitation. As there are no impurities the number of free electrons must be equal to the number of holes.

At OK the valence band is completely filled and the conduction band is empty. The carrier concentration (i.e.,) electron density (or) hole density increases exponentially with the increase in temperature.

Effect of Temperature on Intrinsic Semiconductors

The electrical conductivity of a semiconductor changes appreciably with temperature variations.

(i) At Absolute Zero: At absolute zero, all the electrons are tightly bound to the nucleus (i.e.,). The inner orbit electrons are bound, whereas the valence electrons are bonded, with covalent bond, with other atoms. Hence there won't be any free electrons and the semiconductor crystal behaves as a perfect insulator.

Hence at absolute zero, no valence electron can reach the conduction band to become free electron. Therefore the valence band is completely filled and the conduction band is empty (Fig. 2.3).

(ii) Above Absolute Zero: When the temperature is raised some of the co-valent bonds break due to thermal energy supplied. Due to the breaking of bonds the electrons are released from the co-valent bonds and become free electrons. Now, if a potential difference is applied across the semiconductor, these free electrons moves with respect to field direction and produces a tiny electric current as shown in Fig. 2.4.

This shows that the resistance of semiconductor decreases with the rise in temperature (i.e.,). It has Negative temperature Co-efficient of Resistance.

From the energy band diagram as shown in Fig. 2.5, each time when a valence electron becomes free and enter into the conduction band, a hole is created in the valence band. Therefore the number of electrons that move to conduction band will be exactly equal to the number of holes created in the valence band.

Hole Current

Similar to the normal current which is due to free electrons another current called the hole current also flows in the semiconductor.

Due to thermal energy if one electron enters conduction band from valence band, one hole is created in the valence band. Due to thermal energy hole-electron pairs are created. [Hole → virtual Charge].

Let us consider a valence electron at 'L' has become free electron due to thermal energy and a hole is created at 'L' as shown in Fig. 2.6.

Since, it is the tendency of semiconductor to form covalent bonds, a hole will attract an electron from the neighbouring atom, say an electron from 'M' fills 'L' position. Hence a hole is created at 'M'. Another valence electron (say at N) fills the hole at 'M' thus creating a hole at N similarly hole, moves from L to M, M to N, N to P, P to Q etc (i.e.,) towards the negative terminal of supply voltage. This constitutes hole current.

Definition: It is noted that hole current is due to the movement of valence electrons from one covalent bond to another bond [not by the free electrons in conduction band].

Thus it is clear that the valence electrons move along the path NML, whereas holes move in the opposite direction. (i.e.,) along the path L→M→N etc as in Fig. 2.7.