CONDUCTORS, INSULATORS AND INTRINSIC SEMICONDUCTOR

The elements in the first three groups of the periodic table like alkali metals, noble metals, Aluminum, etc. are good conductors due to the presence of free electrons. Non-metals are bad conductors of electricity due to lack of free electrons. The elements in the fourth group of the periodic table like Si and Ge have greater resistance than good conductors but less than bad conductors. They are known as semiconductors. They behave as bad conductors at absolute zero temperature in their pure form.

The resistivity of the good conductors increases with temperature, while the resistivity of the semiconductors decreases on increasing the temperature unto a certain limit. The conductivity of the semiconductors is changed by making radiation of suitable frequency incident on them.

Two very important semiconductors Ge and Si are discussed here. Both have diamond crystal structure. If an atom of Si is considered at the center of the tetrahedron, then its four nearest neighbours are at the vertices's of a tetrahedron as shown in the figure. Diamond crystalline structure is obtained on extending this arrangement in a three dimensional space.

The electronic arrangement of Si is 1s² 2s² 2p⁶ 3s² 3p². The electrons in 1s² 2s² 2p⁶ completely occupy the K and L shells. 3s² 3p² electrons are the valence electrons. These 2 s orbitals and 2 p orbitals combine to form 4 sp³ complex orbitals. These orbitals combine with similar such orbital’s of the neighbouring atoms and form covalent bonds. Thus, each of the four valence electrons of the silicon forms a covalent bond with its four neighbouring atoms as shown in the figure.

At absolute zero temperature, Si and Ge behave as insulators as the valence electrons are bound in covalent bonds. At room temperature, these bonds break due to thermal oscillations of atoms freeing the electrons which increase conductivity. Deficiency of electron in a bond produces a vacant space which is known as a hole. The hole has the ability of attracting electrons and the randomly moving free electron can get trapped in a hole. Thus hole behaves as a positive charge though it is neither a real particle nor has any positive charge.

On applying p.d. between two ends of a crystal as shown in the figure, electric current gets set up. Now, thermal oscillations and external electric field cause covalent bonds to break and the free electrons produced get trapped in the holes during their motion. Simultaneously, new holes are produced by electrons breaking free from the covalent bonds. The free electrons move towards the positive end and the holes to the negative end. The motion of holes towards the negative end is equivalent to the motion of bound electrons towards the positive end. Thus current in a semiconductor is due to ( i ) motion of free electrons and ( ii ) motion of bound electrons. Both these currents are in the same direction.

The number density of free electrons n_e and holes n_h in a pure semiconductor are equal. Pure semiconductor is called intrinsic semiconductor. Hence electrons and holes are called intrinsic charge carriers and their number density is indicated by n_i. n_e = n_h = n_i.