the third post

Q.4: Define energy band, energy gap, energy level.

Ans: Energy band: In the isolated atomic structure there are discrete energy levels associated with each orbiting electron. As the atoms of a material are brought closer together to form the crystal lattice structure, there is an interaction between atoms that will result in the electrons of a particular orbit of an atom having slightly different energy levels from electron in the same orbit of an adjoining atom. The net result is an expansion of the discrete levels of possible energy states for the valence elections to that of energy band.

Between successive energy band there are forbidden regions where no electron can reside. This forbidden region is called energy gap.

Fig: Discrete levels in isolated atom

Energy level: In the atoms electrons are distributed around the nucleus in different well defined orbits. Each orbit is associated with a certain amount of energy. The energy of an electron in an orbit for away from the nucleus having higher energy, than an electron in an orbit close to the nucleus. A small region near each orbit where electrons reside is called energy level.

Q.5: Brief the characteristic of a semiconductor.

Ans: Characteristic of a semiconductor: Pure semiconductors materials show some properties as listed below:

1. At extremely low temperature, pure semiconductor is insulator.


2. As the temperature rises from absolute zero, an increasing number of valence electrons break covalent bond and become free to conduct electricity.


3. Resistivity of semiconductor materials decrease with increasing temperature.


4. The energy gap between valence and conduction bands of semiconductor materials are in the order of 1ev.


5. Resistivity of a semiconductor material lies between conductor and insulator.

Q.6: Draw the atomic structure of P – type and n – type material. Or, Describe P – type and n – type material with diagram.

Ans: N – type material: If a pure semiconductor material is doped with pentavalent impurity atoms then the resulting semiconductor material is called n – type semiconductor material.

Atomic structure of n – type material where silicon is doped with pentavalent antimony is shown in bellow:

Here, the four covalent bonds are still present. An additional fifth electron, due to the impurity atom which is unassociated with any particular covalent bond. This remaining electron, loosely bound to its parent atom, is relatively free to move with in the newly formed n – type material.

P – type material: If a pure semiconductor material is doped with trivalent impurity atoms then the resulting semiconductor material is called P – type semiconductor material. The P – type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons such as boron, gallium and indium. The effect of one of these elements, boron on a base of silicon is indicated in fig:

Now an insufficient number of electrons to complete the covalent bonds of the newly formed lattice. The resulting vacancy is called a hole and is represented by a small circle or a plus sign, indicating the absence of a negative charge.