Semiconductors are those materials which have electrical properties lies between those of insulators and conductors. According to energy band theory a semiconductor has following properties.
A partially fielld conduction band.
A partially filled valance band.
A very narrow forbidden energy gap.
A pure semi conductor is an insulator at 0 k. At this temperature, it has no electron in the conduction band. But with the increase in temperature the electron jumps from valance band to the conduction band.
Thus a vacancy of electron is produced in valance band which is called hole and behave like a positive charge. So at room temperature, Ge and silicon(Si) crystal becomes the semi conductor.
Intrinsic Semiconductor :
A semi conductor in its pure form is called intrinsic semiconductor. The electrical behaviour of the semiconductor purely depends upon the purity of the semiconductor. Electrical behaviour of intrinsic semiconductor will change by adding small amount of impurity, that process is called doping.
These semiconductors belong to 4th group of periodic table. Therefore they have four electrons in their valance shell. Each atom in the semiconductor material is regular arranged and forms four covalent bonds with its neighbours. This effectively allocates eight electrons to the outermost orbit of each atom which is a stable state. Due to these covalent bonds the electrons are bound in their respective shells.
Extrinsic semiconductor :
The impure semiconductor are called extrinsic semiconductor.
There are two types of extrinsic semiconductors.
- N-type semiconductor
- P-type semiconductor
N-type Materials :
The semiconductor which is formed by adding some pentavalent impurity to pure semiconductor is called N type semiconductor.
When a silicon material is doped with a pentavalent element for example arsenic, antimony or phosphorus etc. , four valance electrons of impurity atom make four covalent bonds with the four neighbours silicon atoms, while the fifth valance electron is called a free electron remains un-bonded.
N type semiconductor refers to this kind of extrinsic semiconductor. The impurity atom is called the donor atom because it donates a free electron. This electron is thermally excited into the conduction band.
P type semiconductor :
The semi conductor which is formed by adding some trivalent impurity to pure a semiconductor is called P type semiconductor.
When a silicon crystal is dropped with a trivalent impurity e.g. aluminium, indium,gallium etc. Three valance electrons of impurity atom from three covalent bonds with the silicon atoms, there is missing electron to form the fourth covalent bond with the fourth neighbour Silicon atom.
In other words there is a vacancy in that bond. This vacancy is called the hole and its considered to have the positive charge. This vacancy accommodate and electron. Such a semi conductor is called P type semiconductor. The impurity atom in this case is called the acceptile impurity because it can accept an electron from a nearby silicon atoms by creating the valance bond.
Electrical Conduction by Electrons and holes:
Consider a Semiconductor crystal lattice of Ge or Si, as shown in figure, The cycle shows the positive ions of semi conductor material and dots are the valance electrons. Covalent bonds hold these electrons together.
So they contain thermal Kinetic motion at room temperature. The covalent bond cannot hold some electrons together because their bond is so strong. So the covalent bond will break, so an elector-hole pair is created, which moves in the semiconductor crystal as explained below;
Conduction in Semi Conductors:
Consider a row of silicon Si atom in a crystal lattice. Whole is present in the valence shell of atom A. Whole is deficiency of electron show it has positive charge. This attracts and electron from neighbouring atom say B. Thurs the electron moves from B to A and the hole shift to B. Now donate don is attracted from C to be and whole is created at C and positive charge at C. This process repeated between the atoms C and D with the result that electron moves from D to see and the holes appear at D. So we can say whole move from one atom to other with the electron moving in opposite direction. The holes are mobile in this case and a moving all appears to be moving like to be positive charges.
