Semi-conductors

Semiconductor is a material which is neither a good conductor of electricity nor a good insulator. Simply, Those solid substance whose elecrical conductivity lies between good conductors and insulator are known as Semiconductors. Its conductivity lies midway between a conductor and an insulator. The resistivity of semiconductors varies from 10^-5 to 10^-4 ohm-m as compared to the values ranging from 10^-8 to 10^-6 ohm-m for conductor and 10^7 to 10^8 ohm-m for insulator.Examples of such substances are the crystalline forms of the fourth group of the periodic table. Germanium(Ge) and silicon(Si) are two very typical substances showing this behaviour.The band gap of semi-conductors varies from 0.2 to 2.5 eV which is quite small as compared to that of insulators. Eg: The band gap of diamond (a typical insulator) is 6 eV. The valence band and conduction bands of metals may even overlap.
At absolute zero temperature, a semiconductor behaves as an insulator because all the electrons are filled in its valence band and the conduction band is empty. But when the temperature increases the electrons starts to jump to the conduction band so that conductivity increases and resistivity decreases. Hence, the increase in temperature has negative coefficient of resistance in semi-conductor.

Currents in the Semiconductor:
There are two types of current in semiconductor which are classified as:
Electron current: The electric current which is set up in the semi-conductor due to the movement of free electrons in its conduction band is called electron current.
Hole current: The electric current which is set up in the semi-conductor due to the movement of the holes is called hole current. Its direction is opposite to the electron current.

Types of Semi-conductor:
There are two types of Semi-conductor:

Intrinsic semiconductor: The semiconductors like silicon (Si) and germanium (Ge) which are found in their pure state are called Intrinsic semiconductors or pure semiconducrors. In Intrinsic semiconductor the electric current is set up by thermally generated electrons and holes which is in very less amount and cannot be used significantly.
Now to increase the electrical conductivity of semiconductor other impurity atoms can be mixed with pure semiconductors. The process of mixing of impurity atoms with pure semiconductor atoms is called dopping and the impurity agent which is mixed is called dopping agent.

Extrinsic semiconductor: The semiconductors which are obtained by mixing with other impurity atoms in suitable amount with pure semiconductors are called Extrinsic semiconductors. Generally, one impurity atom is mixed with 8 pure atoms. Now the electrical conductivity of the semiconductor can be highly increased. On the basis of mixing of impurity atoms there are two types of extrinsic semiconductors which are listed below:

N-type extrinsic semiconductor: The extrinsic semiconductor which is obtained by mixing pentavalent impurity atom like As, P and antimony (Sb) then it is called N-type extrinsic semiconductor. It is called N-type because the majority charge carriers of such semi-conductors are free electrons.
Donor atom: In N-type semiconductor the pentavalent impurity atoms which are mixed to the semiconductor atoms donate an electron for the electrical conductivity, so these are called donor atoms. When they donate an electron, they are positively charged.

P-type extrinsic semiconductor: The extrinsic semiconductor which are formed due to the mixing of trivalent impurity atoms like boron (B), aluminium (Al), galium (Ga) etc with pure semi-conductor atoms are called P-type extrinsic semiconductor. Here P stands for positive. It means the majority charge carriers of P-type semiconductors are holes and minurity charge carriers are free electrons. Only the fraction of total current is obtained from free electrons.
Acceptor atom: When trivalent impurity atom is mixed with pure semiconductor atom, one bond is incomplete and creates a hole. This hole has a tendency to attract the free electrons i.e, it accepts the electrons. So, trivalent impurity atoms are called acceptor atoms.

Why Semiconductor is damaged by the strong current?
Ans: When strong current is passed through the semiconductor, it heats up the atoms in the covalent bonds of semiconductor. Now the covalent bonds are broken and the electrical conductivity of semiconductor increases. It means it loses the property of semiconductor and shows the behave of conductor. Hence, the semiconductor is damaged by the strong current.