1.) INTRINSIC SEMICONDUCTOR
A Semiconductor in its extremely pure form is said to be an Intrinsic semiconductor. The properties of this pure semiconductor are as follows −
- The electrons and holes are solely created by thermal excitation.
- The number of free electrons is equal to the number of holes.
- The conduction capability is small at room temperature.
In order to increase the conduction capability of intrinsic semiconductor, it is better to add some impurities. This process of adding impurities is called as Doping. Now, this doped intrinsic semiconductor is called as an Extrinsic Semiconductor.
Doping
The process of adding impurities to the semiconductor materials is termed as doping. The impurities added, are generally pentavalent and trivalent impurities.
Pentavalent Impurities
·The pentavalent impurities are the ones which has five valence electrons in the outer most orbit. Example: Bismuth, Antimony, Arsenic, Phosphorus
·The pentavalent atom is called as a donor atom because it donates one electron to the conduction band of pure semiconductor atom.
Trivalent Impurities
·The trivalent impurities are the ones which has three valence electrons in the outer most orbit. Example: Gallium, Indium, Aluminum, Boron
·The trivalent atom is called as an acceptor atom because it accepts one electron from the semiconductor atom.
2.) EXTRINSIC SEMICONDUCTOR
An Impure semiconductor, which is formed by doping a pure semiconductor is called as an Extrinsic semiconductor. There are two types of extrinsic semiconductors depending upon the type of impurity added. They are N-type extrinsic semiconductor and P-Type extrinsic semiconductor.
N-Type Semiconductor:
A small amount of pentavalent impurity is added to a pure semiconductor to result in N-type extrinsic semiconductor. The added impurity has 5 valence electrons.
For example, if Arsenic atom is added to the germanium atom, four of the valence electrons get attached with the Ge atoms while one electron remains as a free electron. This is as shown in the following figure.
·In N-type extrinsic semiconductor, as the conduction takes place through electrons, the electrons are majority carriers and the holes are minority carriers.
·As there is no addition of positive or negative charges, the electrons are electrically neutral.
·When an electric field is applied to an N-type semiconductor, to which a pentavalent impurity is added, the free electrons travel towards positive electrode. This is called as negative or N-type conductivity.
P-Type Semiconductor:
A small amount of trivalent impurity is added to a pure semiconductor to result in P-type extrinsic semiconductor. The added impurity has 3 valence electrons. For example, if Boron atom is added to the germanium atom, three of the valence electrons get attached with the Ge atoms, to form three covalent bonds. But, one more electron in germanium remains without forming any bond. As there is no electron in boron remaining to form a covalent bond, the space is treated as a hole. This is as shown in the following figure.
The boron impurity when added in a small amount, provides a number of holes which helps in the conduction. All of these holes constitute hole current.
·In P-type extrinsic semiconductor, as the conduction takes place through holes, the holes are majority carriers while the electrons are minority carriers.
·The impurity added here provides holes which are called as acceptors, because they accept electrons from the germanium atoms.
·As the number of mobile holes remains equal to the number of acceptors, the Ptype semiconductor remains electrically neutral.
·When an electric field is applied to a P-type semiconductor, to which a trivalent impurity is added, the holes travel towards negative electrode, but with a slow pace than electrons. This is called as P-type conductivity.
·In this P-type conductivity, the valence electrons move from one covalent bond to another, unlike N-type.
