What is Doping ?
The conductivity of intrinsic semiconductor is not so good to use them for making electronic devices. To increase the conductivity of intrinsic semiconductor, some impurities are added. This process of adding impurities of intrinsic semiconductor is called doping. The doping is of two types:
- Accepter Doping
- Donor Doping
Accepter Doping (P-type Semiconductor):
If the trivalent element from the III group such as aluminum (Al), boron (B), and gallium (Ga) is added to pure Si as impurity, then the Al atom as impurity, the Al atom occupies the position of the central Si atom in the tetrahedral structure, then one of the bond will, miss an electron. And there will be a vacancy (hole). The numbers of holes is equal to the numbers of impurity atoms added. So plenty of free charge carries (holes) are available for conduction. As the hole is equivalent to positive charge, so it is called p-type material. The trivalent impurities attract electrons, so they are called acceptor impurities. After doping semiconductor is called extrinsic semiconductor.
Majority And Minority Carriers in p-type:
In p-type material, there is plenty of holes for conduction. So the holes are the majority carriers in p-type. There is a few numbers of electrons are present, produced by electron-hole pair production. These electrons are minority carriers in p-type. This type of doping is called accepter Doping.
Donor Doping (N-type Semiconductor):
The elements of V groups are pentavalent such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb). These have five electrons in their valence band. When one of these materials is added to semiconductor, four of its valence electrons form four covalent bonds and the fifth electron becomes free. The numbers of free electrons is equal to the numbers of impurity atoms added. So plenty of free charge carriers (electrons) are available for conduction. As the electrons have negative charge, so it is called n-type material. The pentavalent impurities give electrons, so it is referred to as donor impurities. After doping the semiconductor is called extrinsic semiconductor.
Majority And Minority Carriers in n-type:
In n-type material, there is plenty of free electrons for conduction. So the electrons are the majority carriers in n-type. There is a few numbers of oles also present, produced by electron-hole pair production. These holes are minority carriers in p-type.
The Pn Junction:
If a piece of intrinsic silicon is doped so that half is p-type and half is n-type, a pn junction forms between the two regions. The p-region has majority carriers holes and few minority carriers electrons. The n-region has majority carriers electrons and few minority carriers holes.
Formation of Depletion Region:
At the instant of the pn junction formation, the electrons near the junction in the n-region cross the junction and combine with holes in p-region. The atoms in the n-region lose electrons and becomes positive ions near the junction. The atoms in the p-region accept electrons and loses the holes as the electrons an d holes combine. This create a layer of negative ions near the junction. These two layers of positive and negative ions near the junction form the depletion region. This means that the free charge carriers are depleted form this area. Further diffusion of electrons from n-region into p-region forms another layer of ions on both sides of junction. The negative ions in the p-region prevent further diffusion of electrons.
The atoms in the p-region and n-region with free charge carriers are electrically neutral. The ions across the junction produced an electric field due to which a potential difference is created called potential barrier as its stops diffusion of majority carriers.
If an electron has to move across the junction, it must be supplied energy equal to the height of barrier. The typically potential barrier is approximately 0.7 volts for silicon and 0.3 volts for germanium.
Energy Diagrams of PN junction and Depletion Region:
Due to differences in the atomic characteristics of the pentavalent and trivalent impurity atoms, the valence and conduction bands in an n-type material are at slightly lower energy levels than the valence and conduction bands in a p-type material.
The free electrons from the n-region can easily diffuse across the junction into conduction band of p-region. They then lose energy and fall into holes in p-region valence band. As the diffusion continuous, the depletion regions begins to form and the energy levels of n-region decrease. Thus is due to the diffusion of higher-energy electrons to the p-region. When the overlapping to bands is misaligned, the diffusion stopped and junction is at equilibrium.