Semiconductors are those substances whose electrical conductivity lies in between conductors and insulators. In terms of energy band, the valence band is almost filled and conduction band is almost empty. Further, the energy gap between valence band and conduction band is very small. The semiconductor has filled valence band, empty conduction band, and small energy gap or Forbidden gap between valence and conduction band. Semiconductors virtually behave as an insulator at low temperature. However, even at room temperature, some electron cross over to the conduction band, imparting little conductivity.
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Types of Semiconductor (SC)
A semiconductor (SC) in an extremely pure form is known as an intrinsic semiconductor.
In this case, the holes in the valence band are vacancies created by electrons that have been thermally excited to the conduction band and electron-hole pairs are created. When an electric field is applied across an intrinsic SC, the current conduction takes place by two processes namely; by free electrons and holes. The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bond. Under the influence of electric field, conduction through the SC is by both free electrons and holes. Therefore, the total current inside the SC is the sum of currents due to free electrons and holes. This creates new holes near the positive terminal which again drift towards the negative terminal.
An extrinsic semiconductor is a SC doped by addition of small amount of impurity which is able to change its electrical properties such as conduction, making it suitable for electronic applications like diodes, transistor, etc. This is achieved by adding a small amount of suitable impurity (having 3 or 5 valence electrons) to a SC (having four valence electrons). it is then called impurity or extrinsic semiconductor.
The process of adding impurities to an intrinsic semiconductor is known as doping. The purpose of adding impurity is to increase either the number of free electrons or holes in the SC crystal.
If a pentavalent impurity (having 5 valence electrons) is added to a SC, a large number of electrons are produced in the SC.
If trivalent impurity (having 3 valence electrons) added to the SC, a large number of holes are produced in the SC crystal.
Depending upon the type of impurity added, extrinsic semiconductors are classified into two types i.e.
- P-type semiconductor
- N-type semiconductor
When a small amount of pentavalent impurity is added to a pure SC, it is known as an n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons in the SC crystal. Typical examples of pentavalent impurities are arsenic, antimony, bismuth, phosphorus, etc. Such impurities which produce n-type semiconductor are known as Donor impurities because they donate or provide free electrons to the SC crystal.
Electrons = majority carriers
Holes = minority carriers
When a small quantity of trivalent impurities added to a pure SC it is called P-type semiconductor. The addition of trivalent impurity provides a large number of holes in the SC crystal. Typical examples of trivalent impurities are gallium, indium, Boron, etc. Such impurities which produce P-type semiconductors are known as acceptor impurities because holes created can accept the electrons.
Electrons = minority carriers
Holes = majority carriers
Semiconductor (SC) Materials Types
- Elemental SC materials
- Compound SC materials
- Amorphous SC materials
1. Elemental SC materials
Example: Ge, Si, C, B, Al, Ga, P, As, Sb, Bi, etc.
a). Arsenic (As)
- It is pentavalent SC material.
- It is used as donor N-type semiconductor material.
- When it is alloyed with gallium, then it is used in the fabrication of LED.
b). Selenium (Se)
- It is used in a photovoltaic cell.
2. Compound SC materials
As conductivity and band gap are limited for Elemental SC materials hence their usefulness is limited. So, group III-V, II-VI, IV-IV, IV-VI semiconductors are used to provide better properties.
a). III-V SC material
- They provide a wider range of band gap and extended ampere range of a device.
- The structure is zinc blende and diamond cubic.
- Example: GaAs, AlP, etc.
- large band gap material.
- large electron mobility which helps in high-speed switching.
- direct band gap material
- It is 10 times costlier than Silicon.
- It is 2.5 times faster than a silicon-based device.
- In GaAs crystals, Ga substitute corner and face atoms whereas takes place of 4 inside atoms.
- Satellite amplifier
b). Group II-VI SC material
- The band gap is larger than group III-V SC.
- Examples: CdS, CdSe, CdTe, ZnS, ZnSe, etc.
CdS, CdSe, CdTe can be used as photo conduphotoconductors
IV-IV SC material
- Example: SiC
- The band gap of SiC is 3eV.
- X-SiC can be used for high-temperature devices.
- One drawback is that it is expensive and not easy to manufacture.
d). Group IV-VI SC material
- Examples: PbS, PbSe, PbTe
- In this semiconductor, excess Pb gives rise to N-type semiconductor and less Pb gives rise to a P-type semiconductor.
3. Amorphous SC Material
- The structure is similar to super cooled liquid.
- Atoms up to first nearest neighbours are arranged periodically but the atoms which are away from the first nearest neighbour are found to be arranged randomly.
There are 3 types of Amorphous SC Material
a). Elemental Amorphous SC
Examples: Ge, Si, Se, Te, etc.
b). Covalent Amorphous SC
Examples: Ge, Te, etc.
c). Ionic Amorphous SC
Examples: Al2O3, V2O5, etc.