A conductor has a number of charge carriers, which are ready to take part in conduction after a certain voltage or heat is applied. Non-conductors do not have any free charges to take part in conduction even after applying an external voltage or heat. The materials which exhibit properties of both conductors and non-conductors are called semiconductors. The Semiconductor materials behave as an insulator for a certain voltage level and conduct only after the specific voltage level is applied at the input. The commonly available semiconductor materials are silicon (Si) and Germanium (Ge). The compound semiconductors are prepared by alloying different elements, one of the examples is Gallium Arsenide (GaAs).
There are two types of semiconductors namely intrinsic and extrinsic semiconductors. The classification is based on the type and concentration of carriers that majorly contribute to the flow of current these types of semiconductors.
Silicon and Germanium are tetravalent elements. It means that they have four electrons in its outermost shell. Each atom of Silicon forms a covalent bond with four neighboring Silicon atoms. Thus, with the help of the sharing of atoms the lattice crystal structure of the semiconductor is formed.
Intrinsic Semiconductor :
An intrinsic semiconductor is the purest form of the semiconductor. The doping element or impurity is not added to the intrinsic semiconductor. The electrons are bonded to the parent semiconductor atom, but at a certain voltage or heat is applied, these valence electrons, they leave the parent atom are moved freely in the lattice. Such free electrons constitute current in the intrinsic semiconductor material. The electron from the valence band crosses the forbidden gap to enter into the conduction band. This electron leaves a positive hole or void in its place in the valence band. This void or positive space is termed a ‘hole’.
An extrinsic semiconductor is formed by doping an intrinsic semiconductor. Doping is a process where a very small fraction of impurity atom is added to the intrinsic semiconductor. The extrinsic semiconductors are of two types based on the doping elements used.
Types of Extrinsic Semiconductors are N-type and P-type.
- N-type Semiconductor: The intrinsic semiconductor is doped with a Pentavalent impurity element. Such an impurity element has five electrons in the valence shell. The elements like are Phosphorus (P), Arsenic (As), Antimony (Sb) is the pentavalent impurities used for N-type semiconductor.
- P-type Semiconductor: The intrinsic semiconductor is doped with the Trivalent impurity element. Such an impurity element has five electrons in the valence shell. The elements like Boron (B), Gallium (G), Indium (In), Aluminium (Al) is used for P-type semiconductors.
Key Differences between Intrinsic and Extrinsic Semiconductor
The intrinsic and extrinsic semiconductors can be differentiated on the following parameters:
1) Crystal Lattice Structure:
The schematic of an intrinsic Silicon (Si) semiconductor is shown in Figure 1 which depicts the covalent bonds in neighboring Silicon atoms.
Each Silicon atom shares its four electrons with four neighboring silicon atoms to form covalent bonds.
Figure 3. P-type semiconductor with acceptor impurity
The N-type Extrinsic semiconductor in Figure 2 shows a pentavalent impurity atom of Antimony (Sb) along with the free electron which is freely roaming in the crystal structure and it is ready to conduct. The P-type Extrinsic semiconductor in Figure 3 shows a trivalent impurity atom of Boron (B) along with a void space formed in the covalent bond with a neighboring silicon atom. This hole attracts electrons and participates in conduction.
2) Doping Level:
Intrinsic semiconductor is a pure semiconductor with no doping on the crystal structure. There is an equal number of holes and electrons in an intrinsic material. This is termed as electrical neutrality.
An external semiconductor is a doped intrinsic semiconductor. An N-type semiconductor is doped with a pentavalent impurity and a P-type semiconductor is doped with a Trivalent impurity. The pentavalent impurities are called as Donor impurities as they give an extra electron to the lattice of the semiconductor. The Trivalent impurities are termed as Acceptor impurities as they create a void or positive hole in the crystal structure which can accept an electron.
3) Carrier Concentration:
In intrinsic semiconductors, there is an equal number of holes and electron concentration as no doping is added. Intrinsic conductors have lower conductivity compared to the extrinsic semiconductor.
In an N-type semiconductor, electrons are called the majority carriers as they are more in number and holes are termed as minority carriers. The conduction in an N-type of semiconductor majorly results from the electrons which are majority carriers.
In a P-type semiconductor, holes are called the majority carriers as they are more in number and electrons are termed as minority carriers. In a P-type of semiconductor conduction results primarily because of the holes which are majority carriers.
4) Fermi Level:
For an intrinsic semiconductor the Fermi level is exactly at the mid of the forbidden band.energy band gap for Silicon (Ga) is 1.6V, Germanium (Ge) is 0.66V, Gallium Arsenide (GaAs) 1.424V. In an extrinsic semiconductor
For n-type and p-type extrinsic semiconductors the Fermi levels are a function of doping level density.
In an N-type semiconductor, the Fermi level is near the conduction band as it has more electrons. In a P-type semiconductor, the Fermi level is near valance band as it has more of the holes.
5) Effect of temperature:
In an intrinsic semiconductor at there are no electrons in the conduction band. As the temperature reaches room temperature of the electrons gain energy to cross the valence band and reach the conduction band.
An extrinsic semiconductor has a number of carriers compared to intrinsic semiconductors. Increase in temperature will increase the conductivity of extrinsic semiconductors as more number of carriers are released for conduction.
Although the intrinsic semiconductor is a pure semiconductor it is not used for practical manufacturing as has low conductivity. The number of free charge carriers is less hence it has higher resistance to conduction of charges.
Whereas an extrinsic semiconductor has greater conductivity as it has a number of free charge carriers. Hence external semiconductors are preferred for practical manufacturing of semiconductor components and devices.
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