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Have you ever noticed that some solids can easily conduct electricity but some cannot? Like electric current can be passed through an iron rod but it cannot be passed through a piece of wood. This basically depends on the conductivity, resistivity, and energy bands of the solids. Solids are classified into metals, conductors, and semiconductors on the basis of conductivity, resistivity and energy bands.
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Key Takeaways: Conductors, semiconductors, metals, energy bands, conductivity, resistivity, energy bands, metals
Classification of Solids
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On the basis of resistivity and conductivity and bands of energy, there are three categories in which solids can be classified.
The three categories are
- Metals
- Insulators
- Semiconductors
Classification of Solids
Semiconductor Electronics Class 12 Important Notes PDF
Semiconductor Electronics Class 12 Important Notes
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Classification based on Conductivity and Resistivity
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The solids are broadly divided into three categories based on the relative values of resistivity and conductivity. The relation between resistivity and conductivity can be expressed as-
ρ = 1/σ
Where ρ is the resistivity and σ refers to the conductivity.
On the basis of resistivity and conductivity, metals, insulators and semiconductors are classified as-
- Metals - These are the types of solids that have high conductivity and very low resistivity.
σ ~ 102 – 108 S/m
ρ ~ 10-2 – 10-8 Ωm
- Insulators - These types of solids have very high resistivity and so they have very low conductivity.
σ ~ 10-11 – 10-19 S/m
ρ ~ 1011 – 1019 Ωm
- Semiconductors - These are the types of solids that have values of resistivity and conductivity somewhere between the values of insulators and metals.
σ ~ 105 – 10-6 S/m
ρ ~ 10-5 – 106 Ωm
The semiconductors can be further divided into compound and elemental semiconductors.
- Compound Semiconductors: The semiconductors formed by combining two or more metals together are known as compound semiconductors. They can be further divided into three categories
- Organic such as anthracene, doped phthalocyanine, etc.,
- Inorganic such as CdS, CdSe, GaAs, etc.
- Organic polymers such as polythiophene, polypyrrole, polyaniline, etc.
- Elemental Semiconductors: These are the semiconductors that are naturally available like in Silicon (Si) or Germanium (Ge).
The video below explains this:
Semiconductors and Insulators Detailed Video Explanation:
Classification Based on Energy Bands
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Bohr’s model of the atom expresses that an electron’s energy is determined by the orbit in which the electron revolves. However, in a solid, the atoms are closer together to each other and at times they often overlap with each other. Hence, as compared to an isolated atom, the electrons in a solid have a different motion.
Valence electrons are the electrons that are in the innermost orbits of an atom and conduction electrons are the ones in the outer orbits which don’t completely fill the shell.
Valance Band and conduction Band
So, every electron has a different band of energy as every electron has a varied pattern of charges that surround it and a different position. So there is a valence band and a conduction band. The motion of electrons, when the conduction band’s lowest level is lower than the valence band’s highest level, is such that it shifts from the valence to the conduction band.
On the basis of Band Energy- metals, insulators and semiconductors are classified as-
Classification of metals, insulators and semiconductors on the basis of Band Energy
Metals- In metals, the conduction band and the valence band partly overlap each other and there is no forbidden energy band gap between the conduction band and the valence band. Since there is no forbidden energy bandgap and there are a large number of electrons available for the conduction, that is why electrons can easily move from valence band to conduction band.
Hence, the conductivity of metals is high and their resistivity is very low.
Conduction Band and the Valence Band in Metals
Insulators- In insulators, there is a large energy gap between the valence band and the conduction band. Due to which the electrons which are in the valence band can not move to the conduction band easily.
Since there is no electron for the transfer in the conduction band, the conductivity of the insulators are very low.
Conduction Band, Valence band and Band Gap in Insulators
Semiconductors- In semiconductors, there is a finite but very small energy gap between the valence band and the conduction band. Room temperature is needed for the electrons to acquire energy and jump from the valence band to the conduction band.
Hence, the semiconductors have conductivity at a higher temperature.
Conduction Band, Valence Band and Band gap in Semiconductors
Things to Remember
- On the basis of resistivity and conductivity and bands of energy, there are three categories in which solids can be classified.
- The three categories are Metals, Insulators, Semiconductors.
- The semiconductors can be further divided into compound and elemental semiconductors.
- In insulators, there is a large energy gap between the valence band and the conduction band.
- In metals, the conduction band and the valence band partly overlap each other and there is no forbidden energy band gap between the conduction band and the valence band.
- Bohr’s model of the atom expresses that an electron’s energy is determined by the orbit in which the electron revolves.
Also Read:
Sample Questions
Ques: State the reason, why GaAs are most commonly used in the making of a solar cell. (All India 2008, 1 Mark)
Answer: Gallium arsenide (GaAs) is most commonly used in the making of a solar cell due to its two major properties. These are-
(i) GaAs has high optical absorption (~ 104 cm-1) .
(ii) GaAs have high electrical conductivity.
Ques: Write any two distinguishing features between conductors, semiconductors, and insulators on the basis of energy band diagrams. (All India 2012, 3 Marks)
Answer: Metals (Conductors)- In metals, the conduction band and the valence band partly overlap each other and there is no forbidden energy band gap between the conduction band and the valence band. Since there is no forbidden energy bandgap and there are a large number of electrons available for the conduction, that is why electrons can easily move from valence band to conduction band.
Hence, the conductivity of metals is high and their resistivity is very low.
Semiconductors- In semiconductors, there is a finite but very small energy gap between the valence band and the conduction band. Room temperature is needed for the electrons to acquire energy and jump from the valence band to the conduction band.
Hence, the semiconductors have conductivity at a higher temperature.
Insulators- In insulators, there is a large energy gap between the valence band and the conduction band. Due to which the electrons which are in the valence band can not move to the conduction band easily.
Since there is no electron for the transfer in the conduction band, the conductivity of the insulators are very low.
Ques: Draw the ‘Energy bands’, diagrams for a
(i) pure semiconductor
(ii) insulator(Comptt. All India 2015, 3 Marks)
Answer: Pure Semiconductor- In pure semiconductors, there is a finite but very small energy gap between the valence band and the conduction band. Room temperature is needed for the electrons to acquire energy and jump from the valence band to the conduction band. Hence, the pure semiconductors have conductivity at a higher temperature.
Insulator- In insulators, there is a large energy gap between the valence band and the conduction band. Due to which the electrons which are in the valence band can not move to the conduction band easily.
Ques: How does the energy band, for a pure semiconductor, get affected when this semiconductor is doped with
(a) an acceptor impurity
(b) donor impurity? Hence discuss why the ‘holes’, and the ‘electrons’ respectively, become the ‘majority charge carriers’ in these two cases? Write the two processes involved in the formation of p-n junction. (Comptt. All India 2015, 2 Marks)
Answer: When a semiconductor is doped with an acceptor impurity- The energy band of the semiconductor results in an additional energy level. It gets slightly above the top of the valence band. In this, the electron easily jumps over to the acceptor level and leaves holes behind. These holes become the majority charge carriers.
When a semiconductor is doped with donor impurity- The donor impurity decreases the additional energy level slightly below the bottom of the conduction band. In this, the electrons from the donor level easily jump over to the conduction band. Hence, the electrons become the majority charge carriers.
The two-process involved in the formation of the p-n junction are :
- Diffusion
- Drift
Ques: How can the solids be classified on basis of conductivity? (1 mark)
Answer: On the basis of the values of resistivity and conductivity of a solid, it can be classified into three types which are as such-
- Metals
- Insulators
- Semiconductors
Ques: What are the various types of semiconductors? Explain. (2 marks)
Answer. The semiconductors are classified into two types:
- Compound Semiconductors: They are formed by the mixing of two or more metals together. There are further divided into three types of compound semiconductors which are such as-
- Organic semiconductors such as polyaniline, polypyrrole, etc.
- Organic polymers such as doped phthalocyanine, anthracene, etc.
- Inorganic semiconductors such as CdSe, GaAs, etc.
- Elemental semiconductors: These are the semiconductors that are formed naturally such as Germanium(Ge) and Silicon(Si)
Ques: In which types of solids, there is a very high conductivity, which means low resistivity- (1 mark)
(i) Semiconductors
(ii) Metals
(iii) Insulators
Answer. (ii) In metals, Metals have the conduction band and the valence band partly overlapped with each other and there is no forbidden energy band gap between the conduction band and the valence band. Since there is no forbidden energy bandgap and there are a large number of electrons available for the conduction, that is why electrons can easily move from valence band to conduction band. Hence, the conductivity of metals is high and their resistivity is very low.
Ques: What is a band model? (1 mark)
Answer: The band theory model depicts the behavior of electrons in solids by suggesting the existence of energy bands. It uses a material’s band structure to explain the many physical properties of solids.
Ques: Write the classification of metals, semiconductors, and insulators on the basis of conductivity and resistivity.(3 marks)
Answer: On the basis of resistivity and conductivity, metals, insulators, and semiconductors are classified as-
- Metals - These are the types of solids that have high conductivity and very low resistivity.
σ ~ 102 – 108 S/m
ρ ~ 10-2 – 10-8 Ωm
- Insulators - These types of solids have very high resistivity and so they have very low conductivity.
σ ~ 10-11 – 10-19 S/m
ρ ~ 1011 – 1019 Ωm
- Semiconductors - These are the types of solids that have values of resistivity and conductivity somewhere between the values of insulators and metals.
σ ~ 105 – 10-6 S/m
ρ ~ 10-5 – 106 Ωm
Ques: Give the relationship between resistivity and conductivity. (1 mark)
Answer: The relation between resistivity and conductivity can be expressed as-
ρ = 1/σ
Where ρ is the resistivity and σ refers to the conductivity.
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