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Solid is one of the four essential states of matter. Solids have the least amount of kinetic energy and their molecules are packed tightly together. Apart from general physical properties like definite shape and value, it also has another important property called conductivity. Conductivity is described as the measure of a solid to ease the flow of electrons and conduct electricity. Depending on the electrical conductivity solids are classified into three main categories: conductors, semiconductors and insulators.
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Key Terms: solid, electrical properties of solids, conductivity, electrical conductivity, conductor, semiconductor, insulator, chemistry
Electrical Properties of Solids
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It is measured with reference to conductivity. A substance’s electrical conductivity is characterized as its capability to transmit heat energy or electrical energy and in some cases additionally sound energy. Therefore, a good conductor can easily transmit energy without melting, boiling or altering its composition in some way.
Solids do not have uniform electrical properties. Some of them have high conductivity, while some of them do not conduct electricity at all. On the basis of a range of conductivities in between 10-20 to 107 ohm-1m-1, solids can be classified into three types: conductors, insulators and semiconductors.
Electrical Properties of Solids
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| Related Articles | ||
|---|---|---|
| Definition of matter | Amorphous and crystalline solids | Mechanical properties of solids |
| Mechanical properties of fluids | Imperfection in solids | Three states of matter |
Types of Solids
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The following are the types of solids:
Conductors
Conductors are the solids that allow easy and speedy transmission of heat energy and electric current through them and with conductivities ranging from 104 to 107 (Ω m)-1. Conductors allow this transmission of energy to happen by a free flow of electrons from atom to atom. They have the ability to carry this energy throughout them even when the current is applied only to one part of their body.
The Metals are usually good and strong conductors of electricity and this conductivity is due to the presence of mobile electrons. These electrons are not tightly bound and are free to pass. The conductivity of metals is near the order of 107 (m)-1.
Conductors
Insulators
Unlike conductors, Insulators do not conduct electricity even if a large amount of energy is provided to them. They have a significant bandgap that prevents electricity from flowing. These are the solids with low conductivities ranging from 10-20 to 10-10 (Ω m)-1.
Since insulators are weak conductors, it is used to insulate conductors and semiconductors. For example, copper wires are covered with plastic or some type of polymer. They secure wires without electric current passing through them, it’s called wire insulation.
Insulators
Semiconductors
In semiconductors the value of conductivity lies between that of conductors and insulators. Neither do they allow complete electricity to pass through them nor do they resist the flow of electrons entirely. These are the solids with conductivities ranging from 10-6 to 104 (Ω m)-1. Silicon, germanium and selenium are examples of some good semiconductors.
Intrinsic semiconductors: These semiconductors are classified as undoped semiconductors without any impurities. By increasing the temperature of the material, we can create vacancies in the bands of valence. Under the electric field, electrons will now move in one direction and holes in the opposite direction. However, these conductors are not strong enough and have very few applications.
Extrinsic semiconductors: These are the semiconductors with doping. To increase the conductivity of the product, some impurities are added. There are two types of extrinsic semiconductors named p-type and n-type.
Conduction of Electricity in Metals
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A conductor can conduct electricity by the motion of electrons or ions. In metallic conductors, the flow of electric current is possible due to the flow of electrons and in electrolytes, due to the flow of ions. Metals conduct electricity in both solid and molten states. It depends on the number of valence electrons available per atom. The atomic orbital of metal atoms from molecular orbital which are so close in energy to each other to form a band or energy band.
If the band is partially filled or it overlaps with a higher energy occupied conduction band, then electrons can flow easily under the influence of an electric field and the metal shows conductivity.
If the gap between the filled valence band and the next higher conduction band is large, then electrons cannot jump from the valence band to the conduction band. Therefore, no electrons are available for conduction. Such materials have very small conductivity and it behaves as an insulator.
Conduction Of Electricity In Semiconductors
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In semiconductors, the gap between the valence band and conduction band is small. Therefore, electrons can jump to the conduction band showing some conductivity. The electrical conductivity of semiconductors rises with rising temperatures because more electrons can jump to the conduction band.
Semiconductors that are made from only one or pure semiconductor are called intrinsic semiconductors. Silicon and germanium like materials show this type of behavior. Their conductivity is too low to be of practical use. The intrinsic semiconductor could be made applicable by using the process called doping.
Doping
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The process of adding an appropriate amount of suitable impurity in a semiconductor to increase its conductivity is called doping. It can be done with an electron-rich impurity or electron-deficiency as compared to the intrinsic semiconductor.
By adding electron-rich impurities
Silicon and germanium are present in group 14 of the periodic table with four valence electrons each and they can form four covalent bonds with other neighbor atoms.
When silicon (Si) and germanium (Ge) get doped with electron-rich impurities like Phosphorus (P), the four of five electrons present in P forms covalent bonds with neighboring atoms of Si and Ge. One extra electron left becomes delocalized and is responsible for increasing the conductivity of Si and Ge. The semiconductors built from this type of doping are called n-type semiconductors as the conductivity is increased due to the addition of negatively charged species.
By adding electron deficient impurities
Silicon and germanium can also be doped with a group of 13 elements like Boron (B), Aluminum (Al) or Gallium (Ga) which contains only three valence electrons. Three valence electrons present in these elements form covalent bonds with three electrons of Si and Ge leaving one electron delocalized. The place where the fourth valence electron is missing is called electron-hole or vacancy.
Under an electric field, an electron from nearby atoms comes and fills the vacancy but another electron hole is created at the place of movement of the electron. Under the influence of the electric field, electrons move towards the positively charged plate through electronic holes, but it appears as if electron holes are positively charged and are moving towards the negatively charged plate. These types of semiconductors are defined as p-type semiconductors.
Doping in Semiconductor
Application of p-type and n-type of Semiconductors
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- Various combinations of both types of semiconductors are used in making electronic components.
- Diode which is a combination of both types of semiconductors is utilized as a rectifier.
- Transistors that are used in detecting and amplifying sound signals are made by placing one layer of semiconductor in between two layers of the other type of semiconductor.
- A solar cell is a photodiode that is widely used in converting light energy into electrical energy.
Things to Remember
- Electrical conductivity corresponds to resistivity.
- Conductivity is usually represented by the Greek letter σ (sigma) and the SI unit of conductivity is Siemens per meter (S/m).
- In solid-state physics, the energy band which includes the energy levels of the valence electrons is called the valence band.
- In insulators, change of temperature or addition of impurities does not have any effects on conductivity.
- Insulators have an energy gap of more than 3 eV between the conduction band and valence band.
- Semiconductors have a property that one can control their conductivity; therefore, it is mostly found in electronic applications.
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Sample Questions
Ques. The electrical conductivity of semiconductors increases with an increase in temperature. True or false? (1 Mark)
Ans. The given statement is true as in the intrinsic semiconductors, conductivity goes up or resistivity goes down when the temperature rises.
Ques. In terms of which parameter the electrical properties of solids can be measured? (1 Mark)
Ans. Conductivity and resistivity are used to measure the electrical properties of solids as they both are inversely proportional to each other. Conductivity is the ability of a material to conduct electricity and resistivity is the ability of a material to resist electricity.
Ques. Why are solids good conductors of electricity? (1 Mark)
Ans. Metallic solids have delocalised electrons due to their bonding. These free electrons can move around and can conduct electricity.
Ques. What type of solids are electrical conductors, ductile and malleable? (1 Mark)
Ans. Metallic solids are electrical conductors, ductile and malleable in nature.
Ques. How energy bands are formed in solids? How is the band gap important in explaining the electrical properties of solid elements? (2 marks)
Ans. In molecules, two atomic orbitals combine together to form a molecular orbit with two distinct energy levels to form a band called energy bands.
The bandgap is a major factor defining the electrical property of solid. Insulators have large band gaps, semiconductors have smaller band gaps, while conductors either have very small or no band gaps because valence and conduction bands overlap.
Ques. How solids are good conductors of electricity? (3 Marks)
Ans. The particles present in the solid are closely packed in a fixed location. The force between the adjacent particles is also high which makes it more effective for the heat to transfer and pass by during collision.
Solids are known as conductors as they have great electrical conductivity. As their conductivity broadly depends on their number of atoms of valence electrons, heat and electricity can pass through them very easily.
Ques. What does the electrical conductivity of an object depend on and how does an increase in temperature affect it? (3 Marks)
Ans. The electrical conductivity of an object depends on the ability of its electrons to move within the structure of the material. If the electrons or the other charge carriers of an object are able to move freely within the structure, then it is considered a good conductor of electricity.
The conductivity of an object is directly proportional to temperature. As the temperature increases, the ions start to move faster as their kinetic energy increases. Hence, the conductivity of an object increases.
Ques. Explain the conductivity of semiconductors. (2 Marks)
Ans. Semiconductors act like insulators at low temperatures and conductors at high temperatures because the electrons that gather around the atoms of the semiconductor are able to break away from their covalent bonds with a rise in temperature. We can say that although the electrical conductivity of a semiconductor is not nearly as high as the metals, it is also not as low as the insulator.
Ques. In terms of band theory, what is the difference between Conductor and semiconductor? (5 Marks)
Ans.
| Conductor | Semiconductor |
|---|---|
| Allows the flow of current to pass through them. | It allows the flow of current to pass through them but under some conditions. |
| The conductivity of semiconductors is based on the free electrons due to metal bonding. | The conductivity of semiconductors is based on the free electrons and holes. |
| The outer electron in the atom of a conductor can be easily removed by applying the small potential difference. | The removal of the outer electron in the atom of the semiconductor needs more energy or potential difference. |
| It has a positive coefficient of resistance. | It has a negative coefficient of resistance. |
| The energy gap in a conductor is zero or very small. | The energy gap in semiconductors is small. It is more than conductors and less than insulators. |
| Mixing of impurity causes increases in resistance and a decrease in conductivity. | Mixing of impurities causes a decrease in resistance and an increase in conductivity. |
| Ex: silver, copper, etc. | Ex: silicon, germanium, gallium. |
Ques. In the terms of band theory, what is the difference between Conductor and insulator? (5 Marks)
Ans.
| Conductor | Insulator |
|---|---|
| It allows the flow of current to pass through them. | It resists the flow of current passing through them. |
| They have high heat and electricity allowance and very low resistance power. | They have almost no heat and electricity allowance and high resistance power. |
| It has a very weak covalent bond. | It has a strong covalent bond. |
| The conductor has the positive temperature coefficient and the valence band remains empty. | Insulator has a negative temperature coefficient and the valence band remains occupied with electrons. |
| It does not store energy when kept in a magnetic field. | It stores energy when kept in a magnetic field. |
| Conductors are used in the production, application and mechanism of various electrical equipment. | Insulators are used for insulating electrical devices for security and safety purposes. |
| Ex: gold, copper, aluminium etc. | Ex: wood, paper, etc. |
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