How Do Solids Conduct Electricity? Key Types & Mechanisms Explained
One of the four essential states of matter is solid (the others being liquid, gas and plasma). In a solid, the molecules are packed tightly together and have the least amount of kinetic energy. Structural stability and resistance to a force applied to the surface characterise a solid. A solid material, unlike a liquid, does not flow to take on the form of its container, nor does it expand like a gas to fill the entire volume available.
There are physical as well as electrical properties of matter. Likewise, solids have definite shape and volume. However, depending on their composition and chemical structure, the electrical properties of solids differ to a large degree. Conductors, semiconductors and insulators are classified into three categories.
Electrical Properties of Solids
Conductivity is referred to as the electrical property of a material. A substance's electrical conductivity is characterised as its capacity to transmit heat energy or electrical energy (and in some cases also sound energy). Thus a good electricity conductor can easily transmit energy without boiling, melting or altering its composition in some way.
Solids have different degrees of conductivity, which means that all solids do not have uniform electrical properties. Currently, based on their electrical conductivity, solids can be classified into three different categories. The following are these three categories:
Conductors
Conductors are solids that have strong electrical conductivity. They allow heat energy and electric currents to transmit with ease and speed through them. Conductors allow this energy transfer to take place through free electron flow from atom to atom. When the current is just applied to one part of their body, they have the power to bring this energy all over themselves.
The strongest conductors are understood to be all metals. Their conductivity is dependent on their atoms' number of valence electrons. Such electrons are not tightly bound together and are free to pass. Metals have electrons like this in their atoms, which is why they conduct heat and electricity so well. Metals allow the electric field to transmit in conductivity ranges from 106-108 ohm-1 through them.
Insulators
Unlike conductors, insulators are materials that do not conduct any electrical energy or currents at all. They do not allow any electric charge (or very little) to pass through them. They have a considerable bandgap that prevents electricity from flowing. Glass, wood, plastic, rubber, etc. are some examples.
Since insulators are very weak conductors, there is another use for them. In order to insulate conductors and semiconductors, we use them. You would have seen copper wires, for instance, covered with plastic or some sort of polymer. Without allowing the electric current to go through them, they secure the wires and cables. This is wire insulation.
Semiconductor
The one between conductors and insulators are semiconductors. These are solids that have the ability, but only under certain conditions, to conduct electricity through them. The ability of semiconductors to conduct energy, heat and impurities is impaired by two such conditions.
Intrinsic Semiconductor: These are pure materials, so they are classified as undoped semiconductors with no impurities added. We add thermal energy to the material here and create vacancies in the bands of valence. This makes it possible for the energy to move through. Yet, these conductors are not very strong and have very few applications Extrinsic Semiconductors: These are semiconductors with doping. To boost the conductivity of the products, we add some impurities. There are two kinds of extrinsic semiconductors: n-type and p-type, respectively. Examples are that through this technique, we increase the conductivity of silicon and germanium.
Semiconductors are the most important material due to its property that one can control the conductivity of semiconductors. Due to this reason, semiconductors are mostly found in electronics applications.
Thermal conductivity is nearly related to the electrical conductivity of a substance. We know that metals are good electrical conductors. For a solid to conduct heat, one molecule or atom movement needs to be easily transferred to its neighbour. This type of transfer is relatively easy because of the non-directional nature of the metallic bond, so metals conduct heat well. In a solid network, on the other hand, where the bonds are stiffer and the angles between the atoms are strictly defined, it is more difficult to transfer them. These solids are expected to have low heat conductivity and are known as heat insulators.
FAQs on Electrical Properties of Solids
1. How are solids classified based on their electrical properties as per the CBSE Class 12 syllabus for 2025-26?
According to the CBSE syllabus, solids are classified into three main types based on their ability to conduct electricity, which is measured by their conductivity (ohm⁻¹m⁻¹):
Conductors: These materials have very high electrical conductivity (10⁴ to 10⁷ ohm⁻¹m⁻¹). They allow electricity to pass through them easily. Examples include metals like copper, silver, and aluminium.
Insulators: These materials have extremely low electrical conductivity (10⁻²⁰ to 10⁻¹⁰ ohm⁻¹m⁻¹). They strongly resist the flow of electricity. Examples include wood, rubber, and diamond.
Semiconductors: These have intermediate conductivity values (10⁻⁶ to 10⁴ ohm⁻¹m⁻¹) that lie between those of conductors and insulators. Examples include silicon (Si) and germanium (Ge).
2. What is the fundamental difference between conductors, insulators, and semiconductors explained by band theory?
The difference is explained by the energy gap between the valence band (filled with electrons) and the conduction band (mostly empty):
In conductors, the valence and conduction bands overlap, creating no energy gap. This allows electrons to move freely and conduct electricity with minimal energy.
In insulators, a large energy gap (forbidden gap) exists between the bands. A significant amount of energy is required for electrons to jump this gap, so they normally cannot conduct electricity.
In semiconductors, there is a small energy gap. At absolute zero, they act as insulators, but at room temperature, some electrons can gain enough thermal energy to jump the gap, allowing for a limited conduction of electricity.
3. Why does the electrical conductivity of a metallic conductor decrease when its temperature is increased?
The electrical conductivity of a metallic conductor decreases with a rise in temperature due to increased resistance. In metals, electricity is conducted by the flow of free electrons. When the temperature increases, the metal's positive ions (kernels) in the crystal lattice start to vibrate more vigorously. These increased vibrations obstruct the path of the flowing electrons, causing more frequent collisions and hindering their movement. This increased opposition to electron flow results in higher resistance and, consequently, lower conductivity.
4. How does the conductivity of a semiconductor change with temperature, and why is it opposite to that of a metal?
Unlike metals, the conductivity of a semiconductor increases with a rise in temperature. This is because semiconductors have a small energy gap. As the temperature increases, more electrons gain sufficient thermal energy to jump from the valence band to the conduction band. This process creates more free electrons in the conduction band and an equal number of holes (electron vacancies) in the valence band. Since both electrons and holes act as charge carriers, the increase in their numbers outweighs the effect of increased scattering, leading to higher overall conductivity.
5. What are n-type and p-type semiconductors? Provide an example for each.
N-type and p-type are extrinsic semiconductors created by a process called doping, which intentionally adds impurities to a pure semiconductor like silicon.
n-type semiconductor: Created by doping with an element that has more valence electrons (e.g., adding pentavalent Phosphorus to tetravalent Silicon). The fifth electron is free to move, making electrons the majority charge carriers. The 'n' stands for negative.
p-type semiconductor: Created by doping with an element that has fewer valence electrons (e.g., adding trivalent Boron to Silicon). This creates an electron deficit, or a 'hole', which acts as a positive charge carrier. Here, holes are the majority charge carriers, and 'p' stands for positive.
6. What is the practical importance of doping a semiconductor?
Doping is critically important because it allows us to precisely control the electrical conductivity of a semiconductor. An intrinsic (pure) semiconductor has very low conductivity at room temperature, limiting its use. By introducing a small, controlled amount of dopant atoms, we can dramatically increase the number of charge carriers (either electrons or holes). This controlled enhancement of conductivity is the fundamental principle behind the functioning of most modern electronic devices, including diodes, transistors, and integrated circuits (ICs).
7. Can an insulator ever conduct electricity? Explain the conditions.
Yes, an insulator can be forced to conduct electricity under extreme conditions, though it doesn't behave like a typical conductor. This can happen through a process called dielectric breakdown. If an extremely high electric field (voltage) is applied across the insulator, it can be strong enough to pull electrons from the valence band directly into the conduction band, overcoming the large energy gap. When this occurs, the material starts to conduct a large current, but this process often causes permanent physical damage or structural changes to the insulator.
8. Compare intrinsic and extrinsic semiconductors based on their composition and conductivity.
Intrinsic and extrinsic semiconductors differ in their purity and charge carrier concentration:
Composition: An intrinsic semiconductor is in its purest form, composed of only one element (e.g., pure Silicon or Germanium). An extrinsic semiconductor is an intrinsic one that has been deliberately doped with a specific impurity.
Conductivity: The conductivity of an intrinsic semiconductor is very low and depends solely on temperature. In contrast, an extrinsic semiconductor has much higher conductivity because the dopant atoms provide a large number of majority charge carriers (electrons in n-type, holes in p-type), making its conductivity less dependent on temperature at normal operating ranges.
