Types of Electrical Properties of Solids and Band Theory Explanation
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 in Solid State Chemistry
1. What are the electrical properties of solids?
The electrical properties of solids describe how solids respond to an electric field, mainly in terms of their ability to conduct or resist electric current. These properties depend on the movement of charge carriers such as electrons or holes in the solid.
- Electrical conductivity (σ): Ability to conduct electric current.
- Resistivity (ρ): Opposition to current flow, where ρ = 1/σ.
- Band structure: Arrangement of valence and conduction bands.
- Charge carriers: Free electrons or holes responsible for conduction.
2. What is electrical conductivity in solids?
The electrical conductivity (σ) of a solid is the measure of its ability to allow electric current to pass through it. It depends on the number of free charge carriers and their mobility.
- Unit of conductivity: S m-1 (siemens per metre).
- High in metals due to free electrons.
- Moderate in semiconductors.
- Very low in insulators.
3. What is the difference between conductors, semiconductors, and insulators?
The difference between conductors, semiconductors, and insulators lies in their band gap and electrical conductivity. The band gap determines how easily electrons can move from the valence band to the conduction band.
- Conductors: Overlapping valence and conduction bands; very high conductivity (e.g., Cu, Ag).
- Semiconductors: Small band gap (~1 eV); moderate conductivity (e.g., Si, Ge).
- Insulators: Large band gap (>3 eV); negligible conductivity (e.g., diamond).
4. What is band theory of solids?
The band theory of solids explains electrical conductivity by describing the formation of energy bands from overlapping atomic orbitals in a crystal. In solids, closely spaced atoms cause atomic orbitals to split into continuous energy bands.
- Valence band: Filled with electrons at 0 K.
- Conduction band: Higher energy band where electrons move freely.
- Band gap (Eg): Energy difference between the two bands.
5. What is a semiconductor and how does it conduct electricity?
A semiconductor is a solid with a small band gap that conducts electricity through electrons and holes. At higher temperatures, some electrons gain enough energy to move from the valence band to the conduction band.
- Examples: Si and Ge.
- Conduction occurs via electrons (negative charge carriers).
- Also via holes (positive charge carriers formed when electrons leave the valence band).
6. What are intrinsic and extrinsic semiconductors?
An intrinsic semiconductor is pure, while an extrinsic semiconductor is doped with impurities to increase conductivity. Doping introduces additional charge carriers into the crystal lattice.
- Intrinsic semiconductor: Pure Si or Ge; equal number of electrons and holes.
- Extrinsic semiconductor: Doped semiconductor.
- n-type: Doped with pentavalent atoms (e.g., P in Si); electrons are majority carriers.
- p-type: Doped with trivalent atoms (e.g., B in Si); holes are majority carriers.
7. What is doping in semiconductors?
Doping is the intentional addition of a small amount of impurity to a pure semiconductor to increase its electrical conductivity. The impurity atoms create extra charge carriers in the crystal.
- Pentavalent impurity (e.g., P, As) → forms n-type semiconductor.
- Trivalent impurity (e.g., B, Al) → forms p-type semiconductor.
- Only a small fraction (about 1 in 105–106 atoms) is sufficient.
8. Why does the conductivity of metals decrease with increase in temperature?
The conductivity of metals decreases with increase in temperature because lattice vibrations increase and hinder the movement of free electrons. Higher temperature causes more frequent collisions between electrons and vibrating metal ions.
- Free electrons are the charge carriers in metals.
- Increased vibration reduces electron mobility.
- Resistivity (ρ) increases with temperature.
9. What is the relationship between resistivity and conductivity?
The relationship between resistivity and conductivity is given by σ = 1/ρ, meaning conductivity is the reciprocal of resistivity. If resistivity increases, conductivity decreases, and vice versa.
- Resistivity (ρ) measures opposition to current flow.
- Conductivity (σ) measures ease of current flow.
- Unit of resistivity: Ω m.
- Unit of conductivity: S m-1.
10. What is a p–n junction and why is it important?
A p–n junction is the boundary formed when p-type and n-type semiconductors are joined together, creating a basic semiconductor device. At the junction, electrons and holes recombine to form a depletion region.
- Forms a potential barrier that controls charge flow.
- Allows current in forward bias.
- Blocks current in reverse bias.
