Properties of crystalline solids
Properties of crystalline solids
Electrical properties of solids
Solids exhibit a vast range of electrical conductivities, which can be broadly classified into three categories.
These are solids that have high electrical conductivity, typically in the range of 104 to 107 ohm–1m–1. Metals are examples of good conductors, with conductivities of the order of 107 ohm–1m–1.
These are solids that have very low electrical conductivity, typically ranging between 10–20 to 10–10 ohm–1m–1.
These are solids that have intermediate electrical conductivity, typically ranging from 10–6 to 104 ohm–1m–1. Examples of semiconductors include silicon and germanium, which are widely used in electronic devices.
Conduction of electricity in metals
Metals are good conductors of electricity in both solid and molten states. The electrical conductivity of metals is determined by the availability of valence electrons per atom. The atomic orbitals of metal atoms combine to form molecular orbitals that are so close in energy to one another that they form a band. If this band is partially filled or overlaps with a higher energy conduction band that is unoccupied, then electrons can move easily under an applied electric field and the metal exhibits high electrical conductivity.
On the other hand, if the gap between the filled valence band and the next higher unoccupied band (conduction band) is large, then electrons cannot move to the conduction band, resulting in very low conductivity. Such a substance is classified as an insulator.
Conduction of electricity in semi-conductors
In case of semiconductors, the gap between the valence band and conduction band is small. Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called intrinsic semiconductors. The conductivity of these intrinsic semiconductors is too low to be of practical use. Their conductivity is increased by adding an appropriate amount of suitable impurity. This process is called doping. Doping can be done with an impurity which is electron rich or electron deficient as compared to the intrinsic semiconductor silicon or germanium. Such impurities introduce electronic defects in them.
n-type semiconductors are semiconductors that have been doped with impurities that introduce extra electrons into the crystal structure. These impurities are typically from group 15 of the periodic table, such as phosphorus or arsenic, which have one more valence electron than silicon or germanium. When these impurities are added to the semiconductor crystal, they occupy some of the lattice sites and one of their valence electrons becomes delocalized, contributing to the conduction of electricity. These extra electrons increase the conductivity of the doped semiconductor, and hence it is called n-type semiconductor.
P-type semiconductors are a type of semiconductor that is doped with a group 13 element such as boron (B), aluminum (Al), or gallium (Ga), which has one less valence electron than silicon or germanium. This creates a hole in the crystal lattice structure, which can be thought of as a positively charged vacancy for an electron. These holes can conduct electricity by allowing electrons to flow from one hole to another, creating a flow of positive charge. This process is known as hole conduction. The increased concentration of holes in the p-type semiconductor leads to an increase in conductivity. P-type semiconductors are commonly used in electronic devices such as transistors and diodes.
Applications of n-type and p-type semiconductors
N-type and p-type semiconductors have a wide range of applications in modern electronics. Some of the important applications are:
- Diodes: N-type and p-type semiconductors are used to create diodes, which are electrical components that allow current to flow in only one direction. Diodes are used in many electronic devices such as power supplies, radios, and televisions.
- Transistors: Transistors are electronic components that amplify or switch electronic signals. They are made by combining n-type and p-type semiconductors to create different regions called the emitter, base, and collector. Transistors are used in many electronic devices such as computers, televisions, and radios.
- Solar cells: N-type and p-type semiconductors are used in solar cells to convert light energy into electrical energy. Solar cells are used to generate electricity in many applications, including powering homes and businesses.
- Integrated circuits: Integrated circuits are electronic devices that contain many transistors and other components on a single piece of silicon. N-type and p-type semiconductors are used to create these circuits, which are used in many electronic devices such as computers, smartphones, and digital cameras.
- Light emitting diodes (LEDs): LEDs are electronic components that emit light when a current flows through them. They are made by combining n-type and p-type semiconductors to create a region called the p-n junction. LEDs are used in many applications such as lighting, displays, and traffic signals.
Magnetic properties of solids
- Every substance has some magnetic properties associated with it, originating from the magnetic moment of each electron in an atom.
- The magnetic moment of an electron comes from two types of motion: its orbital motion around the nucleus, and its spin around its own axis.
- Magnitude of the magnetic moment is very small and is measured in the unit called Bohr magneton, µB. It is equal to 9.27 × 10–24A m2.
- Substances can be classified into five categories based on their magnetic properties: paramagnetic, diamagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic.
- Paramagnetic substances are weakly attracted by a magnetic field and are magnetized in the same direction as the field. They lose their magnetism in the absence of a magnetic field.
- Diamagnetic substances are weakly repelled by a magnetic field and are magnetized in the opposite direction as the field. They are shown by substances in which all the electrons are paired and there are no unpaired electrons.
- Ferromagnetic substances are strongly attracted by a magnetic field and can be permanently magnetized. They have metal ions grouped into small regions called domains, each acting as a tiny magnet.
- Antiferromagnetic substances have domain structure similar to ferromagnetic substances, but their domains are oppositely oriented and cancel out each other’s magnetic moment.
- Ferrimagnetic substances have domains aligned in parallel and anti-parallel directions in unequal numbers. They are weakly attracted by a magnetic field as compared to ferromagnetic substances.