Electronics is the branch of physics and engineering that deals with the behavior and control of electrons to process information or control systems. The technology is based on circuits made of components that manipulate electrical signals, and it is the foundation of almost all modern devices, from consumer gadgets to industrial machinery.
Development of electronics has resulted to manufacture of appliances such as television sets, computer motherboards, radio-receivers, hi-fi systems, smart watches, etc. modern electronics devices are based on understanding properties of conducting materials.
Understanding of electricity and conductivity of various materials has enabled us develop electronic components such as diodes and transistors. These are very useful in controlling of electric currents.
Materials used to construct electronic components maybe classified as conductors, insulators and semi conductors. The differences in electrical properties among these materials depends on the force that holds the outermost electrons to the atoms of the material.
Conductors
This are materials with low electrical resistance. They carries electrical charges in them from one point to another. Their conductivity is facilitated by their internal structure.
The outermost electrons of the atoms in a conductor are loosely held such that they becomes detached to move freely through the material. The movement of these electrons facilitates conduction of current.
Resistance of current in metal is as a result of collisions between the freely moving electrons and the vibration of atoms. Increase of temperature increases the speed of vibrating atoms. The increase vibration increases the frequency of vibration. This increases the resistance of conductors hence resistance in metal increases with increase in their temperature. examples of conductors includes iron, copper, aluminium, lead, brass etc.
Insulators
They are materials with very high electrical resistance. Their outmost electrons are held tightly to their atoms and so they do not have free electrons. Insulators do not conduct electric current nor heat as they do not have free electrons to do so. However, insulators are very useful as they help in handling of materials that are carrying current or at high temperatures. Examples of insulators includes rubber, plastics , ceramics and wood.
semiconductors
They are the most useful as far as the electronics is concerned. These are materials with conductivity that is between that of conductors and that of insulators. Semiconductors allows the flow of electric current or heat under certain circumstances only. pure semiconductors have four electrons in their atoms outermost shell. They electrons are tightly held to the atom but the force that hold them is less compared to that in the insulators. However, the force is stronger than that of conductors.
At room temperature, the random atomic vibrations associated with the heat energy gives a small fraction of these electrons sufficient energy to escape from their bond and become free electrons. This causes them to be able to conduct electric current.
The escape of electrons from the structural bond leaves a gap where it was occupying.
The gap left by the escaped electron is known as the hole. Holes can hop from one atom to the other and responds to electrical voltage just like the electrons. However, holes carries positive charge while electrons carries negative charge. The figure below illustrates the movement of a hole during electrical conductivity of a semi-conductor.
Holes are the bonds between atoms where an electron has left the atom. Holes hop from atom to atom as shown:
As the temperature of a semiconductor is raised, the bond that holds electrons is weakened. More electrons are able to escape and so the number of free electrons and holes increases. This means that the electrical resistance of semiconductor decreases with increase of temperature. The reverse in conductivity is also true when temperature reduces.
The conduction band theory
In an atom, each electron has a specified amount of energy it posses. Each electron is thus said to exist in a certain energy level.
According to the energy-band theory, when two or more atoms are brought close to each other, the energy levels split into smaller energy levels called bands. This results from interaction of both electric and magnetic fields of the electrons as they revolve in their energy levels. The energy bands are illustrated below:
In solids, because atoms are close together, energy levels merge into bands of energy. Between the bands are gaps that represents energies electrons cannot have. It is the width of the gap that determines conductivity of the material.
The bands have gaps between them which represents energies electrons cannot have.
conduction band
The conduction band is the lowest energy band in a solid where electrons can move freely and conduct electricity. It is located above valence band and is typically empty or partially filled. When electrons gain enough energy, they can jump from the valence band to the conduction band.
Electrons in the conduction band can move freely through the material under the influence of an electric current.
The outermost electrons of the atoms occupies the conduction band and are not bounded exclusively to any one atom. The slightest potential difference across a metal will make the electrons flow. This makes metal good conductors of electric current and where current flow is proportional to the potential difference across the metal. Conductors have no energy gaps such that conduction band and the valence band overlaps. see the figure below:
valence band
The valence band is the highest energy band in a solid that is filled with electrons at absolute zero temperature. These electrons, called valence electrons, are the outermost electrons of the atoms and are responsible for chemical bonding. In valence band, electrons are not free to move.
Energy bands in semi-conductors
In semiconductors, there exists an energy gap between the valence band and the conduction band. An electron in a covalent bond between two atoms must receive extra energy in order to be lifted into the conduction band.
A significant number of electrons receives enough energy from thermal vibrations to be excited into the conduction band. This is because the gap allows.
When temperature rises, it increases the chance of electrons moving from valence band to the conduction band. Therefore electrical resistance of a semiconductor reduces with increase of temperature.
Energy bands in insulators
Insulators are as important in electronics as the conductors and semiconductors. The gap below the conduction band is very large and normal thermal vibrations are not sufficient to excite electrons into the conduction band. see the figure below.
There will never be any electron in the conduction band as the electrons remains bonded to their individual atoms hence cannot move as current. Temperature will not increase conductivity as there can never be found enough energy to excite an electron into the conduction band.
Related topics
- Introduction to Photo electric effects
- Introduction To The Quantum theory
- Energy of radiations
- Introduction to cathode rays
- Properties of cathode rays
- Electromagnetic induction