Electronics is the branch of physics and technology concerned with the behaviour and movement of electrons.

Electron Emission:

In metals, the electrons from the outermost orbit of atom become free at room temp. However these free electrons cannot leave the surface of metals. For the free electrons to leave the metal surface, external energy is required. Electron emission can be achieved by the following methods-

1. Thermionic Emission: by heating the metal.

2. Photo-electric emission: By illuminating the metal surface with light (electromagnetic waves like ultraviolet rays) of suitable frequency. The electrons so emitted are called photo electrons.

3. Secondary emission: It is the electron emission when highly energetic electron beam is incident on a metal surface. The electron so emitted are called secondary electron.

4. Field or cold emission: This is the process of electron emission by applying a strong electric field across the metal surface.

Diode Valve:

Designed by J. A. Fleming in 1904, diode valve consists of two electrodes placed inside an evacuated glass envelope. One electrode is called cathode which is made up of tungsten on which there is a thin layer of barium oxide. When heated, cathode emits electrons. These electrons flow towards the other electrode called anode or plate, which is at positive potential. As a result an electric current is established in the circuit.

  • The electrons emitted from the cathode are collected in the space around it. This collection of electrons is called space charge which is obviously negative.
  • Diode valve acts a rectifier. Rectifier is a device which converts alternating voltage (current) into direct voltage (current).

Triode Valve:

Designed by Lee de Forest in 1907, triode valve is a modified form of usual diode. It consists of a usual anode - cathode pair and one more electrode called control grid.

  • Triode valve can be used as amplifier, oscillator, transmitter and detector.


Semi conductor are those materials whose electrical conductivity, at room temperature, lies in between that of insulator and conductor. Germenium and Silicon are two important semiconductor. In a crystal lattice of semi-conductor, some of the electrons become free from bond formation. At the sites of these electrons a deficiency of electron exists which acts as a virtual positive charge. These virtual positive charges are called holes. Semi-conductors are used in electronics industry.

Semi-conductors are of two types:

1. Intrinsic Semi-Conductor:

A semi conductor in an extremely pure form is known as intrinsic semi conductor. At absolute zero, an intrinsic semi conductor is a perfect insulator, (conductivity = zero).

2. Extrinsic Semi-Conductor:

If a measured and small amount of chemical impurity is added to intrinsic semi-conductor, it is called extrinsic semi-conductor or doped semi conductor. As a result of doping, there is large increase in its conductivity.

Extrinsic semi conductor are of two types:

a. N type semi conductor: An extrionicsemi conductor in which electrons are majority charge carrier is called N type semi conductor. Such a semi conductor is made by doping a pure semi conductor with pentavalent impurity like Arsenic, Antimony & Phosphorus.

b. P type semi conductor: An extrinsic semi conductor in which holes are the majority charge carrier is called a P type semi conductor. Such a semi conductor is made by doping a pure semi conductor with trivalent impurity like Gallium, Indium, Boron and Aluminium.

Some other information

Doping: Adding of chemical impurity to a pure semiconductor is called doping. The amount and type of impurity is closely controlled.

Donor: Pentavalent impurities are called donor.

Acceptor: Trivalent impurities are called acceptor.

  • The electrical conductivity of a semi conductor increases with the increase in temperature.

Rectifier: Rectifier is a device which converts alternating current into direct current.

Nanotechnology: Study of science of small structure 'less than 100 nanometres) is called nanotechnology.

  • Solar cooker and solar PV (Photo voltaic) cell both receives energy directly from sun.
  • Light emitting diodes works on the principle of electroluminescence. (Combination of electrons and holes at the junction and release of energy)

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