Electronics is the branch of physics and technology concerned with the behavior 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 can not 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 electrode 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 electron flow towards the other railed 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 evaluated 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.