EMISSION OF ELECTRONS AND THERMIONIC DEVICES

 

What are Emissions of Electron?

 Electron emission is described as the liberation of electrons from the metal surface caused by temperature increase, radiation, or a strong electric field. Electrons evaporate from metal surface under the influence of heat.

Types of Emission of Electrons

1. Thermionic Electron Emission

What is Thermionic Electron Emission?

Thermionic emission is the process by which electrons leave a metal surface when heated. In a very short time, the region surrounding the hot metal becomes occupied by space charge electrons.

Space charge electrons are the negatively charged cloud of electrons.

Application of thermionic emission

1.      Thermionic diode

2.      Cathode ray tube

2. Secondary Emission

Secondary emission is the emission of electrons from a metal surface that is bombarded by higher energy primary electrons. Electrons are ejected from a solid due to the bombardment of beam of charged particles.

Application of Secondary Emission

1.      Mass Speedometer

2.      Photomultiplier

3.      Photo ejector

3. Field Emission

Field emission as the emission of elections from the surface of a conductor (metal) caused by strong electric field

OR

Field emission is the emission of electrons that are stripped from parent atoms by a light electric field.

Application of Field Emission

1.      X – ray tubes (for space exploration)

2.      Microwave devices

3.      Carbon nanotube

4) Photo Emission

Photo emission is the emission of electrons from the surface of a metal when electromagnetic radiation of appropriate frequency (wavelength) falls on the metal surface. The electrons emitted are called photoelectrons

Application of Photo Emission

1.      Photo cells

2.      Burglar alarms

3.      Television

4.      Flash camera

5.      Automatic devices for switching on light

Thermionic Devices

What is thermionic devices?

Thermionic Devices is a device that connects heat difference to electric power.

Electrode is a two unlike (dissimilar) metals separated by solution. The positive metal is called anode and the negative metal is cathode.

Examples of Thermionic Devices

(1) Diode 

A diode is a two – terminals device that conducts electricity in only one direction. It has two electrodes, anode (positive) and cathode (negative)

Functions of the Electrodes

a.       When the anode is positive with respect to the cathode, the diode offers low resistance to the flow of current.

b.      When the anode is negative with respect to the cathode, the diode offers high resistance to the flow of current.

A valve is a device (compound) used for signal voltage, amplification, detection and rectification.

Application of Diode

1.      Used as rectifiers (for rectification)

2.      For stabilization

(2) Triode 

A triode is a three – electrode electricity. The electrodes are anode cathode and the control grid.

Function of the Control Grid           

1.      Less negative grid voltage increases the plate current.

2.      More negative grid voltage decreases the plate current.

Advantages of the Control Grid

1. It enables the vacuum tube (valve) to amplify a small voltage at the grid to round more voltage at plate

 

(3) Tetrode: 

Tetrode is a triode with a screen grid. If a screen grid is added to a triode it becomes a tetrode. G₂ – screen grid.

Function of the screen grid

1.      It increases high frequency performance.

2.      The screen grid G₂ minimizes inter electrode capacitance of the triode. Tetrode. amplifies signal more than triode.

(4) Pentrode: 

Pentode is tetrode with a suppressor. When a suppressor is added to a tetrode, it is referred to as a pentrode.

Function the suppressor

1. The suppressor grid reduces (suppressor) the secondary emission effect in pentrode valve.

Application of Pentrode

1.      Trans conductors

2.      Amplification factor

3.      Anode slope resistance

G₁ – Control grid                                                                              

G₂ – Screen grid

G₃ – Amplification factor

Cathode Ray Tube (CRT)

Cathode ray tube is a device used to display waveforms of alternating voltages and currents

Functions of Electrode in CRT

1.      The anode maintains light potential so that the electrons can accelerate very rapidly.

2.      The electrons produce the electrons beam (electron gun)

Diode Constance

Mutual Conductance (gm): It is the rate of change of anode current (Ia) to the charge in grid potential 

Vg. gm = ΔIg va – constant, 

                ΔV𝑔     

gm = ΔIg, Va = constant milli ampere/volts                 ΔV𝑔

Anode Slope Resistance ( ): It is the ratio of the change of anode voltage to the change of a current.

Amplification Factor: The product of mutual conductance to the anode voltage.