INTRODUCTION
What is Electron Conduction?
• Valence electron in conductor atoms are
loosely bound.
• Thermal energy at room temperature
frees these electrons.
• These free electrons move randomly
inside the conductor.
•
How Current Flows?
When an electric field is applied, free
electrons move in an organized manner
This movement forms an electrical current.
Why electron movement matters?
Many electronic devices rely on
controlling electron movement.
Electron behavior changes significantly
in a vacuum.
Electron Emission
Electron Emission:
refers to the process by which electrons are released
from a material’s surface.
• This phenomenon plays a curcial role in many
scientific and technological applications.
The release of electrons occurs
when the material absorbs enough energy to
overcome it’s work function , the energy barrier that
keeps electrons bound to the surface.
Applications:1. Vacuum Tubes and CRTs
2. Electron Microscopes
3. X-rays Tubes
4. Field Emission Displays (FED)
5. Radiation Detection
Types Of Electron Emission
Electron Emission is the process where
electrons escape from a material’s surface
when sufficient energy is provided.
Types:
1. Thermionic Emission
2. Field Emission
3. Photoelectric Emission
Thermionic Emission:Mechanism:
Occurs when the material is heated to
high temperatures, providing electrons
enough energy to overcome the work
function.
Example:
Electron Emission from a heated filament
in a vacuum tubes.
Field Emission:Mechanism:
A Strong electric field reduces the energy
barrier, allowing electrons to tunnel through.
Applications:
Used in electron microscopes and cold
cathodes.
Photoelectric Emission:Mechanism:
Electrons absorb energy from photons and
escape the matrial’s surface.
Applications:
Solar cells, light sensors and photomultiplier
tubes.
Secondary Emission:Mechanism:
High-energy particles (like electrons)
collide with a surface, causing the
emission of additional electrons.
Applications:
Used in electron multipliers and CRTs.
2.3
Thermionic
Emission
Thermionic Emission
• Under normal conditions,the energy of electrons inside the
metal is not enough for them to escape the surface
• Mechanism Of Thermionic Emission:
• 1. When the material (typically metals or semiconductors) is heated, the thermal
energy of the electrons within the material increases.
• 2. Some electrons gain enough energy to overcome the work function and escape
from the material’s surface.
• 3. The emitted electrons create an electric current known as thermionic current.
Factors Affecting Thermionic Emission:
1. Temperature:
Higher temperatures increase the number of emitted electrons.
2. Work Function:
Materials with lower work functions (like tungsten or thorium) emit
electrons more easily.
3. Surface Nature:
A smooth and clean surface enhances thermionic emission.
As we will see in Richardson-Dushman equation
Applications of
Thermionic
Emission:
1.Thermionic Valves:
Used in early electronic devices
like radios and televisions to amplify
signals.
2. Cathode Ray Tubes (CRT):
Utilized in old displays where
electrons are emitted via thermionic
emission to form images.
3. Thermionic Generators:
Convert heat directly into
electrical energy for space or industrial
applications.
4. Microwave Tubes:
Richardson-Dushman equation
• Definition: The amount of thermionic emission increases rapidly as the emitter temperature is
raised .
• Js
Emission current density (current per unit area)
• Basic equation:
• Effect of Temperature:
• Thermionic emission is greatly influenced by temperature:
Increasing the temperature of an emitter (such as a metal) can increase
the emission of electrons by more than 10 million times.
• Effect of Work Function:
• Small changes in the work function (the energy required to
release an electron from the surface) can have a huge impact
on electron emission. In fact, halving the work function has
the same effect on emission as doubling the temperature.
Thermionic Emmitter
Definition:The substance used for electron emission is known as an emitter
or cathode.
How does it work?
The Cathode is heated in an evacuated space to emit electron.
What are it’s characteristics?
A Cathode should have the following properties:
1-Low work function:Cathode should have low work function so that electron emission take place by
applying small amount of heat energy.
2-High melting point:As electron emission take place at very high temperatures (<1500 C) therefore the
substance used as a cathode should have high melting point
3-High mechanical strength:The cathode should have mechanical strength to withstand the bombardment of positive ions.
There are always present some gas molecules which may form ions by impact with electrons when
current flows.
Under the influence of electric field the positive ions strike the cathode.
If high voltage are used the cathode is subjected to considerable bombardment and may e
damaged.
2.5 commonly
used
Thermionic
Emitters
Thermal emitters :
materials or devices designed to release heat energy. They work on the
principle of blackbody radiation, emitting electromagnetic radiation
across a spectrum of wavelengths. The specific wavelength and intensity
of the emitted radiation depend on the temperature of the emitter.
Because vacuum tubes need high temperatures to
emit electrons efficiently.
This limits the materials used to
tungsten
thoriated tungsten
oxide-coated metals
Tungsten:
Tungsten was the first material used as a cathode (the surface from which electrons are
emitted) in electronic tubes.
Advantages:
Disadvantages
1. High melting point
2. High mechanical strength
3. Long lifespan
1. High operating temperature
2. High work function
3. Low emission efficiency
Main applications: Tungsten is primarily used in devices that require very high voltages, such
as X-ray tubes.
Thoriated Tungsten:
Mixture of two metals ,tungsten and thorium.
Cathode manufacturing:
The tungsten filament is impregnated with thorium oxide and
then heated to a very high temperature to convert the oxide to
metallic thorium, which coats the filament surface with a thin
layer.
Advantages:
1. Low work function
2. Thermionic emission
at lower temperature
3. Higher efficiency
Applications:
Intermediate voltage electronic tubes
oxide-coated:
is a fundamental component in many electronic tubes, such as
those used in older radios and televisions.
This cathode consists of :
a nickel strip coated with a thin layer of oxides of certain alkaline
earth metals, like barium or strontium.
Advantages:
Disadvantages
1. Long lifespan
2. High efficiency
1. Sensitive to pressure
2. changesCannot
withstand high
voltages
How does it work?
When this cathode is heated,
the oxide layer on its surface
begins to emit electrons. These
electrons flow through the
electronic tube, generating the
required electrical signal.
2.6 Cathode Construction
- Definition:The cathode is a crucial component in many electronic
devices that rely on electron emission.
- Importance: It plays a vital role in the efficient operation of devices such
as X-ray tubes, cathode ray tubes (CRTs), and electrochemical cells.
Types of Cathodes
1. Directly heated Cathode
2. Indirectly heated Cathode
Directly heated Cathode
Design:
A nickel strip coated with oxide, often referred to as a filament.
Mechanism:
An electric current is passed directly through the strip, causing it to
heat up and emit electrons.
Efficiency:
Highly efficient in converting thermal energy into electron emission.
Structure of Directly Heated Cathode
Components:
1. Filament:Nickel strip coated with
oxide.
2. Glass Stem:Provides structural
support.
3. Glass Envelope:Encases the
components
to protect from external factors.
Indirectly heated
cathode
Definition
is a type of cathode used in electronic
devices, such as vacuum tubes, where the
heating element and the cathode itself are
separate.
The cathode does not directly receive
current to heat it. Instead, it is heated by
thermal radiation or heat transfer from a
separate element called the heater
.
How Does it
Work?
.1Heating Element (Heater):The heater is a wire (usually made
of a high-resistance material like tungsten or nickel) that is
electrically powered. The heater gets hot when current passes
through it.
.2Thermal Transfer:The heater, which is electrically isolated from
the cathode, heats the cathode through thermal conduction
.3Electron Emission:As the cathode heats up, it reaches a high
enough temperature for thermionic emission to occur. This is
when electrons gain enough energy to leave the surface of the
cathode and are emitted into the surrounding space, ready to be
used in the Device
Applications
Vacuum Tubes: Indirectly heated cathodes are
used in a variety of vacuum tubes such as radio
tubes and oscilloscopes
Cathode Ray Tubes (CRTs): These are used in older
television and computer monitor screens.
Electron Microscopes: Indirectly heated cathodes
are also used in electron microscopes, where
precise control over electron emission is critical.
2.7- Field Emission?
Definition:Field emission refers to the
emission of electrons from a
solid surface into a vacuum
under a strong electric field.
Key concept:Based on quantum mechanical
tunneling, where electrons
overcome the potential barrier.
Mechanism of Field Emission
The process involves:-A strong electric field reduces the energy barrier.
-Electrons “tunnel” through the barrier due to quantam effects.
Influencing factors:-Work function of the material.
-Surface geometry (sharp tips enhance emission).
Applications and Visual Insights
Field Emission Applications:
-Scanning Tunneling
Microscopes (STMs)
-Field Emission Displays (FEDs)
-Advanced particle
accelerators.
2.8 Secondary Emission
2. Mechanism:
The intensity of secondary emission depends on factors
such as:
1. The material of the emitting surface.
2. The mass of the colliding electrons.
3. Their energy.
3. Applications:
This phenomenon is utilized in devices such as:
-Cathode ray tubes.
-Electron multipliers.
-Other electronic equipment.
4. Negative Effects:
In certain devices, such as tetrode valves, secondary emission
can cause undesirable effects like negative resistance.
2.9 Photoelectric Emission
Definition:The photoelectric effect is a phenomenon where electrons are emitted
from the surface of a metal when light shines on it.
Condition of happen:
occurs only if the energy of the light is sufficient to overcome the
metal's work function
How Photoelectric Emission Works
--Light photons strike the metal surface.
--If photons energy exceeds the work function, electrons are emitted.
--The emitted electrons are ejected from the metal's surface
Factors affecting the effect:
* Light intensity
* Light frequency
Applications:
Image tubes (television,
film)
Experiment