Types of Electron Emission and Their Applications

Electron emission is the process through which electrons are ejected from surface of material. This occurs through several mechanisms, including thermionic emission, photoelectric emission, field emission and secondary emission. JEE Main exam and IIT JAM entrance exam require students to have the knowledge of this concept.

Every type of electron emission is used in devices such as photomultiplier tubes, where multiplication of electrons is important for detecting low levels of light. This is also used in scanning electron microscopes (SEMs) for imaging surfaces at a higher resolution.

There are four different types of electron emission that we are going to learn about in this section:

1. Thermionic Emission

Whenever a material (mostly metal) is heated, its thermal energy leads to an increase in the kinetic energy of electrons present within material. Some electrons gain energy sufficient enough to overcome the work function of material. 

In simple terms, electrons gain at least the minimum energy required to escape the surface. After gaining sufficient energy, electrons are emitted. The rate of emission depends on the temperature as well as the work function of material. CUET exam students should also learn about this electron emission to answer questions asked in the exam. 

Here, the emission current density J is described by Richardson-Dushman equation: 

$$J = A T^2 e^{ - \frac{W}{ k T } }$$

This type of electron emission is useful in operation of vacuum tubes, cathode ray tubes and thermionic converters. They are also used in some type of electron microscopes and for space propulsion systems such as ion thrusters.

2. Photoelectric Emission

Whenever photons that have energy greater than the work function of a material strike its surface they can transfer their energy to electrons. If the energy is enough, electrons will be emitted from the surface. Energy of the emitted electrons depends on the frequency of the incident light and work function of the material. According to the Einstein's Photoelectric equation, the maximum kinetic energy K of emitted electrons is:

K=hf−W

Here:

• h is the Planck's constant
• f is the frequency of incident light
• W is the work function

Photoelectric emission is used in solar panels, photoelectric cells, photomultiplier tubes, light detection and imaging devices.

3. Field Emission

When a strong electric field is present, the potential barrier at surface of a material is distorted. This allows electrons to tunnel through barriers and escape the surface. There is no requirement for thermal energy for this process and it can even occur at room temperature. Field emission current density J is described by Fowler-Nordheim equation: 

$$J = A E^2 e^{ - \frac{B}{E} }$$

Here:

• A and B are the constants related to the properties of material
• E is the electric field strength

This is one of the types of electron emission used in electron guns in electron microscopes, field emission displays and in field emission microscopes. IISER exam and NEET exam students must learn about it in detail since it is also important for the development of high-resolution electron sources and vacuum electronic devices.

4. Secondary Electron Emission

In this type of electron emission, material is bombarded with primary electrons or ions. Due to this bombardment, particles can transfer energy to electrons in material. In case, the transferred energy is sufficient, secondary electrons will be emitted from the surface. 

The number of secondary electrons emitted per primary particle is known as secondary electron yield. It is used in devices such as photomultiplier tubes in which the multiplication of electrons is important for detecting low levels of light. 

For CBSE board students, knowing about the applications of electron emission is important. The following points explain applications of electron emission:

1. Applications of Thermionic Emission:

• One of the types of electronic emission is thermionic emission which is used in amplifiers, oscillators as well as other electronic devices. While these are being replaced by solid-state devices, they are still used in high-power applications like radio transmitters.
• Thermionic emission was also used in older television sets and computer monitors, where beam electrons were used for creating images on a fluorescent screen.
• This type of electronic emission provides electron beams that are used in scanning electron microscopes (SEMs) and transmission electron microscopes (TEMs) for high-resolution imaging. 

2. Applications of Photoelectric Emission

• It is used in light detection as well as in automatic lighting systems like streetlights that turn on at dusk. This electron emission converts sunlight into electrical energy to provide a renewable energy source for a range of applications.
• Photoelectric emission is used in the operation of charged-coupled devices (CCDs) and complementary metal-oxide semiconductor (CMOS) sensors.

3. Applications of Field Emission

• It provides high-resolution images of the surfaces at the atomic level. This is particularly useful in surface chemistry and material science. Field emission is also used in particle accelerators for producing high-quality electron beams. 
• This electron emission is also used in high-frequency and high-power applications like microwave amplifiers and oscillators.

4. Applications of Secondary Electron Emission: 

• This is used for creating detailed images of surface topography of samples.
• It amplifies weak light signals by multiplying the number of electrons through secondary emission, which is important in applications such as particle detection and medical imaging.
• Secondary electron emission amplifies low-light images for night vision devices as well as other low-light imaging applications.

Let us take a look at some of the important FAQs that are related to electron emission: