Thermionic emission and diffusion theory

In summary: This is why you often see thermionic emission currents dominate current through PN junctions (even though they're diffusion transport processes).
  • #1
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What is the main physical difference behind these two theories?

1) I understand that the thermionic emission theory is applied in metal-semiconductor contacts and heterostructures where the energy band off-sets are large. Whereas the diffusion theory is applied in a simple homojunction, of which the gradient of the quasi-fermi levels are continious.

2) On the other hand, I understand that the diffusion current and the thermionic emission current work in series, where the slowest process limits the current.

There seems to be contradictions between statement 1 and 2. What have I misunderstood?
 
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  • #2
CKwT said:
What is the main physical difference behind these two theories?

1) I understand that the thermionic emission theory is applied in metal-semiconductor contacts and heterostructures where the energy band off-sets are large. Whereas the diffusion theory is applied in a simple homojunction, of which the gradient of the quasi-fermi levels are continious.

2) On the other hand, I understand that the diffusion current and the thermionic emission current work in series, where the slowest process limits the current.

There seems to be contradictions between statement 1 and 2. What have I misunderstood?

Thermionic emission crosses the barrier created by band gap difference through ballistic transport (due to raising the Fermi distribution tail over the barrier height) or tunneling or both, where both are fast in terms of mobility. Traditional vacuum tube thermionic emission is primarily ballistic transport across the cathode/vacuum barrier formed by the discontinuity of the metal interface to vacuum. The cathode is heated to raise the Fermi tail explicitly with heat rather than hot carriers. If you are getting ballistic or tunnel transport across an entire heterojunction layer that is forming the barrier, this is very fast.

Diffusion transport involves multiple collisions where transport is simply akin to that of a bulk semiconductor and PN junction between one or another heterojunction layer, so it's slow in terms of net mobility because it's minority carrier diffusion transport, recombination and majority carrier diffusion transport. In the limit as heterojunctions approach identity, you approach bulk diffusive transport. The dimensions of the layers are greater than the mean free path length for it to be diffusive.

You can get a superposition of several transport regimes depending on materials and physical dimensions.

I'm not sure how the two statements are problematic but maybe the mechanisms explain things. In series, the transit time of the slowest layer transport process certainly will dominate the total transit time of the entire heterojunction structure.
 
  • #3


Thermionic emission and diffusion theory are both theories that explain the flow of current in semiconductor devices. The main physical difference between these two theories lies in the mechanism by which electrons are transported in the material.

Thermionic emission theory is based on the principle that electrons can escape from a solid material when heated to a high temperature. This process is known as thermionic emission and is commonly observed in vacuum tubes. In the context of semiconductors, this theory is applied in metal-semiconductor contacts and heterostructures where there is a large energy band offset. In this case, the electrons can overcome the energy barrier and flow from the metal into the semiconductor.

On the other hand, diffusion theory is based on the principle of diffusion, which is the movement of particles from an area of high concentration to an area of low concentration. In semiconductor devices, this theory is applied in simple homojunctions, where there is a continuous gradient of quasi-Fermi levels. In this case, the electrons diffuse from the high concentration side to the low concentration side, resulting in a flow of current.

The statement that the diffusion current and thermionic emission current work in series is correct. This means that both processes contribute to the overall current in the device. However, the slowest process will limit the overall current. In the case of a metal-semiconductor contact with a large energy band offset, thermionic emission will be the slower process and will limit the current. On the other hand, in a homojunction with a continuous gradient of quasi-Fermi levels, diffusion will be the slower process and will limit the current.

In summary, the main difference between thermionic emission and diffusion theory lies in the mechanism by which electrons are transported. While thermionic emission relies on the escape of electrons from a solid material, diffusion relies on the movement of particles due to concentration gradients. Both theories are important in understanding the flow of current in semiconductor devices.
 

1. What is thermionic emission?

Thermionic emission is the process by which electrons are emitted from a heated surface or material. This is due to the thermal energy provided by the heat, which overcomes the attractive forces between the electrons and the material's surface, allowing them to escape.

2. How does thermionic emission relate to diffusion theory?

Thermionic emission is a key concept in diffusion theory, as it describes the initial release of electrons from a heated material into a vacuum or gas. These emitted electrons then contribute to the diffusion of charge carriers, which is the movement of charged particles in response to an electric field.

3. What factors affect the rate of thermionic emission?

The rate of thermionic emission is affected by several factors, including the temperature of the material, the work function of the material (the minimum energy required for an electron to escape), and the electric field strength at the surface of the material.

4. How is thermionic emission used in practical applications?

Thermionic emission has a wide range of practical applications, including in vacuum tubes and cathode ray tubes, where it is used to generate and control electron beams. It is also used in electron microscopes, x-ray tubes, and thermionic converters, which convert heat energy into electrical energy.

5. What is the relationship between thermionic emission and the Richardson-Dushman equation?

The Richardson-Dushman equation is a mathematical expression that relates the current density (the amount of current per unit area) to the temperature and work function of a material. It is based on the principles of thermionic emission and is often used to calculate the expected emission current from a heated material.

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