# Why is aligning the vacuum level necessary in band diagram creation?

• Swapnil
In summary, the vacuum level is aligned first in order to have a reference point for comparing the energy changes in the band diagram when two semiconductors are put together. This alignment is necessary for understanding the changes in band energies, work functions, and band bending. After making contact, thermal equilibrium is reached and the Fermi level is also aligned, indicating no net charge transfer in the interface. This alignment is important for understanding the overall behavior of the interface.
Swapnil
Why is it that we align the vacuume level (when drawing the band diagram) first before we put two pieces of semiconductor together?

(My book talks about how the energy of the two free electrons should be the same when each of them jumps out of its respective material...but it is not entrirely clear)

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The vacuum level is the reference level. All calculated energies are expressed with respect to this level. It needs to be aligned because otherwise one cannot compare what happens to the bands (change in energies, work functions, band bending etc etc) when contact is made.

Also, after making contact, thermal equilibrium is reached and the Fermi level (Ef) is aligned and thus the same throughout the entire interface. The Fermi level has the same value (ie no net charge transfer in the interface). For example if the Ef is -6 eV it means that the Ef is 6 eV below the vacuum level which is the 0 reference level. you see ?

marlon

I see. Thank you.

## 1. What is a band diagram?

A band diagram is a graphical representation of the electronic energy levels of a material. It shows the energy levels of electrons in the material as well as the energy required for them to move from one level to another.

## 2. What is the significance of band diagrams in materials science?

Band diagrams are important in materials science because they help us understand the electronic properties of materials. By looking at the band diagram, we can determine if a material is a conductor, insulator, or semiconductor, and how it will behave under different conditions.

## 3. What is the difference between the conduction band and the valence band in a band diagram?

The conduction band is the highest energy band that contains free electrons, while the valence band is the lowest energy band that is completely filled with electrons. The energy gap between these bands determines whether a material is a conductor, insulator, or semiconductor.

## 4. What is the vacuum level in a band diagram?

The vacuum level is the energy level of the electrons in a material when there are no external influences, such as an electric field or other materials, affecting them. It serves as a reference point for the energy levels in the material.

## 5. How does the vacuum level affect the electronic properties of a material?

The vacuum level can affect the electronic properties of a material by influencing the position of the energy bands. If the vacuum level is shifted, it can change the energy gap between the conduction and valence bands, altering the material's conductivity and other properties.

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