# Semiconductor -- Conduction and Valence bands

• pj33
In summary, the conversation discusses the basics of semiconductor conduction and the movement of electrons and holes. The formation of bands and the energy difference between the valence and conduction bands is explained, as well as the role of energy input in exciting electrons. The motion of electrons in a potential difference is described, as well as the flow of current and holes. Additional resources are suggested for further understanding.
pj33
TL;DR Summary
How does a semiconductor conducts
I am a new to this and I try to understand the basics.
So initially once the atoms of silicon come together to form a solid, due to Pauli law no electrons can exist in the same energy state,thus many energy states are formed which together make the bands.

My problem starts at this stage where I try to understand the conduction in macro and micro state.
1) In the silicon the valence and conduction band have some energy difference, but once there is some energy input, the electrons get excited if there is sufficient energy, then they move to the conduction band (does this energy correspodn to the 0.7V required by the semiconductors such as diodes, to conduct?)
2) Can someone explain me the electron motion once the potential difference is applied ( I was shown a picture same as the first picture here https://ecee.colorado.edu/~bart/book/eband4.htm). I was told that the electrons roll down because there available energy levels, but I can't see how this work.
3) Then, once the potential difference is applied the electrons move towards the positive electrode thus current flow and the holes flow towards the negative electrode (does the flow of holes cause anything?)
4) What happens to the electrons left in the valence band?

Thank you in advance!

pj33 said:
Summary:: How does a semiconductor conducts

I think you should reference Chenming Hu's excellent & free semiconductor textbook.

For 1: You're close but your terminology is wrong. Read section 1.3 very closely. The 0.7V is something else. You cover it when you review PN junctions.
https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch1-3.pdf

For 2 to 4: Chapter 2 of has an excellent description of drift. I think especially section 2.2 will help you.
https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch2-2.pdf

pj33

## 1. What are conduction and valence bands in a semiconductor?

The conduction and valence bands are energy levels in a semiconductor that determine its electrical conductivity. The valence band is the highest energy level occupied by electrons, while the conduction band is the next highest energy level that is unoccupied. The energy gap between these two bands is what allows a semiconductor to conduct electricity.

## 2. How do electrons move between the conduction and valence bands?

Electrons can move between the conduction and valence bands through processes such as thermal excitation, where they gain enough energy to jump to the conduction band, or through the absorption of photons in the form of light, which can also excite electrons to the conduction band.

## 3. What is the difference between a conductor and a semiconductor?

A conductor is a material that has a high number of free electrons, allowing it to easily conduct electricity. In contrast, a semiconductor has a lower number of free electrons, but can still conduct electricity under certain conditions, such as when excited by an external energy source.

## 4. How do impurities affect the conduction and valence bands in a semiconductor?

Impurities, also known as dopants, can be intentionally added to a semiconductor to alter its electrical properties. This is because the addition of impurities can create extra energy levels within the energy gap, allowing for easier movement of electrons between the conduction and valence bands.

## 5. What is the role of the band gap in a semiconductor?

The band gap, or energy gap, in a semiconductor is crucial for its electrical properties. It determines the minimum amount of energy required for electrons to move from the valence band to the conduction band, and also affects the material's ability to absorb and emit light. A larger band gap usually means a lower conductivity, while a smaller band gap can result in higher conductivity.

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