Semiconductor -- Conduction and Valence bands

[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!

 

[eq1]

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