# What is the Difference Between Valence and Conduction Bands in semiconductors?

1. Nov 18, 2014

### nst.john

I am learning the basic make-up of LED's and they mentioned conduction and valence bands. What I am unsure of is which of these bands are either the p-type or n-type materials within the LED/semiconductor, and/or which band is positive with holes or negative with the free electrons. Can I just get a simple definition of what is in each band and what they mean?

Thank You!

2. Nov 19, 2014

### Tejas Rathod

Each atom consists of bounded electrons in discrete energy levels (The electrons don't have continuous variation in energy when bounded to a single atom). when many such atoms come close to each other, the discrete energy levels now become energy bands. Within one band electron can have continuous variation of its energy. Many such band levels are formed. The topmost band which is completely filled with electrons is called valence band. Above valance band we have conduction band (here electrons if exists are considered free to move when external electric field is applied. That is they are not bounded to any particular atom or atoms, they are termed as free electrons).
In LEDs (or simply Diode for time being) We have P type semiconductor and n type semiconductor joined by some process to form a pn junction. The valence band of p type is electron deficient so rich in holes. And the conduction band of n type is rich in electons. When pn juction conducts, the electrons from n type conduction band falls in holes of p type valence band, the difference in energy is emmitted as light or many times heat.

3. Nov 19, 2014

### ZapperZ

Staff Emeritus
I'm unsure what the exact problem is here. What about the simplest and most obvious description here:

If the doping level is closer to the conduction band than the valence band, it is a n-type. Dominant charge carrier are electrons.
If the doping level is closer to the valence band than the conduction band, it is p-type. Dominant charge carrier are holes.

I'm guessing this description is provided in the material that you are learning. So where in here exactly did you not understand?

Zz.

4. Nov 19, 2014

### nst.john

Well I am really teaching myself and got confused and my teacher was unsure as well so I checked here. This was all I needed and my question has been answered! Thank you!

5. Nov 19, 2014

### ZapperZ

Staff Emeritus
May I suggest that, if by now you still haven't been told of it, you check out the Hyperphysics website? It has a very clear description that would have answered your question:

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html

This goes for other topics as well.

Zz.

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