Please Help! The Band Theory! (1 Viewer)

[cyne]

What is the concept/definition of Band Theory?! I was looking through the textbook and couldn't find it

could anyone please provide me some information about Band Theory and how it links to semi-conductors?! Thanks heaps!

 

[veloc1ty]

It should definitely be in your textbook. Perhaps reading the biki page could help you find it:

http://www.boredofstudies.org/wiki/...tom_resulted_in_the_invention_of_transistors.

 

[Captain Gh3y]

What is the concept/definition of Band Theory?!

Actually you don't even need to know what the Band Theory is, you just need to know this:

2. describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance

The valance band is the outmost shell with electrons in it. The valance band holds the valance electrons. The conduction band is above the valance band. No electrons are present in the conduction band, however the conduction band is where the electrons jump to and from when they conduct.


If an electron is in the conduction band and a potential difference is present, then current will flow. Electrons need to be in the conduction band for current to flow. Bands are energy levels, so to move an electron from the valance band to the conduction band energy must be given to the electron to allow it to be in the conduction band. This can be in the form of another electron, light, heat, etc.


In insulators there is a large gap between the valance band and the conduction band meaning that electrons need a lot of energy supplied to move then up an energy level and into the conduction band. With a semiconductor the gap is smaller meaning that the electrons need a bit of energy supplied to move then up an energy level and into the conduction band. In a conductor the valance band and conduction band are close together meaning that electrons can easily go from the valance band to the conduction band and hence easily conduct. In metals the conduction band and the valance band overlap meaning that the material very easily conducts.

or

Background

When two atoms are close enough to interact with each other the allowed energy levels that the electrons can occupy splits into two distinct, but closely spaced, energy levels. In a three atom system there are three energy levels, and so on. In a crystalline solid there are so many atoms interacting that the energy levels are very close to each other. The electrons in the structure are restricted to one or other of these energy levels. Depending on the nature of the chemical bonding, electrons at particular energy levels can be grouped into bands. There are several types of bands, including:

* the conduction band where the electrons are free to move
* the valence band, which contains electrons that, given the right conditions, can be induced to move into the conduction band
* between these two bands is often a third bans or region which prevents electrons moving between the conduction and valence bands (forbidden energy band).

Energy bands for conductors, semiconductors and insulators

* In a conductor, the conduction and valence bands overlap. This allows the valence electrons to easily move along the conduction band giving the material low electrical resistance.

* In insulators, there is a large forbidden energy band, which makes it difficult for valence electrons to move into the conduction band giving the material a high electrical resistance.

* In semiconductors, the forbidden energy band is not too wide. Under certain conditions, electrons in the valence band can gain sufficient energy to cross the gap. This reduces the electrical resistance of the material.

 

[Captain Gh3y]

Also note that the Band Theory is just some crap some Honours guy came up with because his lecturer told him so, and now we have to learn it.

It doesn't show/explain WHY, it's just a different way of approaching the phenomenon.

It also happens to be true. I've measured the band gap in germanium myself.

It's just you don't learn the 'why' part in the HSC.

I wrote a post with a little bit about it (and semiconductrs too) just here: http://community.boredofstudies.org/showpost.php?p=3462658&postcount=63

if you're interested in a better-than-hsc explanation that's a good summary. To get the 'why' part you need Quantum Mechanics.

 

[cyne]

Actually you don't even need to know what the Band Theory is, you just need to know this:

2. describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance

The valance band is the outmost shell with electrons in it. The valance band holds the valance electrons. The conduction band is above the valance band. No electrons are present in the conduction band, however the conduction band is where the electrons jump to and from when they conduct.


If an electron is in the conduction band and a potential difference is present, then current will flow. Electrons need to be in the conduction band for current to flow. Bands are energy levels, so to move an electron from the valance band to the conduction band energy must be given to the electron to allow it to be in the conduction band. This can be in the form of another electron, light, heat, etc.


In insulators there is a large gap between the valance band and the conduction band meaning that electrons need a lot of energy supplied to move then up an energy level and into the conduction band. With a semiconductor the gap is smaller meaning that the electrons need a bit of energy supplied to move then up an energy level and into the conduction band. In a conductor the valance band and conduction band are close together meaning that electrons can easily go from the valance band to the conduction band and hence easily conduct. In metals the conduction band and the valance band overlap meaning that the material very easily conducts.

or

Background

When two atoms are close enough to interact with each other the allowed energy levels that the electrons can occupy splits into two distinct, but closely spaced, energy levels. In a three atom system there are three energy levels, and so on. In a crystalline solid there are so many atoms interacting that the energy levels are very close to each other. The electrons in the structure are restricted to one or other of these energy levels. Depending on the nature of the chemical bonding, electrons at particular energy levels can be grouped into bands. There are several types of bands, including:

* the conduction band where the electrons are free to move
* the valence band, which contains electrons that, given the right conditions, can be induced to move into the conduction band
* between these two bands is often a third bans or region which prevents electrons moving between the conduction and valence bands (forbidden energy band).

Energy bands for conductors, semiconductors and insulators

* In a conductor, the conduction and valence bands overlap. This allows the valence electrons to easily move along the conduction band giving the material low electrical resistance.

* In insulators, there is a large forbidden energy band, which makes it difficult for valence electrons to move into the conduction band giving the material a high electrical resistance.

* In semiconductors, the forbidden energy band is not too wide. Under certain conditions, electrons in the valence band can gain sufficient energy to cross the gap. This reduces the electrical resistance of the material.

Thanks heaps, they really help a lot

but would you please give a bit of information of Band Theory and how band theory links to semiconductors too?
I need the Band Theory part just because of my physics assessment /assignment need an explanation of it and it contains 20 marks out of 50 !!

Thanks heaps!

 

[cyne]

It also happens to be true. I've measured the band gap in germanium myself.

It's just you don't learn the 'why' part in the HSC.

I wrote a post with a little bit about it (and semiconductrs too) just here: http://community.boredofstudies.org/showpost.php?p=3462658&postcount=63

if you're interested in a better-than-hsc explanation that's a good summary. To get the 'why' part you need Quantum Mechanics.

hey thanks it helps to understand
it's just that sometimes teachers cut out the part which made it easier to understand and for us to understand we just memorise it.

 

[Glenjamin]

I was wondering how this theory has been tested and vadiated

 

[Captain Gh3y]

Basically you heat up a semiconductor and measure how its conductivity changes with temperature, then you can use the relationship between conductivity and thermal energy to literally work out the size of the band gap. I can attach some lab notes for this experiment if you want.

In terms of why are there bands in the first place, the splitting of energy levels actually comes from diffraction of electrons within a crystal lattice according to Bragg's Law. Electron diffraction has been demonstrated experimentally and Bragg's Law works (I've used it to measure the distance between carbon atoms in graphite)

If you do the Quanta to Quarks elective you'll learn about wave-particle duality and deBroglie's equation for matter-waves: lambda = h/mv. It all happens because the electrons in the lattice are waves

 

[Glenjamin]

thanks for the response