What's the effect of temperature on extrinsic semiconductors???

[MCR]

HELP! I've never posted something on this forum before, so I didn't know where to post this...

 

Anyway, does anybody know the effect of temperature on extrinsic (both p-type and n-type) semiconductors, because I can find aboslutely nothing on the internet!!!

 

Thanks

[5614]

Ah, the trick question.

 

With a standard seminconductor increasing the temperature allows more electrons to enter the conduction band so it increases conductivity.

 

However with an extrinsic seminconductor electrons still jump into the conduction band, however this effect is negligible relative to the reduction in conductivity caused by the lattice vibrating more. This is the same effect as when you have a normal conductor who's conductivity decreases with temperature.

 

So the effect is a decrease in conductivity.

 

I assume it is the same for p and n types (note the word "assume", although it seems logical that it would be correct).

[MCR]

So, an increase in temp. means an increase in conductivity and a decrease resistance...but does the conductivity increase more/ quicker (whatever the correct term may be) in an extrinsic semiconductor than in an intrinsic one??? This only seems logical to me...

[5614]

I don't really know for certain in a quantitative way, but in all seminconductors when you heat them electrons jump into the conduction band.

 

But with extrinsic semiconductors (whilst there are more electrons in the conduction band) there is a massive decrease in conductivity because when you heat it the lattice structure of the seminconductor vibrates more, so increases resistance. The increase in resistance is very big compared to the few extra electrons in the conduction band, the net effect is that the conductivity decreases.

 

Another way of looking at is that in an extrinsic semiconductor you have the effects of a intrinsic seminconductor and a normal metallic conductor all in one. You have the extra electrons in the conduction band (like an instrinsic seminconductor) and you have the increase in resistance (like a normal conductor). The important factor is that the increase in resistance is very large relative to the effect of more electrons in the conduction band, so the net effect is the decrease in conductivity.

 

Summary:

 

Upon heating:

 

Intrinsic: Increase conductivity due to extra electrons in conduction band.

 

Extrinsic: Decrease conductivity due to increased resistance due to increase vibrations in the lattice/semiconductor.

[MCR]

Thank you sooooooooo much...you can't believe how much that has helped me- I could literally find nothing about that on the internet

[MCR]

one final thing...so, does scattering of electrons have an effect on extrinsic semiconductors?

[5614]

You're welcome.

 

And I don't follow the second post... What do you mean "scattering of electrons"?

 

Are you referring to the vibrations of the lattice? Which causes an increased resistance, which means the electrons collide and I suppose you could say scatter?

 

Nah, I don't think that's what you mean.

[MCR]

Yeah, with scattering I mean the collision of the electrons, thus causing an increase in resistance...I have not really come across the term being referred to as 'vibrations of the lattice', but is that basically the same thing?

[5614]

I haven't thought of resistance in terms of electron scattering so let me explain the vibrations thing. When you heat something up you know the molecules vibrate faster.

 

In a semiconductor there is an ordered structure known as a lattice. For example scroll down to the second image here:

http://en.wikipedia.org/wiki/Crystal_lattice

Where the dots are atoms and the lines are bond. It is a type of structure.

 

"Latice: an arrangement of points or particles or objects in a regular periodic pattern in 2 or 3 dimensions"

definition from: http://wordnet.princeton.edu/perl/webwn?s=lattice

 

So having a vibrating lattice means basically that all of the atoms (which are structured) are vibrating more. The idea of atoms vibrating when they are heated is quite a simple idea I assume you have come across. So ye... the lattice vibrates, or the atoms vibrate which increases the probability that an electron will interact with it. The interaction between electrons and atoms in any conducting material is the cause of resistance.

 

So more thermal energy -> atoms vibrate -> more interactions between electrons and atoms = more resistance. And I keep saying "atoms" whereas in reality they would be ions, but you know what I mean.

 

Now I'm not quite sure how to relate this to electron scattering. In a lattice electrons are scattered in a predictable way. Now heating (ie. increasing vibrations) will probably change the pattern produced from the electron scattering. So maybe that is a link between the two.

 

I suppose using my method of thinking the increase vibrations increases probability of interactions, using your method of thinking these interactions would cause the electrons to scatter. This is the cause of resistance. I think we're talking about the same thing. It isn't solely the fact that ions are vibrating which makes the resistance increase... it's the fact that when they vibrate electrons are more likely to interact which I think is the cause of the electron scattering you refer to.