Differences in conduction mechanisms between different classes of materials can lead to some interesting differences. For instance, the conductivity of silicon decreases with temperature. This is because as you heat silicon, the crystalline of the structure is degraded. As we know, it is the crystalline structure that allows such a well defined band structure, leading to conduction electrons.
Conversely, organic semiconductors rely on a different type of conduction. Electrons are conducted via "hopping" between molecules. This can only happen when molecules are very close to each other. An increase in temperature increases the rate of these phononic interactions, thus increasing the conductivity.
I think that this is a good example of how a relatively simple experiment, like temperature dependence of conductivity, can lead to some insight into much more fundamental electronic processes. Can you guys think of some other examples? Or perhaps some other fundamental differences between seemingly similar materials?
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This paper here presents a few different examples between organic semiconductor equations and regular semiconductors http://prola.aps.org/pdf/PR/v133/i6A/pA1668_1.
ReplyDeleteI was initially seeing whether there might be a similar process that happens with thermal conductivity, and whilst there are differences they aren't as pronounced as the electrical conductivity. Still, even a different gradient to what is expected needs to be explained, and that's what that paper seeks to do.
I'm not sure how well the theory there agrees with experiment, as they just have a small section at the end stating that these new calculations match up quite well, and it was released over 50 years ago so a lot of those formulae would be revised.
For metals when you heat them up conductivity decreases because the collision between electrons due to random thermal motion increases. I thought that in semiconductors the reverse was true that if you increase the temperature, the conductivity increases cause more electrons have energy to hop over the band gap into the conduction band.
ReplyDeleteThinking about this, I have come up with a hypothesis. As temperature increases, the crystal structure, and therefore the band structure, becomes less well defined. This creates more states at higher energy than the valence band. More specifically, the decrease in crystallinity causes a "blurring" of the bands. I hypothesise that these states that are created are not conducting states. Furthermore, the rate of creation of these states, is higher than the rate of excitation of electrons to the conduction band. In other words, the increase in temperature creates more states, which are filled with thermally excited electrons. The rate of creation of these non conducting states is greater than the rate of promotion of electrons to the conduction band.
ReplyDeleteOnce again, this is just a theory I thought up in my head. I'll try to find some evidence to back it up (or debunk it...) when I get to uni tomorrow (I can't be bothered setting up VPN right now :P )
What do you guys think?
David, I don't understand what you mean by the band structure becoming less defined as the temperature increases.
ReplyDeleteRegardless, suppose there are more states at higher energy than the valance band.
What if we then fill up all of a band above the valance band?
I suppose what I'm asking is: when something is heated, does the conduction band move up too?
(Intuition would say no since heating things increases conductivity - refer to linear coefficient from Drude)
Sidenote: setting up the UQ VPN is rather straight forward.
Otherwise, to access articles through the library, have the address of the form
journal.com.ezproxy.library.uq.edu.au/article