physics- conductors and semi-conductorsWatch
properties and structure
- Study Helper
Firstly you need to know about band theory and binding energies. The binding energy of an electron to its atom is how much energy you need to remove it from the atom, its the same thing as the ionization energy. Band theory in a nutshell, just states that atoms which are in the outer shell of their atom are in the valence band (hence the name valence electrons) and electrons which have enough energy to leave their atoms (but not the actual structure - they're not beta particles) are in the conducting band. These bands are just another way of saying the range of energies an electron exhibiting certain behaviors can have. As soon as its energy goes outside those boundaries it will start doing something different.
Conductors all work in pretty much the same way. It doesn't matter whether you've got a metal rod or a nanotube or a block of graphite, all of them contain permanently delocalised electrons (specifying that they're permanently delocalised is very important, you'll see why when I talk about semiconductors). Delocalised is a nasty word that just means that the electron is in the conducting band. If you're studying AS or A2 chemistry you'll have come across orbitals before (if not you might want to look them up, they're awesome). The most common conductors are metals because in most cases they have d or f orbitals in their ground state (most stable state). The d and f orbitals are much bigger than s and p orbitals, and contain more electrons. So more electrons are spin-paired, there's a larger distance between the electrons and the nucleus and since the electrons are closer together they experience more repulsion from other electrons in the subshell. Because of this, the electrons in the d and f orbitals tend to have quite a bit of energy, and since atoms prefer to do whatever gives them the lowest energy, they can easily lose these electrons for very short amounts of time. The electron becomes delocalised. It doesn't leave the structure though, because as it approaches a neighboring atom the repulsion between itself and one of that atom's valence electrons increases, causing it to transfer its excess energy to the valence electron, delocalising it and taking its place. Of course that then leaves a positive ion where it originally came from, and that just attracts a delocalised electron or causes one from a neighboring atom to be delocalised and then settle in the positive ion, making it an atom again. These attractions and electron swaps keep the metallic structure together.
In the case of covalent structure conductors, the delocalised electrons come from it being easier for the atoms to delocalise it than form a bond with it. A lot of nanotubes are like this, and some other structures like graphite. If you look at the bonding in graphite you'll see that each carbon forms 3 bonds, but has 4 valence electrons. The fourth is just delocalised in graphite. When a potential difference is applied across the conductor it causes these delocalised electrons to "atom-hop" towards the cathode (positive terminal/end) which is a current. In metals the current is also from the delocalised electrons "atom-hopping" towards the cathode. There is an increase in the number delocalised at a given time though, because their energy is proportional to the p.d across the material, so more become delocalised.
Semiconductors aren't too different to conductors in the way that they transmit current, but unlike conductors they need a bit of persuasion to get them to work. There's loads of different types of semiconductors. The most common ones are light sensitive, heat sensitive and voltage sensitive (I'm not sure what else to call these, other than the ones found in transistor gates and diodes). The pure element ones tend to be metalloids, but a lot of other covalent structures are used, nitrogen or boron doped nanotubes for example.
I'll start off with the voltage sensitive type (to be honest all semiconductors are voltage sensitive, here I mean those that aren't particularly light or heat sensitive) because I love transistors. A bog standard transistor is generally a bipolar junction transistor, but we don't need to get into all that. Basically, transistors turn on when they receive a high enough voltage at their gate pin. Its the same in diodes and numerous other components. The material in modern transistors is generally silicon. The silicon is able to hold onto its valence electrons until the voltage reaches a specific value, then they hop into the conducting band. As I said before, the electron energies are proportional to the p.d so the material becomes conducting when the voltage is high enough to promote the electrons to the conducting band. Germanium used to be used instead because it requires less energy input to become conductive, but was replaced by silicon because the transistors would melt quite often due to germanium's low melting point.
The light and heat sensitive semiconductors work in exactly the same way. However these materials require certain wavelengths of light to start conducting. In heat sensitive ones infra-red or higher is required, and in light sensitive ones it needs visible light or higher. This is how LEDs and thermistors work, as well as loads of other cool stuff.
Sorry its so long, it's a big topic. I've given a fairly simplified view of it here but if you want to know more you should definitely look up the orbital structures of atoms, the photoelectric effect and band theory.
Still I am putting this, hope this helps a bit.