What is the difference between a conduction and valence band ? How an electron will behave different in conduction and valence bands ? When an electron leaves an atom the atom becomes ion. Does it mean that during conduction those atoms become ions whose electrons are taking part in conduction ? Are free electrons are electrons which take part in conduction ?
[Physics] Conduction and Valence band
electronic-band-theory
Related Solutions
One could write a novel about those questions... I'll try to nail down the most important facts.
Regarding what you figured out so far:
Basically correct. I would say: Every system of atoms has a quantum mechanical ground state. You can approximately assign an energy to each of the electrons (depending on the approximation you are using, e.g. Hartree-Fock or density-functional theory).
Bands are a fancy way of plotting those levels in case of a periodic crystal lattice. The k-axis, called crystal momentum, should only be understood as a quantum number or an index. It is NOT the momentum.
The forbidden band is not an actual band - it marks the absence of bands. That's why you call it band gap. The band gap/forbidden band is between the valence bands (=lower, filled bands) and the conduction bands (=upper, not filled bands). The band gap may also be nonexistent (metals).
It's not "closest to the atom", and there are no electrons there because there are no states that they can occupy (it's a gap).
The valence band is essentially fully occupied. This implies (nontrivially) that for every electron moving in one direction, there is an electron moving in the exact other direction. Therefore, there is no conduction. If an electron jumps (for whatever reason) into the conduction band, it doesn't have the aforementioned partner there - thus, it conducts. The same goes for the hole it leaves behind. One can show that a single "absent electron" behaves like a positive charge governed by the same equations as the electron. That's what you call a hole.
It's below the band gap (if the latter one even exists).
Essentially, all electrons of the atom can move throughout the material. The probability that this happens is not equal for all electrons though. So, doesn't really take them any energy.
That's correct. I'm not aware of a temperature dependence of the band gap. Temperature makes it easier to jump over the gap, though. (Edit by @lemon: the band gap actually decreases almost linearly with increasing temperature (at least for silicon and germanium))
Concerning the questions:
Like I mentioned before, if you remove an electron from a band, it leaves behind a quasiparticle that acts like an electron with the opposite charge. This is what you call a hole.
One band can always hold 2 electrons per crystal unit cell. If a crystal has 8 atoms per unit cell, there will be four filled bands. This comes from the Pauli principle which states that a quantum mechanical state can only be occupied by 1 electron, or 2, if you count spin degeneracy. When a state in the band structure is occupied by 2 electrons, there can't be another one there. The states will be filled from bottom to top (concerning energy). The energy of the topmost filled state is called Fermi energy.
All the electrons in the system are free to move, in principal. The problem is, as I explained in 3), that a full band does not conduct. Only if an electron "jumps" into the valence band, it can conduct (and the hole it leaves behind will also conduct).
Every electron can move. Holes can move as well as electrons. Mind though, that a hole is only a missing electron which behaves equally to an electron with opposite charge (imagine that you have 100 people in a room, and everyone has a ball. Nothing will ever change. If you take away one ball, the person without a ball can be given a ball from the person next to him, and it will be as if the "hole" moved).
The picture illustrates the concept of a band structure quite nicely:
- The k axis (horizontal axis) is the k vector, it's just a quantum number/an index. I won't go into detail about that (look into the Bloch theorem if you want to know more).
- There are some energies below the pictures which "belong" to the core electrons (1s). Their probability of moving between the atoms is very small, and the energy needed to get them to the valence band is very high (so they can't get up).
- Every point in the diagram that belongs to a solid line marks a quantum state. The white spaces between don't have states. Only the solid lines.
- The gray regions are the forbidden regions = band gaps. As you can see, there are no bands there.
- The dashed line marks the Fermi energy, the highest occupied energy. The white area below marks the energy region where there are occupied states (=solid lines). This area is full of electrons, it's the valence band.
- The bands in the top white area are the conduction states. If an electron doesn't jump up from the valence band, there is no electron up there.
Mind that "band" can mean "one solid line" as well and "a bunch of solid lines". The conduction band and the valence band are actually a bunch of bands (=a bunch of solid lines).
Semiconductors can be split into two groups. Intrinsic semiconductors have a band gap that is around thermal energies, so a few electrons can be promoted from the valence to conduction band at room temperature. This corresponds to the third picture from the left in your post.
Extrinsic semiconductors have had a dopand added, and this creates new states in the band gap. These extra states can either accept electrons from the valence band or donate electrons to the conduction band. In the former case you get conduction due to holes in the valence band (p type) and in the latter you get conduction due to electrons in the conduction band (n type). This corresponds to the rightmost picture in your post, though whether the dopant states form a band is debatable, though maybe this is just terminology. Note that conduction is movement on holes or electrons in the semiconductor valence or condustion bands, and not due to transport in the dopant states.
Now to your questions:
I suppose all semiconductors have some intrinsic semiconduction, but for an extrinsic semiconductor this is usually negligable. The conductivity is dominated by the doping.
I think this is covered by the into above. An extrinsic semiconductor has either holes in the valance band and an empty conduction band, or electrons in the conduction band and a full valence band, but not both.
In a p type semiconductor I suppose you could excite electrons from lower bands into the holes in the valence band, but the energies required are far greater than thermal energy so this doesn't happen at room temperature. In n type or intrinsic semiconductors you can excite electrons from inner bands into the valence band because the valence band is full.
Extrinsic semiconductivity isn't based on electrons jumping between the valence and conduction bands.
Best Answer
One has to keep in mind that electrons and atoms belong to the realm of quantum mechanics.
In quantum mechanics one has possible solution of the appropriate QM equations with the correct boundary conditions and potentials. Some are approximate models,like the band theory for solids, but they have been very successful in describing and predicting the behavior of solid state matter built up in lattices.
The basic premise in the band theory is that the whole ensemble of mocules and atoms are in one quantum mechanical state, where some of the electrons are bound to the atoms/molecules in the lattice positions, and some are shared by the whole lattice.
What is the difference between a conduction and valence band ?
The assignment of mobility to the higher energy levels of the lattice solution, as in the image.
Because the electron in the valence band is not shared with the whole lattice but is localized at the lattice points. One has to study the mathematics to get a true understanding.
That is true for free atoms.
Handwaving: the atom shares the conduction electrons that leave its neighbors in the lattice.
There are no free electrons in a solid. Just some electrons are bound by the whole solid lattice and shared with the atoms/molecules (conduction band) and the rest are localized at the lattice site of the atom (valence).