0
$\begingroup$

Does doping a semiconductor with acceptor impurities affect the number of states in the valence and conduction bands? How about doping with donor impurities?

It seems to me, that in the former case the answer should be yes, adding acceptors should decrease the number of band states while in the latter I think the answer should be no. The donor impurities add electrons to the system as well as new levels inside the gap, while the accpetor impurities decrease the number of electrons and convert some of the band states into acceptor states inside the gap. In this way, at zero temperature we always have all valence band states full and all conduction band states empty. Otheriwise, we would end up with conduction electrons or holes present even in the ground state. I am not sure however if this is correct.

$\endgroup$
2
  • $\begingroup$ Recall that typical dopant concentrations are in the parts per million range. The bit about zero temperature makes no sense, since any 'excess' carriers will be captured by the dopant levels you introduced in the first place. $\endgroup$ – Jon Custer Feb 9 at 20:47
  • $\begingroup$ @JonCuster i think the zero temperature that is referred is about the example of Boron in the answer i provided. You are correct about the PPM range. I mean if someone wants to change certain attributes of semi-conductors then a precise recipe of impurities would occur and it should be in the form of PPM. $\endgroup$ – user141306 Feb 10 at 18:12
0
$\begingroup$

In general, doping a semiconductor with impurities affect the number of states in the valence and conduction bands. The type of impurities determine the band (valence/conduction) and the number of states.

Bulk band structure for Si, Ge, GaAs and InAs generated with tight binding model. Note that Si and Ge are indirect band gap with minima at X and L, while GaAs and InAs are direct band gap materials.Band diagram of PN junction operation in forward bias mode showing reducing depletion width. Both p and n junctions are doped at a 1×1015/cm3 doping level, leading to built-in potential of ~0.59 V. Reducing depletion width can be inferred from the shrinking charge profile, as fewer dopants are exposed with increasing forward bias.

The following is taken from wikipedia's Doping and Boron

Doping a semiconductor in a good crystal introduces allowed energy states within the band gap, but very close to the energy band that corresponds to the dopant type. In other words, electron donor impurities create states near the conduction band while electron acceptor impurities create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-site bonding energy or EB and is relatively small. For example, the EB for boron in silicon bulk is 0.045 eV, compared with silicon's band gap of about 1.12 eV. Because EB is so small, room temperature is hot enough to thermally ionize practically all of the dopant atoms and create free charge carriers in the conduction or valence bands.

Boron is a useful dopant for such semiconductors as silicon, germanium, and silicon carbide. Having one fewer valence electron than the host atom, it donates a hole resulting in p-type conductivity.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.