Difference between Intrinsic & Extrinsic Behaviour of a Semiconductor

[Claude Bile]

1. What is the difference between extrinsic and intrinsic behaviour in a semiconductor? How can you determine what temperature a semiconductor material such as Silicon will revert to intrinsic behaviour, given the donor, intrinsic carrier concentration and the energy gap?

- I am thinking that intrinsic/extrinsic behaviour is related to the Fermi level and the donor and acceptor concentrations at a temperature, T. Intrinsic behaviour has the Fermi level close to midway between the valence and conduction bands, with nearly equal concentrations of donor and acceptor ions. Extrinsic behaviour has the Fermi level close to the conduction band (or valence band). I am looking for confirmation as to whether this is correct, and if not, where I am going wrong.

- I do not have an equation that gives a direct answer to this question. The best I have managed is gaining approximate expressions for the chemical potential for both intrinsic and extrinsic behaviour and showing at which temperature they are both valid. My main difficulty is that we are only given the intrinsic carrier concentration at room temperature and are not told how it varies with temperature. Is there an equation that gives this also, with only the information given above as stated in the 1st paragraph?

 

[Valerie Park]

The difference between extrinsic and intrinsic behaviour in a semiconductor is that extrinsic behaviour occurs when donor or acceptor impurities are added to the semiconductor, while intrinsic behaviour occurs without any impurities. The temperature at which a semiconductor material such as Silicon reverts to intrinsic behaviour can be determined by calculating the Fermi Level of the material at different temperatures using the donor, intrinsic carrier concentration and the energy gap. By plotting the Fermi Level against temperature, the temperature at which the Fermi Level is midway between the conduction and valence band can be determined, which is the temperature at which the semiconductor reverts to intrinsic behaviour.