Newbie Questions on Band Gap and Fermi Level

[mzottola]

I am new to solid state physics, having been in computational chemistry (quantum mechanics) for the larger part of my career. Hence have a lot of newbie like questions that are not easily answered by google/wikipedia searching.

1)In viewing a 1-dimensional band gap diagram, crossing the Fermi line from from either the conducting to non conducting zone or vice versa, is indicative of a metallic substance. Is this correct?

2) In band gap diagrams where there is no crossing of the Fermi line/level, the band gap can still be calculated by measuring the energy difference between the closest approach of the bands in the conducting and non-conducting zones?

3) Are there generally accepted band gaps that one can say a system is either conducting/semiconducting/nonconducting?

4) If the Fermi level for a system changes from a positive value to a negative value, is it a reasonable conclusion that the the system with the lower Fermi level would be more likely to allow charge migration, i.e. a current flow?

Any help and recommendations for an easily read primer are also greatly appreciated.

 

[M Quack]

1) yes

2) yes

3) There is a grey zone between non-conducting and wide-gap semiconductors in the range 3-5 eV. Diamond is usally seen as a pretty good insulator, but in some applications it is used and treated like a semiconductor. Similarly, if there is a very small band gap, smaller than kT, then the system may behave like a "bad metal".

4) not always. Think of an electron-doped semiconductor. You would have to increase the Fermi energy to increase the number of carriers. For a hole-doped semiconductor it is the opposite, of course.

 

[mzottola]

Thanks - I appreciate your timely and clear response.

 

[M Quack]

my pleasure