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The energy band diagram is a model for describing why an insulator is not a good conductor compared to a semiconductor: in the first case, electrons would need to pass a very large band gap in order to promote from the valence band to the conduction band. The energy required for this is simply too large (and would lead to too much heat in the material).

For the semiconductor, on the other hand, the energy is smaller and electrons can move from the valence band to the conduction band by thermal excitation.

Fine, but which of the following figures is "more" correct, the one where the conduction is of the same thickness, or the one where the conduction bands' upper levels are the same?

The first one is more often used, but that might not mean too much. Any thoughts or ideas?

Energy band diagrams

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I made this one. I modified an earlier version a bit (mostly to reduce the length of the vector graphics code). My figure is not trying to say that the upper levels are the same. The figure tries to show that there is no upper limit for the conduction band, that there is no next gap (in the usual case, like here).

enter image description here

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    $\begingroup$ Interesting point; my initial thought also was that there is no upper limit for the conduction band, what I not thought of is the "lower-reaching" valence band $\endgroup$ – Alf Feb 12 '20 at 9:11
  • $\begingroup$ @Alf The position of the valence band was not very intentional. The nice thing about this compact SVG file is that one can understand the code (you can see it in your browser by "view source") and that anyone can modify it with a simple text editor. $\endgroup$ – user137289 Feb 12 '20 at 9:40
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The upper bound of conduction band is $\infty$ so it doesn't matter which of the figure is correct and which one is incorrect.

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  • $\begingroup$ doesn't your answer (upper band of conduction band being infinity) then implies that the left version of my plot is misleading, as the upper end of the conduction bands is different for each type of conductor? $\endgroup$ – Alf Feb 12 '20 at 12:07
  • $\begingroup$ For real band structures (not the cartoon versions above), unfilled bands are of finite extent until you get close to the vacuum continuum. $\endgroup$ – Jon Custer Feb 12 '20 at 14:03
  • $\begingroup$ YesI think your first figure is little bit misleading as Upper bound get lower and lower. $\endgroup$ – Young Kindaichi Feb 12 '20 at 16:55

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