# Conduction, conduction and valence bands, and energy bands

I am currently studying the textbook Physics of Photonics Devices, Second Edition, by Shun Lien Chuang. In a section discussing the basic concepts of semiconductor band and bonding diagrams, I was reminded of some questions that I had in a previous post. Specifically, in that post, in the comments to Ruslan's answer, I had wondered whether conduction is something that occurs in both the valence and conduction bands, or whether it only occurred in one of the bands (only through the conduction band, or only through the valence band). It seems to me that we came to the conclusion that conduction can occur through either of the bands; that is, conduction can occur through either the conduction band or the valence band. My understanding is that, in order for conduction to be possible, we require that there be capacity (holes) available in both the valence and conduction bands, as well as electrons available, obviously, since we, by definition, require charge in order to perform the conduction. Is this understanding correct? Could someone please take the time to give a clear explanation of this, so that my mind is put to rest on this idea?

Note: I was actually going to post a similar, but different, question regarding something I read in the textbook, but I thought it a good idea to post this first, so that I can ensure that I am understanding this concept correctly. This will in turn ensure that my main question is more focused and of higher quality.

• You need partially filled band or bands. Holes have charge. You can have conduction through one carrier type in one band only. Commented Feb 4, 2020 at 18:31
• @JonCuster Yes, that was what I was trying to allude to: partially filled bands. But my impression was that conduction requires and occurs through both bands together; are you saying that conduction can occur through a single band, without the other band? This is contrary to what I thought. Also, what do you mean by "... conduction through one carrier type in one band only"? Commented Feb 4, 2020 at 18:34
• Simple metals, such as Cu and Ag, have a very simple band structure. There are completely filled bands, with no holes in them, which you might call a valence band. There is a half-filled conduction band. That is where conduction takes place, and Hall measurements indicate that the carriers are negative (electrons). Other metals have less-than-ideal (ranging to truly weird) band structure that may have multiple partially filled bands, bands disconnected in $k$-space, and conditions under which Hall measurements indicate positive charge carriers. See Ashcroft & Mermin. Commented Feb 4, 2020 at 20:44
• For metals, there usually is no purpose to naming a 'valence' band - bands are full or they aren't, and the ones that aren't are conduction bands. For semiconductors, distinguishing between valence and conduction bands is important for establishing hole and electron populations and their characteristics such as intrinsic carrier densities, use of doping, etc. Metals don't need that distinction. Commented Feb 4, 2020 at 22:59
• Holes in the valence band, electrons in the conduction band. Commented Feb 5, 2020 at 1:58