Physics Help Forum Band gap

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 Dec 7th 2016, 10:10 PM #1 Junior Member   Join Date: Oct 2016 Posts: 3 Band gap ZnO is a semiconductor material that has a band gap of 310 kJ/mol. What is the longest wavelength of light, in nm, that will excite an electron from the valence band to the conductance band?
Dec 8th 2016, 10:19 AM   #2

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 Originally Posted by starrysky ZnO is a semiconductor material that has a band gap of 310 kJ/mol. What is the longest wavelength of light, in nm, that will excite an electron from the valence band to the conductance band?
Your unit is wrong. A band gap is defined as the energy to promote an electron from the top/bottom of one band (valence, conduction, etc) to the bottom/top of another. kJ/mol is not an energy.

310 kJ is an awesomely huge band gap. Might we be talking about 310 x 6 x 10^{-23} kJ? That's an awesomely small band gap. I'll leave that one for you to talk to your instructor about.

Anyway, we know that the energy of the photon is $\displaystyle E = \frac{hc}{\lambda}$. So the largest possible $\displaystyle \lambda$ will be for the smallest possible energy jump. So what's E?

-Dan
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 Mar 28th 2019, 11:03 PM #3 Junior Member   Join Date: Mar 2019 Posts: 2 I would like to question here. What is band gap ?
 Mar 29th 2019, 03:22 AM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 964 Electrons in a material can adopt certain energies, but not others. An analogy might be an organ pipe It will give a basic note, but if you blow harder it can be made to give a higher note, harder still gives an even higher note, etc... (until the pipe bursts) But it won't give you any notes in between (that needs a different organ pipe). Similarly for electrons, if you pump energy into the material, the electrons can jump from their normal energy level to a higher energy level. However they can only take certain very particular energy levels. The band gap is the difference between two electron energy levels, particularly in a semiconductor material. topsquark likes this. __________________ ~\o/~
 Mar 29th 2019, 08:59 PM #5 Junior Member   Join Date: Mar 2019 Posts: 2 Thanks woody for the answer. __________________ GRE Classes
 Apr 7th 2019, 11:17 AM #6 Junior Member   Join Date: Apr 2019 Location: Oregon,US Posts: 11 Hmm we cant be sure .We dont know the exact energy of the valence electron.

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