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Old Dec 7th 2016, 10:10 PM   #1
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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?
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Old Dec 8th 2016, 10:19 AM   #2
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Originally Posted by starrysky View Post
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?

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Old Mar 28th 2019, 11:03 PM   #3
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I would like to question here.
What is band gap ?
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Old Mar 29th 2019, 03:22 AM   #4
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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.
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Old Mar 29th 2019, 08:59 PM   #5
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Thanks woody for the answer.
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Old Apr 7th 2019, 11:17 AM   #6
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Hmm we cant be sure .We dont know the exact energy of the valence electron.
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