Why is aligning the vacuum level necessary in band diagram creation?

[Swapnil]

Why is it that we align the vacuume level (when drawing the band diagram) first before we put two pieces of semiconductor together?

(My book talks about how the energy of the two free electrons should be the same when each of them jumps out of its respective material...but it is not entrirely clear)

 

[Swapnil]

anyone? please...

 

[marlon]

The vacuum level is the reference level. All calculated energies are expressed with respect to this level. It needs to be aligned because otherwise one cannot compare what happens to the bands (change in energies, work functions, band bending etc etc) when contact is made.

Also, after making contact, thermal equilibrium is reached and the Fermi level (Ef) is aligned and thus the same throughout the entire interface. The Fermi level has the same value (ie no net charge transfer in the interface). For example if the Ef is -6 eV it means that the Ef is 6 eV below the vacuum level which is the 0 reference level. you see ?

marlon

 

[Swapnil]

I see. Thank you.