will the change in the offset vary the conductivity. Also, can the value for any of the offset be negative?
Conduction band offsets, as well as valance band offsets, generally define how easily an electron and hole can be transported across the oxide layer. It is basically the energy difference of the band alignment between the oxide layer and the semiconductor substrate. A higher conduction band and valance band offset are required to limit the leakage current issue encountered by oxide materials. The best way to understand the concept of band offset is by referring to the band diagram of a typical MOS capacitor structure.
Thank you! @abdul shekkeer km, it was really helpful. Is there any other property that we can determine using this.
Also, how do we get the value of vacuum level Eo to find the electron affinity?
Hi Nishant Singh Jamwal,
The existence of energy band-offsets comes from the model describing the junction (heterojunction) between two semiconductor materials SC1and SC2 with energy bandgaps Eg1 and Eg2 and with electron affinities X1 and X2, respectively, and both with the same vacuum level Vl1=Vl2=Vl which is the absolute reference level (theory of the electron affinity rule).
Now if we suppose that Eg1X2 (practical case), then the energy band offsets can be easily deduced as :
1. Conduction band offset: delta Ec= X1-X2
2. Valence band offset : delta Ev= (Eg1+X1)-(Eg2+X2)
Here in our example X1>X2, we deduce that :
1. delta Ec>0, since X1>X2 and, therefore Ec10, then Ev2>Ev1 and now the holes are also confined in SC2. This means that the holes have to overcome an energy barrier Ebv=delta Ev in order to go into SC1. We define such a heterojunction where electrons are confined in SC1 and holes are confined in SC2 as a type II-heterojunction.
4. if (Eg1+X1)-(Eg2+X2)>0 and (Eg2+X2)Ec1 which means that holes in SC2 have energies in Ev2 of SC2 higher than electrons in Ec1 of SC1. Therefore holes can move freely from SC2 to SC1 through Tunnel Effect, and such a heterojunction is defined as type III- heterojunction (with tunnel effect).
You can easily apply similar method for other cases depending on actual values of X1, X2, Eg1, Eg2, ..of interest in each case. But in all cases, you keep the vacuum level as the absolute reference level, i.e : Vl1=Vl2
You can also easily generalize to any more complex heterojunction with SC1+Sc2+SC3, ...and so on.
Hope this may help you.
Best regards,
Professor A. Kadri
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