GATE EC 2019 Set 1 — Question 18
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Electronic Devices → Semiconductor Physics → Energy Bands
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Question
Which one of the following options describes correctly the equilibrium band diagram at of a Silicon configuration shown in the figure?
Correct answer
(A) [figure]
Solution
In thermal equilibrium, the Fermi level () must be constant (flat) throughout the entire semiconductor device. This immediately eliminates option (B), where the Fermi level is not flat.The device has a configuration. The position of the Fermi level relative to the band edges ( and ) depends on the doping type and concentration:
1.-type region: is located near the valence band edge .
2.-type region: is located near the conduction band edge .
3.-type region (heavily doped ): is even closer to than in the moderately doped region.
4.-type region (very heavily doped ): is very close to or even inside the valence band .
Evaluating the remaining options:- Option (A): Shows a flat . In the region, is near . In the region, is near . In the region, the conduction band moves even closer to . In the region, the valence band moves very close to . This perfectly matches the doping profile.
- Option (C): Shows a flat , but it fails to distinguish between the and regions or the and regions correctly in terms of band bending relative to the Fermi level.
- Option (D): Shows in the middle of the bandgap for the region, which would represent an intrinsic semiconductor, not a -doped one.
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