GATE EC 2016 Set 1 — Question 21
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Electronic Devices → Semiconductor Physics → Energy Bands
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Question
A small percentage of impurity is added to an intrinsic semiconductor at K. Which one of the following statements is true for the energy band diagram shown in the following figure?
Correct answer
(A) Intrinsic semiconductor doped with pentavalent atoms to form n-type semiconductor
Solution
The energy band diagram shows a 'New Energy Level' at eV below the conduction band (). This energy level is very close to the conduction band.
(A) Intrinsic semiconductor doped with pentavalent atoms to form n-type semiconductor: This matches our analysis.
(B) Intrinsic semiconductor doped with trivalent atoms to form n-type semiconductor: Trivalent atoms form p-type semiconductors, not n-type.
(C) Intrinsic semiconductor doped with pentavalent atoms to form p-type semiconductor: Pentavalent atoms form n-type semiconductors, not p-type.
(D) Intrinsic semiconductor doped with trivalent atoms to form p-type semiconductor: Trivalent atoms form p-type semiconductors, but the energy level shown is for an n-type material.The final answer is .
1.Donor Level: An energy level very close to the conduction band is characteristic of a donor impurity. Donor impurities are atoms that can easily donate electrons to the conduction band.
2.Pentavalent Atoms: Pentavalent atoms (like Phosphorus, Arsenic, Antimony) have five valence electrons. When introduced into a silicon (tetravalent) crystal lattice, four electrons form covalent bonds, and the fifth electron is loosely bound. This fifth electron requires very little energy to move into the conduction band, creating a donor energy level just below the conduction band.
3.n-type Semiconductor: When an intrinsic semiconductor is doped with donor impurities (pentavalent atoms), it becomes an n-type semiconductor, where electrons are the majority carriers.
Therefore, the diagram represents an intrinsic semiconductor doped with pentavalent atoms to form an n-type semiconductor.Let's evaluate the options:(A) Intrinsic semiconductor doped with pentavalent atoms to form n-type semiconductor: This matches our analysis.
(B) Intrinsic semiconductor doped with trivalent atoms to form n-type semiconductor: Trivalent atoms form p-type semiconductors, not n-type.
(C) Intrinsic semiconductor doped with pentavalent atoms to form p-type semiconductor: Pentavalent atoms form n-type semiconductors, not p-type.
(D) Intrinsic semiconductor doped with trivalent atoms to form p-type semiconductor: Trivalent atoms form p-type semiconductors, but the energy level shown is for an n-type material.The final answer is .
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