Carrier motion in an intrinsic semiconductor under applied voltage When a potential difference is applied across a pure (intrinsic) semiconductor, in which direction do the positively charged 'holes' drift inside the semiconductor body?
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Atowards positive terminal
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Baway from positive terminal
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Cin the external circuit only
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Dnone of the above
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Etowards ground only
Answer
Correct Answer: away from positive terminal
Explanation
Introduction / Context:Charge transport in semiconductors involves electrons and 'holes'. Holes are vacancies in the valence band that behave like positive charges. Knowing their drift direction under an applied electric field is fundamental for understanding diode and transistor operation, current flow, and device polarity conventions.
Given Data / Assumptions:
- Intrinsic (undoped) semiconductor with equal electron and hole concentrations.
- A DC potential is applied across the sample, establishing an internal electric field.
- Hole motion is considered inside the semiconductor, not in external metal leads.
Concept / Approach:
By definition, a hole carries an effective positive charge. In an electric field E directed from the positive terminal to the negative terminal, positive charges accelerate in the direction of E, i.e., toward the negative terminal. Equivalently, they move away from the positive terminal. This is consistent with conventional current direction: holes drift in the same direction as conventional current; electrons drift oppositely.
Step-by-Step Solution:
Apply voltage: establishes E pointing from + to − inside the semiconductor.Force on a positive charge q: F = q * E → motion along E.Therefore, holes drift toward the negative terminal (i.e., away from the positive terminal).Hence the correct qualitative choice is: 'away from positive terminal'.Verification / Alternative check:
PN junction diagrams show hole current flowing from P to N under forward bias, consistent with holes moving toward lower electric potential (negative terminal) within the device bulk.
Why Other Options Are Wrong:
'Towards positive terminal' reverses the direction for positive charges; 'in the external circuit only' is incorrect because holes are carriers inside semiconductor crystals; 'none of the above' and 'towards ground only' are not generally valid statements.
Common Pitfalls:
Confusing electron drift with hole drift; mixing internal semiconductor behavior with external metallic conductor current, where only electrons move physically.
Final Answer:
away from positive terminal