For a P–N diode to conduct in the forward direction, which polarity must be applied to the anode and to the cathode?
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ANegative on the cathode and positive on the anode (forward bias)
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BNegative on the anode and positive on the cathode
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CPositive on both anode and cathode
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DNegative on both anode and cathode
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EAlternating polarities at high frequency only
Answer
Correct Answer: Negative on the cathode and positive on the anode (forward bias)
Explanation
Introduction / Context:Understanding diode biasing is essential for rectifiers, clippers, clampers, and switching circuits. Forward biasing a diode allows current to flow easily from anode to cathode, while reverse biasing blocks current (except for a small leakage current).
Given Data / Assumptions:
- Ideal P–N junction diode model is assumed.
- Forward bias condition lowers the junction barrier.
- Real silicon diodes exhibit approximately 0.7 V drop when forward-conducting at moderate current.
Concept / Approach:A diode conducts when its P-side (anode) is at a higher electric potential than its N-side (cathode). Equivalently, the cathode must be more negative than the anode so that carriers overcome the junction barrier and the device conducts.
Step-by-Step Solution:1) Identify terminals: anode (P-type), cathode (N-type).2) Forward bias rule: anode at higher potential than cathode.3) Translate to polarities: positive on anode, negative on cathode.4) Conclude that the correct statement is “negative on the cathode and positive on the anode.”
Verification / Alternative check:In rectifier circuits, the diode conducts when the anode side of the AC cycle goes positive with respect to the cathode. This practical observation matches the forward-bias rule.
Why Other Options Are Wrong:Negative on the anode and positive on the cathode: this is reverse bias.Positive on both terminals: does not guarantee forward bias; polarity difference matters.Negative on both terminals: again, no forward conduction is guaranteed.High-frequency alternation only: frequency does not change the basic biasing requirement.
Common Pitfalls:Confusing the arrow on diode symbols (used for transistors) or thinking the stripe mark (cathode) should be tied to positive for conduction. It is the potential difference that matters: anode must be more positive than cathode.
Final Answer:Negative on the cathode and positive on the anode (forward bias)