When the diode current is large, the bias is _____.

Question

Examoogle · Accepted Answer

Forward

Explanation: When a semiconductor diode is forward-biased, the external voltage opposes the built-in potential, drastically reducing the depletion region width. This allows a large number of majority charge carriers to diffuse across the junction, resulting in a large diode current.

Working Principle: In forward bias, the positive terminal of the battery is connected to the p-type region and the negative to the n-type region. This configuration pushes holes toward the junction and electrons toward the junction, shrinking the potential barrier. Consequently, the exponential relationship defined by the Shockley diode equation governs the current flow, allowing it to increase rapidly once the threshold voltage is exceeded.

Key Formulas: $I_D = I_S (e^{\frac{V_D}{\eta V_T}} - 1)$ — Shockley diode equation relating diode current to voltage; $V_T = \frac{kT}{q}$ — Thermal voltage at room temperature

Key Points: Forward bias narrows the depletion region width.; The resistance of a forward-biased diode is very low.; Current in forward bias follows an exponential dependence on voltage.; Threshold voltage (knee voltage) is typically 0.7V for Silicon diodes.

Advantages: Efficient current conduction in forward direction.; High switching speed compared to mechanical switches.

Disadvantages: Small power loss due to forward voltage drop ($V_f 	imes I_f$).; Thermal runaway risk if not properly heat-sinked.

Applications: Rectification in power supplies.; Signal demodulation in communication circuits.; Voltage clamping and clipper circuits.

Additional Info: At large currents, the diode behaves approximately like a constant voltage source in series with a small bulk resistance.. Option B, C, and D are incorrect because 'Inverse' and 'Reverse' bias result in negligible (leakage) current, and 'Poor' bias is not a technical term.

Answer: A is correct — A large diode current occurs when the diode is in forward bias, as the potential barrier is effectively reduced to permit high charge carrier flow.

Exam Tip: Always remember the exponential relationship $I \propto e^V$; it explains why forward current increases so rapidly compared to reverse leakage current.

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