What is the primary difference in the depletion region under forward and reverse bias?

Under forward bias, the external voltage pushes majority carriers towards the junction, effectively neutralizing some of the immobile ions in the depletion region. This causes the depletion region to narrow significantly. Conversely, under reverse bias, the external voltage pulls majority carriers away from the junction, exposing more immobile ions. This action causes the depletion region to widen considerably. The width of the depletion region directly impacts the potential barrier and the diode's ability to conduct current.

Why does a p-n junction conduct current easily in forward bias but not in reverse bias?

In forward bias, the applied external voltage opposes the internal potential barrier, effectively reducing its height. This allows majority charge carriers (electrons from n-side, holes from p-side) to easily overcome the reduced barrier and cross the junction, leading to a large current. In reverse bias, the applied voltage reinforces the internal potential barrier, increasing its height and widening the depletion region. This makes it extremely difficult for majority carriers to cross, thus blocking significant current flow. Only a small leakage current due to minority carriers flows.

What is the significance of the 'cut-in voltage' in forward bias?

The cut-in voltage (also known as knee voltage or threshold voltage) is the minimum forward bias voltage required across a p-n junction to cause a significant increase in forward current. Below this voltage, the current is very small because the applied voltage is not yet sufficient to overcome the internal potential barrier. For silicon diodes, it's typically around $0.7, ext{V}$, and for germanium diodes, it's about $0.3, ext{V}$. Once the applied voltage exceeds the cut-in voltage, the current rises exponentially.

What is reverse saturation current and why is it important?

Reverse saturation current ($I_0$ or $I_S$) is the small, almost constant current that flows through a reverse-biased p-n junction. It is primarily due to the thermal generation of minority charge carriers (electrons in the p-type, holes in the n-type) which are then swept across the junction by the strong electric field. It's important because it indicates that even in reverse bias, a diode is not a perfect insulator. Its magnitude is highly temperature-dependent and is a crucial parameter in diode specifications, especially for low-power applications.

What happens if the reverse bias voltage exceeds the breakdown voltage?

If the reverse bias voltage exceeds the breakdown voltage ($V_{BR}$), the diode experiences a sudden and sharp increase in reverse current. This can occur due to Zener breakdown (direct pulling of electrons from bonds in heavily doped junctions) or avalanche breakdown (collision ionization in lightly doped junctions). While this can damage a conventional diode if the current is not limited, Zener diodes are specifically designed to operate in this region to provide voltage regulation, maintaining a nearly constant voltage across their terminals despite varying current.

Forward and Reverse Bias

Physics

NEET UG

Version 1Updated 23 Mar 2026

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Forward bias and reverse bias are the two fundamental modes of operation for a p-n junction diode, dictating its ability to conduct electric current. When a p-n junction is forward biased, an external voltage is applied across it such that the p-type semiconductor is connected to the positive terminal and the n-type to the negative terminal of a power source. This configuration reduces the width o…

Quick Summary

Forward and reverse bias are the two operational modes of a p-n junction diode. In forward bias, the p-type is connected to the positive terminal and the n-type to the negative terminal of a voltage source.

This configuration reduces the internal potential barrier and narrows the depletion region, allowing majority charge carriers to flow easily across the junction, resulting in a large forward current once the applied voltage exceeds the cut-in voltage (e.

g., $0.7,\text{V}$ for silicon). The current increases exponentially with voltage. In reverse bias, the p-type is connected to the negative terminal and the n-type to the positive terminal. This increases the potential barrier and widens the depletion region, effectively blocking the flow of majority carriers.

Only a very small reverse saturation current, primarily due to minority carriers, flows. This current is largely independent of voltage until the breakdown voltage is reached, where the current sharply increases.

The I-V characteristics show this rectifying behavior, making diodes essential for converting AC to DC and other electronic functions.

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Forward Bias — P-side to positive, N-side to negative.

- Depletion region: Narrows. - Potential barrier: Decreases ( $V_0 - V_F$ ). - Current: Large, exponential (majority carriers). - Diode: Low resistance, 'ON'.

Reverse Bias — P-side to negative, N-side to positive.

- Depletion region: Widens. - Potential barrier: Increases ( $V_0 + V_R$ ). - Current: Very small (minority carriers, $I_0$ ). - Diode: High resistance, 'OFF'.

Cut-in Voltage ($V_{knee}$) —

approx 0.7,\text{V}

(Si),

approx 0.3,\text{V}

(Ge).

Reverse Saturation Current ($I_0$) — Doubles for every

10^circ\text{C}

rise.

Breakdown Voltage ($V_{BR}$) — Sudden current increase in reverse bias.

Forward Bias: For Big Current, Narrow Depletion, Lower Barrier. (P to +, N to -) Reverse Bias: Rare Bit of Current, Wide Depletion, Higher Barrier. (P to -, N to +)