Forward and Reverse Bias — Core Principles

Core Principles

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.

Important Differences

vs Reverse Bias

Aspect	This Topic	Reverse Bias
Connection Polarity	P-type to positive terminal, N-type to negative terminal.	P-type to negative terminal, N-type to positive terminal.
Depletion Region Width	Narrows significantly.	Widens significantly.
Potential Barrier	Decreases (effective barrier $V_0 - V_F$).	Increases (effective barrier $V_0 + V_R$).
Current Flow	Large current due to majority carriers (exponential increase after cut-in voltage).	Very small current (reverse saturation current) due to minority carriers (almost constant until breakdown).
Diode Behavior	Acts like a closed switch (low resistance).	Acts like an open switch (high resistance).
I-V Curve Quadrant	First quadrant.	Third quadrant.

Forward bias facilitates current flow by reducing the potential barrier and narrowing the depletion region, making the diode behave like a conductor. This is achieved by connecting the p-side to a higher potential. Conversely, reverse bias obstructs current flow by increasing the potential barrier and widening the depletion region, causing the diode to act as an insulator. This occurs when the p-side is connected to a lower potential. The fundamental difference lies in how the external voltage interacts with the internal electric field of the junction, either aiding or opposing majority carrier movement.