p-n Junction — Core Principles

Core Principles

A p-n junction is formed by joining p-type and n-type semiconductors. At the interface, electrons from the n-side and holes from the p-side diffuse and recombine, creating a 'depletion region' devoid of mobile charge carriers but containing immobile ions.

These ions establish an internal electric field and a 'barrier potential' (e.g., $0.7\,\text{V}$ for Si, $0.3\,\text{V}$ for Ge) that opposes further majority carrier diffusion. When forward biased (p-side positive, n-side negative), the external voltage reduces the barrier and depletion width, allowing significant majority carrier current.

When reverse biased (p-side negative, n-side positive), the external voltage increases the barrier and depletion width, allowing only a tiny 'reverse saturation current' due to minority carriers. Beyond a certain reverse voltage, breakdown occurs (Zener or Avalanche), leading to a sharp increase in current.

This unidirectional conduction makes the p-n junction a fundamental component in diodes and other semiconductor devices.

Often confused with

Side-by-side differences the NEET paper likes to test.

p-n Junction vs Forward Bias vs. Reverse Bias of a p-n Junction

Aspect	p-n Junction	Forward Bias vs. Reverse Bias of a p-n Junction
External Voltage Polarity	Positive terminal to p-side, negative to n-side.	Negative terminal to p-side, positive to n-side.
Effect on Barrier Potential	Reduces the effective barrier potential.	Increases the effective barrier potential.
Effect on Depletion Region Width	Decreases the width of the depletion region.	Increases the width of the depletion region.
Current Carriers	Primarily majority carriers (electrons from n-side, holes from p-side).	Primarily minority carriers (electrons from p-side, holes from n-side).
Magnitude of Current	Significant current, increases exponentially after knee voltage.	Very small, almost constant reverse saturation current.
Resistance Offered	Low resistance.	High resistance (ideally infinite until breakdown).
Application	Allows current flow, used in rectification, LEDs.	Blocks current flow, used in Zener diodes (at breakdown), switching.

The fundamental distinction between forward and reverse biasing of a p-n junction lies in how the external voltage interacts with the internal barrier potential. Forward bias reduces the barrier, narrows the depletion region, and facilitates a large current flow primarily by majority carriers, exhibiting low resistance.

Conversely, reverse bias increases the barrier, widens the depletion region, and allows only a minuscule current due to minority carriers, presenting very high resistance. This differential behavior is what enables the p-n junction to act as a diode, a crucial component for controlling current direction in electronic circuits.

Why it is tested: For NEET, understanding this difference is paramount as it explains the diode's rectifying action, its I-V characteristics, and the operational principles of various semiconductor devices. Questions frequently test the effects of biasing on depletion width, barrier potential, and current flow direction/magnitude.