Formation of the depletion region in a p–n junction What physical process primarily creates the depletion region at a freshly formed p–n junction?

Introduction / Context:
The depletion region is the heart of p–n junction behavior. It defines the built-in electric field and barrier that control current flow in diodes and transistors.

Given Data / Assumptions:

• Freshly joined p-type and n-type regions with concentration gradients.
• No external bias initially (thermal equilibrium).

Concept / Approach:
Immediately after forming a p–n junction, carriers diffuse from high- to low-concentration regions: electrons diffuse from n to p, and holes from p to n. As carriers recombine, they leave behind fixed, ionized dopant atoms (donors positive, acceptors negative). This creates a space-charge region devoid of free carriers—the depletion region—and sets up an internal electric field that opposes further diffusion. The resulting potential is the built-in (barrier) potential.

Step-by-Step Solution:
Initial condition: carrier concentration gradient across the junction.Process: diffusion of electrons and holes across the boundary.Outcome: recombination leaves uncovered ions, forming space charge.Equilibrium: built-in field and barrier potential balance diffusion → depletion region established.

Verification / Alternative check:
Solving the Poisson equation with dopant profiles shows a space-charge region whose width depends on doping levels, confirming diffusion-driven formation.

Why Other Options Are Wrong:
Doping: sets concentrations but does not by itself create the region.Barrier potential: a result of diffusion and charge separation, not the initiating process.Ions: they are the immobile charges left behind, again a result, not the cause.

Common Pitfalls:
Mixing cause and effect; diffusion creates the charge separation that yields the barrier and depletion region.

Final Answer:
diffusion