Formation of a pn junction — immediately after a p-type region and an n-type region are brought into contact, which microscopic process dominates to establish the depletion region and built-in potential?

Introduction / Context:
Creating a pn junction by joining p-type and n-type semiconductors triggers charge-carrier movement that sets up a depletion region and a built-in electric field. Recognizing the first microscopic events at the interface helps explain rectification and diode I-V characteristics.

Given Data / Assumptions:

• Freshly formed pn junction at equilibrium (no external bias).
• p-type: high hole concentration; n-type: high electron concentration.
• Lattice is already crystallized; we are examining carriers at the interface.

Concept / Approach:
Once the regions touch, majority carriers diffuse: electrons from the n-side move into the p-side, and holes from the p-side move into the n-side. When an electron and a hole meet, they recombine, eliminating free carriers in a narrow zone. This creates the depletion region, leaving behind fixed ionized donors and acceptors whose charges establish the built-in electric field and potential barrier. The process continues until drift due to the electric field balances diffusion, reaching equilibrium.

Step-by-Step Solution:

Verification / Alternative check:
Standard band diagrams show band bending and a built-in potential V_bi with a space-charge region depleted of mobile carriers, consistent with initial recombination and diffusion.

Why Other Options Are Wrong:

• Covalent bonding reformation: The lattice is already bonded; the key early phenomenon is carrier diffusion and recombination.
• Bulk crystallization: Happens during wafer growth, not junction formation in processed material.
• Catastrophic avalanche breakdown: A high-field failure mode; not a normal initial event at equilibrium.

Common Pitfalls:
Thinking that breakdown or large currents are necessary to form depletion; in reality, equilibrium diffusion and recombination suffice.

Final Answer:
recombination across the junction interface