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?
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Arecombination across the junction interface
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Bcovalent bonding reformation of the entire lattice
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Cbulk crystallization of silicon
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Dcatastrophic avalanche breakdown
Answer
Correct Answer: recombination across the junction interface
Explanation
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:
Contact p and n regions → start of majority-carrier diffusion.Electron-hole pairs meet at the interface and recombine.Recombination removes mobile carriers near the junction, forming depletion.Fixed charges create an internal field and built-in voltage; equilibrium occurs when diffusion equals drift.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