Formation of a PN junction: A PN junction is created at what point in semiconductor fabrication or structure?

Introduction / Context:
The PN junction is the fundamental building block of diodes, BJTs, many FET structures, LEDs, and solar cells. Understanding how a junction forms clarifies depletion regions, built-in potentials, and rectification behavior observed under bias.

Given Data / Assumptions:

• We consider crystalline silicon as the base material.
• One side is doped p-type (acceptors); the other, n-type (donors).
• Initially at thermal equilibrium (no external bias).

Concept / Approach:
When p-type and n-type regions meet, carriers diffuse: electrons move from n to p, holes from p to n. This leaves behind fixed ionized dopants, creating a space-charge region (depletion region) and a built-in electric field. The physical interface where these regions meet is the PN junction; the depletion region is a consequence of formation, not the definition of the junction itself.

Step-by-Step Solution:

Verification / Alternative check:
Capacitance–voltage profiling reveals the depletion width and confirms junction location. Rectifying I–V behavior under forward and reverse bias further verifies the presence of a PN junction.

Why Other Options Are Wrong:

• Depletion region: A result of the junction, not its definition.
• Large reverse biased region: Bias condition, not formation.
• Forward voltage drop: Operating behavior under forward bias, not formation.

Common Pitfalls:
Equating the depletion region with the entire junction; the junction is the interface, whereas depletion is the charge-separated zone surrounding it.

Final Answer:
the point at which two opposite doped materials come together