PN junction physics — origin of the depletion region: What primary factor creates the depletion layer at a semiconductor junction?

Introduction / Context:
The depletion region is central to diode operation. It dictates junction capacitance, barrier potential, and conduction behavior. Understanding what physically constitutes this region clarifies how diodes and transistors function.

Given Data / Assumptions:

• Standard PN junction formed by adjacent P-type and N-type regions.
• Thermal equilibrium without external bias.
• Basic semiconductor processes apply.

Concept / Approach:
Initially, carriers diffuse across the junction, and many electrons and holes recombine. This leaves behind uncovered, immobile ionized donors on the N side and ionized acceptors on the P side. These fixed charges (ions) create an internal electric field and the barrier potential that opposes further diffusion. Hence, the depletion layer is the region depleted of mobile carriers but rich in fixed ions.

Step-by-Step Solution:
Carrier diffusion occurs from high to low concentration.Recombination removes mobile carriers near the junction.Uncovered dopant atoms become fixed ions, establishing space charge.This space charge region is the depletion layer.

Verification / Alternative check:
Textbook cross-sections show ionized donors/acceptors as the source of the built-in field; the field magnitude corresponds to the barrier potential at equilibrium.

Why Other Options Are Wrong:
(a) Doping enables the effect but does not itself constitute the layer; (b) recombination is a process that leads to ions being exposed; (c) the barrier potential is a consequence of the ionized region, not its cause.

Common Pitfalls:
Confusing cause and effect: recombination and diffusion lead to the ionized region; the ions create the field and barrier, which then limit further diffusion.

Final Answer:
Ions.