PN junction internal regions: Ionization near the PN interface creates fixed charges on either side. What is the name of the region that forms and becomes depleted of free carriers?

Introduction / Context:
The PN junction is the building block of diodes, BJTs, and many IC structures. Upon contact, diffusion and drift lead to a region with fixed ionized dopants and few mobile carriers. Knowing the name and behavior of this region is key to understanding rectification and junction capacitance.

Given Data / Assumptions:

• A p-type and an n-type semiconductor are brought into contact.
• Thermal equilibrium initially, no external bias.
• Standard silicon or similar semiconductor.

Concept / Approach:

Electrons diffuse from n to p and holes from p to n. They recombine near the interface, leaving behind fixed ionized donors and acceptors. This region lacks mobile carriers and is called the depletion region (or space-charge region). The internal electric field that develops creates a potential barrier (built-in voltage) opposing further diffusion.

Step-by-Step Solution:

Verification / Alternative check:

C-V (capacitance-voltage) measurements reveal depletion width changes with applied bias, confirming the existence and properties of the depletion region.

Why Other Options Are Wrong:

• Junction: generic location where p and n meet; not the specific region depleted of carriers.
• Barrier voltage/forward voltage: potential values across the junction, not the region itself.
• Space-charge neutrality zone: the depletion region is not neutral; it contains fixed charges producing the electric field.

Common Pitfalls:

• Confusing built-in potential with applied forward voltage; they are related but distinct.
• Assuming depletion region contains no charge; it contains fixed ionic charge but few free carriers.

Final Answer:

depletion region