A PN junction forms where a p-type region (rich in holes) meets an n-type region (rich in free electrons). The moment they are joined, the steep difference in carrier concentration drives diffusion across the boundary.

• Free electrons from the n-side diffuse across and recombine with holes on the p-side.
• Holes from the p-side diffuse across and recombine with electrons on the n-side.
• This recombination sweeps a thin zone straddling the junction clean of mobile charge carriers — it is left with essentially no free electrons or holes.

What remains in this zone are the immobile ionised atoms: negatively charged acceptor ions on the p-side and positively charged donor ions on the n-side. These fixed charges set up a built-in electric field (and the associated barrier potential) that opposes further diffusion and brings the junction to equilibrium. Because it is depleted of mobile carriers, this zone is called the depletion region (also known as the space-charge region), which is the correct choice.

The other options name regions that still contain plenty of mobile carriers. The bulk n-type region has abundant free electrons, and the bulk p-type region has abundant holes, so neither is carrier-free. An ohmic terminal interface is simply the low-resistance metal-to-semiconductor contact used to wire the diode into a circuit; it is deliberately made non-rectifying and is not the carrier-free layer being described.