Final answer:
The charge density in the depletion region of a PN junction diode is due to the presence of immobile ions, which remain after the free electrons and holes have recombined. These ions create an electric field that increases the resistance of the region.
Step-by-step explanation:
In the depletion region of a PN junction diode, charge carriers such as free electrons and holes are indeed negligible due to recombination. However, there is still charge density present. This charge density is due to the presence of immobile ions. When the junction is formed, the free electron from the n-type region combines with the free hole from the p-type region, creating a region void of free charge carriers but leaving behind charged dopant ions that are fixed in the lattice and cannot move. These charged ions create an electric field which opposes further movement of free charges across the junction, establishing a depleted zone where no free charge carriers exist. This field is responsible for the large resistance of the depletion region.