Final answer:
The resistance in a p-n junction is concentrated in the depletion region due to the lack of free charge carriers and the electric field that impedes further movement of charges, enhancing resistance.
Step-by-step explanation:
The resistance of a p-n junction is concentrated in the depletion region because it is an area devoid of mobile charge carriers. When p- and n-type materials are joined, the free electrons from the n-type material and the holes from the p-type material combine near the junction. This combination of electrons and holes leaves a region empty of free charge carriers, known as the depletion region. As a result, an electric field is formed between the uncovered positive ions in the n-type region and the negative ions in the p-type region, which acts to limit further movement of charge carriers and thus increases the resistance.
During the operation of the diode, applying a positive voltage to the n-side increases the width of the depletion layer, enhancing the electric field and thus increasing resistance to charge carrier movement across the junction. This condition is known as reverse bias and allows negligible current flow. In contrast, when the positive voltage is applied to the p-side, the depletion layer narrows, and the resistance decreases, which is the forward bias condition and enables the current to flow easily.