In an unbiased p - n junction electrons diffuse from n-region to p-region because:

In an unbiased p-n junction, there is no external voltage applied. To understand why electrons diffuse from the n-region to the p-region, we need to recall the nature of the two regions.

In the n-type semiconductor region, the majority charge carriers are electrons, meaning there is a high concentration of free electrons. In the p-type semiconductor region, the majority carriers are holes, so the concentration of free electrons is low because electrons are the minority carriers there.

Diffusion is a fundamental process where particles move from a region of higher concentration to a region of lower concentration to achieve equilibrium. Since the electron concentration in the n-region is much higher than in the p-region, electrons naturally diffuse across the junction from the n-region to the p-region due to this concentration gradient.

Now, let's evaluate the options:

Option A states that electrons travel due to a potential difference. However, in an unbiased junction, there is no external potential difference applied. While an internal potential barrier develops after diffusion starts, the initial movement is driven by concentration difference, not potential difference. Thus, this is incorrect.

Option B claims that only electrons move from n to p and not vice versa. This is false because while electrons diffuse from n to p, holes simultaneously diffuse from p to n. Additionally, electrons can move in both directions due to drift and diffusion, but the net diffusion of electrons is from n to p. The "only" and "not vice versa" part is inaccurate.

Option C correctly identifies that the electron concentration in the n-region is higher than in the p-region. This concentration gradient is the primary reason for electron diffusion from n to p.

Option D suggests that holes in the p-region attract electrons. Although holes are positively charged and electrons are negatively charged, electrostatic attraction is not the main driver for diffusion in an unbiased junction. The diffusion occurs due to the concentration gradient, and the attraction becomes relevant only after the formation of the depletion region and the built-in electric field. Hence, this is not the correct reason.

Therefore, the correct explanation is that electrons diffuse from the n-region to the p-region because the electron concentration in the n-region is higher than in the p-region.

Hence, the correct answer is Option C.