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

• electrons concentration in n-region is more as compared to that in p-region.
• holes in p-region attract them.
• electrons travel across the junction due to potential difference.
• only electrons move from n- to p- region and not the vice-versa.

In p-type materials, holes are majority carriers, and electrons are majority carriers in n-type materials. When the two types of semiconductor materials are joined together, the electrons from the n-type material diffuse into the p-type material and combine with holes as their concentration is higher in the n-type layer. This creates a layer of negative ions near the junction in the p-type material. Negative ions are formed because the trivalent impurities (e.g., Aluminum) now have an extra electron from the n-type material. Similarly, the holes from the p-type material diffuse into the n-type material, resulting in a layer of positive ions in the n-type material. These negative ions create an electric field in the direction from the n-type to the p-type. As more electrons diffuse into the p-type material, the electric field strength increases. The electrons from the n-type material now diffusing into the p-type material will have to overcome the electric field due to negative ions. At one point, the electric field becomes sufficiently strong to stop further diffusion of electrons.