What is the main cause of electron's diffusion from n-type region to p-type region, even when there is no external supply used?

In an n-type semiconductor, the majority carriers are electrons i.e., electrons per unit volume is more whereas, in a p-type holes are the majority charge carriers. It is because of this concencentration gradient across the n- side and p- side that, the electrons diffuse from n-type region to p-type region. Mobility of electrons also contribute to the diffusion of electrons from n-type to p-type region.