The potential barrier in the depletion region is due to

The potential barrier in the depletion region is due to ions.

Explanation:

A pn junction diode forms a depletion region or layer when the charge carriers of the respective sides gets away from their diffusion region. For example, say the p-sides in diode has holes as charge carriers and n-sides in diode has electrons as charge carriers.

So when current passes via the diode, at one point, the charge carriers will be drifted away from the diffusion region and their corresponding ionized minority carriers will be forming a layer. This layer will be resisting the flow of current and is termed as depletion region.

So the potential barrier is the potential difference required to diffuse these ionized electrons from one side to another. Thus, the potential barrier at the depletion regions is due to ions.