While there is a depletion of charge carriers close to the surface of most semiconductors, some semiconductors for example InAs, show a electron accumulation layer at their surface, where the charge neutrality level lies within the conduction band. Such electron accumulation layers induce the downward band bending at the surface. I understand that by n(p) doping the upward(downward) band bending are induced due to the electron transfer from the bulk(surface) to the surface(bulk). But I do not know why the downward band bending occurs by the surface accumulation layer.
Please tell me the reason!