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!


1 Answer 1


The downward bending is actually caused by surface states that lie within the conduction band. Carriers from the bulk state these and counteract the bending as you stated, trying to make the band flat.

The difference between a material with a surface depletion and surface accumulation later is just the position of the Fermi level on this ladder of surface states on an energy scale.

The top part of the figure shows the surface states "disconnected" from the bulk material. You can see that the Fermi levels don't match. Now if you "bring them into contact", the charge carriers diffuse until the Fermi levels match, resulting in band bending. This can be upwards or downwards, depending on the energy and fill level of the surface states.

Band bending due to surface states

  • $\begingroup$ Thanks. I understand the carrier transfer occurs in order to make the band flat. But I am not sure the reason why surface states within the conduction band cause the downward bending. Could you explain that? $\endgroup$
    – hitgirl
    Jul 21, 2020 at 5:38
  • $\begingroup$ I'll look for some figure that illustrates the band bending. $\endgroup$
    – engineer
    Jul 21, 2020 at 14:59
  • $\begingroup$ Sorry, took a while. Couldn't find a suitable figure, so I made a sketch. Hope this helps and hope it's not too late. $\endgroup$
    – engineer
    Jul 25, 2020 at 13:17
  • $\begingroup$ Thank you for your explanations. This sketch means that the surface accumulation layer causes the downward bending at the surface because the charge carriers diffuse from the surface to the bulk. It was very helpful for me to understand that. $\endgroup$
    – hitgirl
    Jul 27, 2020 at 1:58

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