## Semiconductor surfaces Unlike the bulk, the potential that a surface feels is different. Since the periodic boundary condition (BC) is no longer valid for electrons at the surface. Therefore, we have the dangling bonds and surface reconstruction, which create surface states. (Actually, surface reconstruction is the result of automatically rearranged bonds/saturation of states, and due to the asymmetry, it could be characterized by nonlinear optics techniques) Due to the existence of surface states, it is possible that the Fermi energy is different from the surface and the bulk. This causes the band bending near the surface and the **Fermi level pinning effect**. - Band bending: due to the system trying to maintain the same $E_F$. - Fermi pinning: when applying an external field or doping, $E_F$ gets pinned due to newly incoming electrons first choosing to fill the surface states. The sketches below show how band will shift and the Fermi pinning effect. ![[Drawing 2024-09-02 17.57.11.excalidraw.svg]] >[!Info] >For contact with metal, see [[Metal electrodes on semiconductor surfaces]].