Energy levels localized in the @S06187@ of semiconductors, which do not bear any direct relation to the bulk energy distribution, but which can exchange electrons with the bulk. In such a case a @S05776@ may arise even when \(\sigma \) (the free @C00988@) is zero, i.e. \(\sigma =\sigma _{\mathrm{sc}}+\sigma _{\mathrm{ss}}\), where \(\sigma _{\mathrm{sc}}\) is the space @C00988@ and \(\sigma _{\mathrm{ss}}\) is the @C00988@ associated with the surface states.