FIGURE 1. The band structure of a silicon-on-insulator membrane.

Trap states (short lines) at the silicon–silicon-dioxide interface in Zhang and colleagues' thin silicon membrane¹ depress the Fermi energy (E_F) of the membrane to below its intrinsic level (E_i), hindering conduction. But surface effects create allowed energy states (yellow) just above the valence band, into which electrons (blue circles) can be excited thermally, producing 'holes' (white circles) that can propagate through the valence band. This surface effect in silicon works equivalently to the doping effect in bulk silicon in enhancing the material's conduction properties. (Adapted from ref. 1.)