Silicon nanomembranes (SiNMs) are very thin, large, free-standing or free-floating two-dimensional (2D) single crystals that can variously be flat, rolled into tubes, or made into any number of odd shapes, cut into millions of identical wires, used as conformal sheets, or chopped into tiny pieces. Because SiNMs are mostly surface or interface and little bulk, they have very interesting properties. We describe electrical conductivity in SiNMs. Because of trap states at the Si/SiO₂ interface, bulk dopants become irrelevant to electronic transport when the membrane is thin enough. Replacing the oxide at one interface with the clean-Si surface reconstruction dramatically increases the nanomembrane conductivity. We provide a model for this behaviour. The dimer reconstruction surface states provide a means of 'surface doping'. Other materials with proper highest-occupied molecular orbital (HOMO) or lowest-unoccupied molecular orbital (LUMO) bands, when deposited on the Si surface, should produce the same conductivity effect, affording a broad opportunity for membrane-based sensors.