The really interesting thing about semiconductors is not the size of the gap between conduction & valence energy bands but the fact that two different types of semiconductor can be created that have overlapping conducting / covalent energy levels. A junction of these two different materials (one doped to give extra holes, one doped for extra electrons), will allow electrons to pass through _only_ one way. For the electrons to flow backwards (reverse bias), there must be enough Potential for them to tunnel through the barrier, which happens in zener diodes and chips when they go 'phwttt' but not much else. Conductivity of semiconductors is _much_ worse than any metal, but is the fact that semiconductor's can make a switch, where the tunneling energy is reduced at command of a small electric flow, is the only reason that they are valuable.