Bandgap

The problem has been the lack of what is known as a bandgap. Semiconductors have bands of higher and lower energies and a point – the bandgap – at which excited electrons can hop from one to the other. This effectively allows switching on and off of the flow of current, so it is either conducting or not conducting, creating the binary system of zeroes and ones used in digital computers. […] Now, Walter de Heer at Georgia Tech in Atlanta and his colleagues have created graphene with a bandgap and even demonstrated a working transistor, an on/off switch that either prevents or allows current to flow through it.

The propagation of an acoustic wave inside a phononic crystal is forbidden in the bandgap frequency range due to Bragg scattering.