As the source-to-drain distance, d , of a transistor approaches the nanometre scale, quantum-tunnelling-mediated transmission ( ) through the potential energy barrier that creates an off state increases exponentially, leading to high leakage current and degrading the device subthreshold swing ( S s-th ). The source-drain leakage becomes increasingly problematic at the molecular scale (<5 nm) unless interference between two coherent conduction channels acts to suppress transmission. For two quantum-coherent transport channels (with transmission coefficients 1 , 2 , where i=|i|e^{-i{i}} ), total transmission can be completely suppressed if | 1 | = | 2 | and their phase difference, = (through 2 = | 1 + 2 | 2 = | 1 | 2 + | 2 | 2 + 2| 1 || 2 |cos ), providing a route to regain desirable characteristics of mesoscopic transistor geometries even with a few-nanometre channel length.
