Origin of unipolarity in carbon nanotube field effect transistors

Several mechanisms have been proposed for the origin of unipolar p-type behavior in carbon nanotube (CNT) transistors including (i) O ₂ molecules trapped in the metal/CNT contact, (ii) OH radicals on the SiO ₂ surface, and (iii) O ₂/H ₂O coupling on the CNT surface. Nevertheless, the underlying mechanism is far from being clearly understood. Using density functional calculations, we propose that the SiO ₂ substrate induces wetting of moisture, which enhances the formation of CNT-OH complexes via several intermediate chemical reactions. As a consequence, the CNT-OH complex provides deep (occupied and unoccupied) acceptor levels near the valence band, acting as an electron trap center, which is the main cause of the asymmetric unipolar behavior of CNT transistors. Based on the theory, I-V characteristics are proposed in terms of gate bias and work function difference between metal and CNTs.