Various methods of polarity control of FETs based on CNTs and graphene. a, Redox potential of nanotubes as a function of the diameter (left). This Reproduced with permission . Copyright 2010, Royal Society of Chemistry. Array of n-type CNT transistor by precisely positioning an air-stable BV. Reproduced with permission . Copyright 2011, American Chemical Society. b, Effect of oxygen on p-doping. I-V curves of originally p-type CNT FET, with the nanotube capped with PMMA, have been converted to n-type. Reproduced with permission . Copyright 2001, American Chemical Society. c, The type conversion of CNT FETs by trap layer-induced electrostatic doping. Adapted with permission . d, I-V characteristic of an initially p-type characteristic in SWCNT FET, gradually changed to n-type caused by increasing amounts of K. Reproduced with permission . Copyright 2001, American Chemical Society. e, Polarity control by metal (Pd and Al) work function. Reproduced with permission . Copyright 2005, American Institute of Physics.