Nonideal Diode Behavior and Bandgap Renormalization in Carbon Nanotube p-n Junctions

The p-n junction diodes are formed by electrostatic doping using two gate electrodes positioned beneath individual, suspended single-walled carbon nanotubes (CNTs). These devices exhibit nearly ideal diode behavior within a small bias voltage range near 0 V. At higher bias (>\vert 0.2 V\vert), nonideal diode behavior is observed arising from Schottky contacts formed between the nanotube and its metal contact electrodes and the presence of electron tunneling between the N- and P-doped regions. We introduce a back-to-back diode model to explain the observed current versus voltage (I-V) characteristics. The reverse saturation current, parallel resistance, and open-circuit voltage dependence on gate voltage provide quantitative evidence for the theoretically predicted doping-induced bandgap shrinkage in CNTs. The minority carrier lifetimes are also estimated from this model.