TABLE OF CONTENTS Electroluminescence is the process by which current injection causes electron-hole recombination and photon emission. In conventional devices such as light emitting diodes, the simultaneous presence of electrons and holes in the same spatial region is achieved by operating a p- n junction in forward bias, thereby injecting electrons in the p region where they recombine with holes. In nanotube field-effect transistors, one can take advantage of the gate modulation of the contacts and the band-bending, to simultaneously inject electrons and holes in the nanotube. Thus, in nanotube devices doping is not required to observe electroluminescence. The situation is illustrated in Fig. 7.29 which shows a sketch of an ambipolar carbon nanotube transistor. The device consists of a single semiconducting nanotube contacted by Ti at the source and drain, creating Schottky barrier contacts. A heavily doped Si backgate, separated from the nanotube by a 150 nm thick oxide controls the device conductance. When operated as a Schottky barrier transistor, the device shows ambipolar behavior, with nearly equal ON currents at positive and negative values of the gate voltage (Fig. 7.29 (b)). However, by choosing the gate-source and drain-source voltages appropriately, the band-bending can be such that the electric field at the source and drain contacts have the same sign. This arises when the gate-source voltage is less than the drain-source voltage. In this regime, the device essentially behaves as a forward biased p- n junction, with an exponential turn-on of the current with drain-source...
