IBM Corp. researchers say they have characterized four types of carbon nanotube field-effect transistsor defects that can stimulate nanotube FETs to emit light. The first light-inducing defect was discovered on the end contacts, where a natural Schottky barrier exists at the semi- conducting nanotube interface with its metal electrode. The second was found anywhere that charge had been inadvertently trapped in the oxide-covered silicon wafer; trapped charge locally inverted the carriers in the nanotubes atop it by forming a light-emitting intratube npn or pnp junction. The third defect characterized was on a nanotube loop where the tube bent around and crossed over itself. At the crossover point, hot carriers could tunnel from one leg to the other, where they affected other carriers and caused light emission. The first three defects were studied by randomly laying down nanotubes on a substrate and looking for those anomalies. But the fourth type of defect was intentionally made by partially covering a nanotube with a polymer. Where the polymer ended, a voltage drop caused carriers to collide, inducing electroluminescence.