NEGATIVE CHARGING IN ULTRATHIN METAL-OXIDE-SILICON TUNNEL-DIODES

A negative charging effect is studied in small, ultrathin metal-oxide-silicon diodes biased at constant voltage. The charging is seen as a gradual decrease in the magnitude of the tunnel current over time. Along with the charging, the current noise power decreases. Results are described from measurements of the temperature and voltage dependence of the current transients. A physical model is presented in which filling and emptying of electron traps in the as-grown oxide cause the noise, and the conductance decrease is caused by the transformation of these traps into fixed negative charge storage centers. From fits of the model to measured data, we obtain a thermal activation energy of 0.3 eV for the charging process. A low-temperature annealing treatment at 200-degrees-C restores the conductance of a previously charged device to its initial value. On the other hand, the charging effect can be eliminated by a 350-degrees-C anneal in nitrogen. Together, these experiments suggest that the traps are water- or hydrogen-related defects.