BACKGROUND CHARGE FLUCTUATIONS IN SET-TRANSISTORS

We have studied fluctuations of background charges in single electron tunnelling (SET) transistors. Changes in the charge distribution of th e substrate and other dielectric materials near the transistor cause u ncontrolled changes in its conductivity. We have observed big differen ces in the type and frequency of the conductivity fluctuations between different samples, but no systematic dependence on the substrate mate rial has been found. To obtain information about the location of the s ource of the charge noise, we performed coincidence measurements on tw o separate SET transistors which were made very near to each other. Th e results suggest that the conductivity fluctuations are caused by cha rges which are in the immediate vicinity, within about 300 nm of the t ransistor.