5.2.3 Random Background Charge

     This term denotes any charge which is located close enough to the circuit to be disturbing, but which is outside of our direct control, making it virtually random. These are charged impurities, traps, parasitic and stray capacitances and other circuit parts, which may induce charges on sensitive quantum dots. Further worsening the problem is the time dependence of these background charges. Impurities can migrate, trapped electrons might get detrapped and unrelated circuit parts produce unpredictable changes in induced charges. Single-electron devices are extremely charge sensitive, which can be exploited for highly sensitive electrometers, but which is in most devices a deadly feature, destroying desired device functions.

Currently there are three approaches under investigation, which deal with this problem. One possibility is to find process technologies which allow production of impurity free materials or materials where impurities accumulate in regions where they are not disturbing device behavior. A second approach is to use SET features which are independent of random background charge, like Coulomb oscillations. Such a design is investigated in Section 5.2.9. The third solution is to use, instead of a single island, an array of similar islands which show extraordinary resistance to random background charge. A design with these features is investigated in Section 5.2.10.

Random background charges are for the time being the worst problem single-electron devices face today. In the past room temperature operation was in the forefront. Since the first granular production processes showed room temperature or near room temperature operation it is not of so much concern anymore; reliable operation will be achieved in the next years. However, the search for solutions to the random background charge problem continues.