Quantization of charge in metallic or semiconductor islands is usually not directly noticeable. However, when the size of such islands is in the nano-meter regime, that is when the total capacitance becomes very small and the charging energy is larger than the thermal energy, then the change in free energy associated with the addition or subtraction of a single electron from an island, or a quantum dot, becomes significant.
New phenomena, such as the Coulomb blockade, which is a suppression of current flow at low bias, and Coulomb oscillations, a time or space correlated transfer of electrons through tunnel junctions, appear. With these new quantum-effects it is possible to control the movement and position of single electrons. Beside the wanted characteristics of controlled transfer of single electrons, unwanted effects arise, too. These are for instance co-tunneling, a simultaneous tunneling of two or more electrons in different tunnel junctions, or the sensitivity to uncontrollable impurities that are dispersed through out the material, and which are disturbing the charge distribution and hence the Coulomb blockade.
Single-electron technology is a very promising alternative to CMOS technology. It has ultimate low power consumption, is down-scalable to atomic dimensions and fast. The result could be micro-chips with ultra large scale integration in combination with strongly reduced power consumption where one electron carries one bit of information. We developed a simulator to support the design of single-electron devices and circuits and address especially the problem of simulating co-tunneling, because this effect is beside thermal fluctuations a major source of errors in these circuits.