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Abstract

Quantization of charge in metallic or semiconducting structures in conventional electronic devices is usually not directly noticeable. However, when the smallest feature size is in the nano-meter regime, and the total capacitance becomes very small and the charging energy is larger than the thermal energy, the change in free energy associated with the addition or subtraction of a single electron to and from an island, or a quantum dot, becomes significant. Novel effects due to charge quantization have raised the prospect of a new electronic technology which needs new analysis tools. Therefore, this thesis deals with the simulation of such new electronic devices.

Electrons can enter or exit islands only via tunnel junctions, which is why this field is often referred to as single-electron tunneling. Charging effects result in time- and space-correlated transfer of electrons. New phenomena appear, such as 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. With these new quantum-effects, it is possible to control the movement and position of single electrons. Besides the desired characteristics i.e. controlled transfer of single electrons, undesirable effects arise and reduce or even eliminate the Coulomb blockade. These are, for instance, co-tunneling; a simultaneous tunneling of two or more electrons in different tunnel junctions, or the sensitivity to uncontrollable impurities dispersed throughout the material and to trapping-detrapping events which disturb the charge distribution.

This thesis describes how the transport of electrons through single-electron devices and circuits can be simulated. Two important methods, a Monte Carlo approach and a Master Equation approach which treat the device as a capacitive equivalent circuit under the action of discrete tunnel events, are compared. This very behavior of single-electron devices makes new simulation techniques mandatory. This thesis deals with the issue of how to simulate co-tunneling, which is a numerically challenging problem along with other implementation issues, for instance, how to accelerate the simulator. Besides the fundamental theory underlying single-electron tunneling, a detailed study about single-electron memories is presented. Simulation results are shown, to exemplify the inherent possibilities of single-electron technology and to show the capabilities of simulation. Many important questions are raised and examined. Is room temperature operation achievable? Are quantum fluctuations and co-tunneling suppressible? And is the sensitivity to random background charges controllable?

Single-electron devices show many very promising characteristics, such as ultimate low power consumption, down-scalability to atomic dimensions and high switching speed. The result could be micro-chips with ultra large scale integration in combination with dramatically reduced power consumption. These promising characteristics give single-electron devices the potential to partially replace conventional CMOS devices in the near future.


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Next: Acknowledgments Up: Dissertation Christoph Wasshuber Previous: Kurzfassung

Christoph Wasshuber