Coulomb blockade devices
A comprehensive programme of basic theory, device and system modelling is being pursued to investigate the feasability of a future technology based on so-called Coulomb blockade devices which function as single electron transistors thereby raising the prospect of digital logic with one bit on one electron. This work is also intended to underpin experimental work in single electronics both in metal-insulator systems (which are capable of high temperature operation) and semiconductor quantum point contact structures (presently requiring very low temperature operation).Part of the work is directed at possible quantum computing devices.
Coulomb blockade devices are epitomised by a capacitative tunnel junction, formed for example as a metal-insulator-metal structure (MIM) or as Schotthy dot (metal) on a semi-insulating semiconductor substrate. Viewed as a capacitor such a device will have a charging energy Q2/2C. Provided sufficient thermal energy kT >> e2/2C = kTc can be acquired from the lattice it will be possible for a single electron to tunnel across the junction and the junction will display typical tunnelling current-voltage characteristics. If the capacitance is made for small, typically by reducing the area of the junction, it becomes feasible for the charging energy due to the passage of one electron to significantly exceed the thermal energy and tunnelling will become energetically impossible unless a finite bias is applied to supply the necessary energy eV = e2/2C. Evidently. if T < Tc = e2/2kC, the current-voltage characteristics will become offset by e/2C. Although this is moderately interesting a more profound effect occurs if we consider an array of tunnel capacitors undr Coulomb blockade conditions. Then one finds that the presence of an additional electron on one of the “electrodes” induces a polarisation field across the entire array which together with the excess charge acts as a soliton. The Coulomb charging energy constraints provide a stringent discipline on the behaviour of electrons which are injected into the array from the source: the electron tunnelling flow becomes highly correlated in space and time. Essentially, the tunnelling electrons behave like a queue of traffic on a road system, only a certain number can be present at any one time and electrons must wait before entering a full array until other electrons have left.
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