The integration density in microelectronics is progressing in a breath-taking pace despite all predictions about technological and physical limits and will continue at least up to the Gbit generation. But nevertheless pricipal physical limitations are visible. One problem is the power consumption of complex chips. In some cases it has reached values, which cannot be further increased. As the conventional devices and circuits work with large numbers of charge carriers, there should be a great potential for saving energy. E.g. a single bit in DRAM is represented by about 10000 electrons. In contrast, future electronic circuits may handle single electrons, utilizing the so called Coulomb blockade of electron transfer. This effect occurs when the energy to charge a capacitance C by a single electron, e 2 /2C, is larger than the energy of thermal fluctuations k b T, and the tunnel resistance is larger than the resistance quantum h/e 2 At gate voltages less than e/2C, electron tunneling through the tunnelbarriers onto the island is suppressed and the Coulomb blockade is observed.
Devices based on quantum effects, which are now within the reach of present day technology, have been often discussed in relationship with locally interconnected architectures like cellular automata or cellular neural networks. In particualar, a new computational principle based on the concept of Quantum Cellular Automata (QCA) has been recently proposed by Lent, Tougaw and Porod at the University of Notre Dame [1]. The basic cell is made up of five quantum dots, four on the corners of a square and one in the center, and is occupied by only two electrons. Coulomb repulsion between the electrons in a single cell causes the charge in the cell to align along one of two directions. These two alignment states, or "polarizations", represent binary data (see Fig.). Tunneling of the electrons occurs between nearest neigbour and next-nearest neighbour dots.
The whole circuit is basically a two-dimensional array of such cells,
that interact with eachother only by means of the capacitive coupling between
adjacent cells. Inter-cell tunneling is not allowed, due to the thick barriers
separating cells. It has been demonstrated that in a chain of cells, if
the first cell is forced into a given polarization, all other cells tend
to move into the same polarization, in order to minimize the total electrostatic
energy. Therefor, such chains can be used to transmit information, with
a velocity that does not depend on electron moblilty, but on inner cell
dynamics and on the capacitive coupling between the cells. The possibility
of performing basic logic funtions has also been demonstrated.
In this group we are working on the realization of such small cell of quantum
dots and the demonstration of the feasibility of this approach for future
nanoelectronic logic devices.
[1] P. Tougaw, C.S. Lent, J. Appl. Phys. 74, 1818 (1994)