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Yoshihito Amemiya, and their colleagues at the University of Hokkaido. They
started to develop semiconductor planar circuits (chips), simulating Belousov-
Zhabotinsky chemical reaction-diffusion media. Several variants of circuit design
to implement these models were proposed. Apparently the most interesting one is
the use of single-electron oscillators.
Earlier, in Chap. 4 , it was pointed out that qualitatively a Belousov-Zhabotinsky
medium can be represented as a set of unit cells with dimensions smaller than the
diffusion length, interacting with each other through the diffusion of medium
components. The main regimes of the unit cells are concentration fluctuations and
the excitable regime. For semiconductor modeling of Belousov-Zhabotinsky
media, a one-electron oscillator constructed on the basis of the tunnel transition
and possessing analogous properties was selected as an elementary cell.
Let there be a tunnel junction—a thin layer of nonconductive material between
two electrodes. Let us apply a small direct current voltage to the electrodes. Since
the transition, as the simplest capacitor, has a capacitance, it starts to charge, and at
a certain voltage on the electrodes, electron tunneling takes place. This process is
repeated over and over again (Fig. 7.7 ). This process of reducing the resistance of
the device at low bias voltages is called Coulomb blockade. Often such a process,
occurring in the tunnel junction, is compared with the sequential formation of
droplets in a loosely closed water tap. Figure 7.8 shows a schematic diagram of
such an oscillator and its modes, which are characterized by nanosecond times. To
create a model of the reaction-diffusion medium, these oscillators were combined
by capacitive coupling (Fig. 7.8 ), which plays the role of diffusion. Real chips
implementing this scheme were not produced. Their functional features were
determined by computer modeling. It turned out that in a device containing
100
100 oscillators, the quality of the recorded wave process leaves much to be
desired (Fig. 7.9 ). Results improved significantly when the scheme was modified. It
used oscillators built on several series-connected tunnel junctions.
The developed reaction-diffusion chip is a quasi-flat structure—a layer
containing oscillators and their connection elements. The authors of the device
then made the next step. They tried to create a multilayer device in which quasi-
planar structures were merged into a spatial circuit. Computer modeling of the
functionality of this device led to interesting results. The quality of the Voronoi
diagram, initiated and designed at the first level of the device, dramatically
improved on the third layer of the device, connected with the first one (Fig. 7.10 ).
This is consistent with the views on the importance of multilevel information
processing by reaction-diffusion systems.
Today it is difficult to determine the practical significance of the work done by
the Japanese researchers. More needs to be done to understand the real capabilities
of the developed semiconductor devices and technological features of their
manufacturing.
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