Christoph Wasshuber

Recent advances and future prospects in single-electronics

This paper introduces new developments in single-electron logic technology and review a few clever applications made possible when single-electron transistors are combined with CMOS.

Circuits and Applications

Single-electronics needs applications which make good use of inherent properties of the Coulomb blockade, tunnel junctions, and quantum dots. Many problems that plague single-electronics today can be removed or alleviated by intelligent circuit design. In order to capture the status quo of single-electronic circuit design a fairly complete list of devices and circuits is presented. We will look at fundamental building blocks, such as the ubiquitous single-electron transistor, pump, and turnstile, we will analyze metrological applications like supersensitive electrometers and primary thermometers, and we will study single-electron memories and logic applications. Finally we will assess the merits of neuronal networks, Boltzmann machines, and other specialized circuits.

Simulation Methods and Numerical Algorithms

Together with the exponential growth of computer performance, simulation became a cheap and widespread engineering and research practice. It is unthinkable to live without it in today’s high-technology society. And singleelectronics is no exception. This chapter includes tried and proven algorithms for the simulation of single-electron devices and circuits. Where possible, I present more than one algorithm for one task to better illustrate advantages and disadvantages. And there is seldom only one right way to do something.

Random Background Charges

The most serious deficiency with single-electronics is the so-called random background charge problem. Quantum dots are highly susceptible to nearby charges which can stem from several sources: charged impurities in the surrounding material, charged traps on surfaces and grain boundaries, charges on nearby conductors, and ionizing radiation. On top of that, these background charges will be in general mobile and move and change over time. Thus in both, space and time, we have to deal with random background charges. These random charges are so devastating, because they can fully suppress the Coulomb blockade, which is equivalent with a destruction of intended device behavior. As a simple example let us take a look at the Coulomb blockade dependence on background charge for a single-electron transistor. Figure 120 shows the I – V characteristic for a symmetrically biased SET for three different background charges.

Manufacturing Methods and Material Systems

So far we have looked at the physics of single-electronics, at possible applications, and at methods to analyze them. What is missing is how to fabricate them. This final piece is a very important one, if not the most important one, for a successful single-electron technology. It means that most likely new and uncommon materials will have to be introduced and new processes have to be made reliable, reproducible, and economical. The following gives an overview of fabrication methods and material systems for single-electron devices. At this point no single method can be said to be ready for mass fabrication or can be considered superior to any other method. A compatibility with CMOS processes is desirable to allow mixing of CMOS and single-electronics.