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
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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. The Coulomb blockade is largest for zero background charge, or more precisely for background charges of integer multiples of e. Increasing the background charge starting from zero reduces the size of the Coulomb blockade until at a background charge el2 the Coulomb blockade is entirely suppressed . Since in almost all single-electron devices the presence of a Coulomb blockade is mandatory, random background charge is the number one problem . In other words, your single-electron circuit would not work anymore if background charges are unfortunately positioned . Luckily , not all hope is lost. In the following I will describe some ideas to cope with this difficult problem. Not all of these ideas are fully explored and developed, and some are just hypothetical arguments or not very practical suggestions, but they could show the way to a practical solution. The reward of a good solution to the random background charge problem would be huge . Hence, I would like to encourage everyone to try his wildest ideas on this challenge.
S. Selberherr ed., Computational Microelectronics © Springer-Verlag/Wien 2001
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Fig. 120. a /- V-characteristic ofa symmetrically biased single-electron transistor for three different background charges. For qo = 0 the maximum Co ulomb blockade is achieved . For qo = e/2 the Cou lomb blockade vanishes . h Difference in Co ulomb blockade for a symmetrica lly biased single-electron transistor and an asymmetrically biased transistor. In both cases the gate is grounded . Increasing the background charge from zero decreases the Coulomb blockade in the symmetrically biased case, whereas in the asymmetrically case the Cou lomb blockade first shifts to the right and then reduces
5.1 The Good Side of High Charge Sensitivity The random background charge is such a big problem
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