NANOSCALE DEVICES: APPLICATIONS AND MODELING - Bio-Inspired and Nanoscale Integrated Computing - page 42

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Resonant peaks

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Figure 2.6. Conceptual representation of the current-voltage characteristics of
an RTD.

2.4.1.2. Single-Electron Transistors. In the operation of the RTD, we
looked at the problem as if there were only one electron in the system. In other
words, we neglected electron-electron interactions. Electrons repel each other
through Coulomb forces; if an electron is put inside the device, it will be harder for
a second one to enter since it will have to face the repulsive force of the first one.
This is called Coulomb blockade. In other words, if multiple electrons are to enter
the device at a time, each subsequent electron will have to be put at an energy not
only determined by the level quantization shown in Figure 2.3, but also by the
charging energy of the device due to its capacitance. As a consequence, the
difference between the energy of the electrons will be approximately the sum of
the two contributions. One effect may be more dominant than the other based on
the actual device size and material. In RTDs the device has a relatively large
capacitance such that the charging energy e 2 /C is much smaller than the level
quantization energy discussed above. In an SET, however, the structure is such
that the charging energy is much larger than the level quantization energy. Note
that although the end result of both effects (resonant tunneling and Coulomb
blockade) is that transport happens through discrete energy states and gives rise to
unusual device characteristics, the two effects are fundamentally different: one
effect is due to the wave nature of particles (quantum mechanical), and the other
one due to the fact that the value of charge is quantized (which could be regarded
as a somewhat more classical effect). A review of single-electron transistor devices
can be found in [16]. Such structures could also be operated under an alternating
voltage, and the frequency and amplitude of this signal is such that only one
electron is transferred per cycle through the device. Such a device is called a single-
electron turnstile [17].

2.4.1.3. Quantum Dots. There seems to be some degree of ambiguity in the
literature as to the exact definition of a QD. In particular, the terms SET and QD
often seem to be used one instead of the other. A QD could be thought of as a
confined region between two potential barriers such as in Figure 2.4, although
confinement in all three dimensions is typically assumed. In a sense, RTDs
and SETs can be considered special forms of QDs. Both energy level quantization