NANOSCALE DEVICES: APPLICATIONS AND MODELING - Bio-Inspired and Nanoscale Integrated Computing - page 43

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Figure 2.7. The staircase-like current-voltage characteristics of a QD. The width
of the plateaus is defined by the separation of the energy levels due to quantum
confinement and/or Coulomb blockade.

(due to the wave or quantum mechanical nature of electrons) and Coulomb
blockade (due to the discrete nature of charge) are present in the electronic
characteristics of a QD. The authors in [15] present the clear distinction that if
these two effects are on the same order of strength and significance, the device is a
QD; otherwise it is an RTD or SET depending on which effect is dominant. In so
doing, they correlate these effects to the number of ''small'' or confined dimen-
sions in each device.
What is really important in any case is the clear understanding of both these
effects and their fundamental difference. Also, note that typically the structure of
an RTD is such that as the bias is increased, only one level at a time is in the
energy ranges where there are available electrons for transport, giving rise to the
peaks in the conductance characteristics of the device and the negative differential
conductance after each peak (Fig. 2.6). However, in what we usually think of as
QDs or SETs, often a wider energy range can be available for the incoming
electrons (compared to the level spacing inside the well); as the bias is increased,
first one level enters that range, then a second one, a third one, and so on, without
the previous levels leaving that range. Thus, instead of a number of peaks in the
current-voltage characteristics, one can have a staircase-like profile: every time a
new level enters the allowable energy ranges of the contacts, the current goes up by
a finite amount (Fig. 2.7). Because electron energy is quantized in a QD (like the
energy levels in an atom) quantum dots are sometimes referred to as artificial
atoms. For more detail on the operation of QDs the reader can consult [18, 19].
The above quantum devices can be realized in various ways. One popular
method is to create a so called two-dimensional electron gas (2DEG) in the
interface region of a heterojunction of III-V semiconductors with different band
gaps. A set of electrodes is patterned on top of this 2DEG, and by the application
of proper bias to these electrodes, some regions of this 2DEG can be depleted
(Fig. 2.8). As a result, an ''island'' of electrons is created that is connected to the
reservoirs on the sides through potential barriers (the depleted regions).

2.4.1.4. Temperature Dependence. Table 2.1 shows the first few allowed
energy levels in a potential well with different widths calculated using the formula