NANOSCALE DEVICES: APPLICATIONS AND MODELING - Bio-Inspired and Nanoscale Integrated Computing - page 39

such small devices to perform, say, electrical measurements on them. Usually this
can be done by making ''interface'' structures to connect the nano world to our
macroscopic world. For instance, in the case of the device in Figure 2.2, the metal
electrodes on the sides can be connected to large pads that one can use for wire
bonding to the outside world. Another option is to use high resolution motion
probes, such as in an SPM tool, to move, manipulate, and contact objects at the
nanoscale. The second challenge is determining whether a measurement or
perceived effect is really due to the nanodevice and not other surrounding
structures, noise, inaccuracies, etc. This is true in any experiment in general, but
it becomes particularly important at the nanoscale since one is dealing with very
small signals. Creative techniques need to be developed constantly for such
experiments. This is an ongoing challenge in nanotechnology.
2.4. NANODEVICE APPLICATIONS
Although nanotechnology is still very much in a research phase, many applica-
tions have been suggested and even demonstrated at the laboratory level for
nanoscale devices. It should be noted that many of the more traditional micro-
devices have also found their way into nanotechnology. Their scaled down
versions could well be considered nanodevices, especially since the reduction in
scale results in major differences in characteristics and operation, such as the
operating temperature. In this section we will first review some of the more
established devices in this category, and then present some of the less traditional
nanodevice ideas.
2.4.1. ''Traditional'' Nanodevices
The fundamental building blocks of our computers are field-effect transistors
(FETs), which act as switches to perform logic operations. In parallel with efforts
to shrink those devices in the well-established silicon technology, as well as in new
types of materials (discussed later in this chapter), there have also been many
efforts in making alternative devices that could perform such operations. Here we
will look at some of the most common devices in this category, namely resonant
tunneling diodes (RTDs), single-electron transistors (SETs), and quantum dots
(QDs). An interesting overview of these devices can be found in [15]. As discussed,
a fundamental aspect of nanodevices is that quantum effects are directly visible in
them. We will take a closer look at this here.
2.4.1.1. Resonant Tunneling Diodes. Consider the problem of a ''particle
in a box.'' By this we mean a particle, say an electron, confined to a small region in
space by a potential energy distribution such as the one shown in Figure 2.3. This
represents what is called an infinite potential ''well''—namely, a region where the
electron is trapped by two barriers on the sides.