AN INTRODUCTION TO NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 8

An electron wave becomes a decaying
signal as it tries to penetrate a barrier.

If the barrier is thin enough, the wave
can continue on the other side.

Figure 1.5. Visualization of the tunneling phenomenon. Mathematically, the
wave changes into an exponential decay when it enters a region of ''high
potential''—the barrier—and resumes as a wave after it exits. The probability
that an electron will tunnel across the barrier is related to how much amplitude
the wave has left once it exits the barrier.

structure of silicon, electrons can move through a transistor more easily. This can
be done by adding materials on top of transistors that naturally want to bend, thus
pulling or pushing on the silicon. The so-called strained silicon [4] has quickly
become a standard technique to improve the performance of transistors at 90 nm
or less. Another example is the development of better insulating materials, known
as high-K dielectrics [5]. The right combination of conducting and insulating
materials can reduce the amount of undesirable tunneling between the gate and
channel, even when the barrier is only a few layers of molecules thick. This
advancement has been the key towards 45-nm technology. In the future, it may be
necessary to use multiple gates to reliably control the current along a wire.
FinFETs [6] or trigate transistors [7] are two multigate variations of transistors
that may take us beyond 45 nm.
There are several more limitations when using tiny transistors that motivate
the nanocomputing ideas presented in this topic. First, the wiring that inter-
connects transistors is becoming a very signficant limitation for performance,
power, and size of devices. There are even theoretical limitations about how much

Gate Gate

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Figure 1.6. One of many nonideal effects in a transistor is that electrons in the
gate may tunnel into the wire. This occurs more often as the thickness of the
insulating barrier decreases.