Page 18 - Semiconductor For Micro- and Nanotechnology An Introduction For Engineers

The System Concept
A semiconductor can be viewed as consisting of many subsystems. For
one, there are the individual atoms, combining to form a chunk of crys-
talline material, and thereby changing their behavior from individual sys-
tems to a composite system. The atomic length scale is still smaller than
typical length scales that a designer will encounter, despite the fact that
sub-nanometer features may be accessible through modern experimental
techniques. The subsystems that this book discusses emerge when silicon
atoms are assembled into a crystal with unique character. We mainly dis-
cuss three systems:

• the particles of quantized atom vibration, or phonons;
• the particles of quantized electromagnetic radiation, or photons;
• the particles of quantized charge, or electrons.

There are many more, and the curious reader is encouraged to move on to
other books, where they are treated more formally. The important feature
of these subsystems is that they interact. Each interaction yields effects
useful to the design engineer.

Why do we emphasize the system concept this much? This has a lot to do
with scale considerations. In studying nature, we always encounter scales
of different order and in different domains. There are length scales that
play a significant role. Below the nanometer range we observe single
crystal layers and might even resolve single atoms. Thus we become
aware that the crystal is made of discrete constituents. Nevertheless, on a
micrometer scale—which corresponds to several thousands of mona-
tomic layers—the crystal appears to be a continuous medium. This
means that at certain length scales a homogenous isotropic continuum
description is sufficient. Modern down-sizing trends might force us to
take at least anisotropy into account, which is due to crystal symmetry, if
not the detailed structure of the crystal lattice including single defects.
Nanotechnology changes all of this. Here we are finally designing at the
atomic length scale, a dream that inspired the early twentieth century.

Semiconductors for Micro and Nanosystem Technology 17