Page 77 - Nanotechnology an introduction

7.2. Digital Information Processing
As already mentioned, a device (or component) is fundamentally an information processor, in other words a transducer that encodes (or decodes)
information. As noted by Szilard, the elementary “quantum” of information is the binary digit or bit. Binary or Boolean logic, based on zero or one,
true or false, presence or absence, and so forth, has very modest physical requirements. There is essentially no intrinsic lower limit to the size of the
physical embodiment of “1”. Of all technologies, information technology is the one most suited to miniaturizing down to the nanoscale. The main
residual problem is heat dissipation (see Section 7.5).
The fundamental component of a digital information processor is the switch, or relay (Figure 7.2). Several relays can be connected together to
create logic gates, for example a not–and (NAND) gate, a fundamental component of a binary logic processor. Its characteristics can be
summarized in the following truth table:
input 1 input 2 output
0 0 1
1 0 1
0 1 1
1 1 0

Figure 7.2 A relay or switch. When the coil C is energized by applying a voltage across the input terminals I, it pulls the movable contact arm above it to link voltage +V to the output terminals O. If the restoring spring S is present,
setting the input I to zero will also cause the output to return to zero. Alternatively, a second appropriately placed coil could be used to move the contact arm in the opposite direction. The device is then bistable and would be suitable for
use as a memory element.
Figure 7.3 shows a transistor inverter, the simplest logic gate (NOT), and Figure 7.4 a NOR gate as examples of physical realization.

Figure 7.3 An inverter. If the voltage V b on the base is below a certain threshold, corresponding to logic level 0 (typically 0.7 V), the transistor remains in the off state, the current is small and the voltage V ce on the collector is
approximately equal to the supply voltage E, typically 5 V, corresponding to logic level 1. If V b exceeds the threshold then the transistor enters the active region of its characteristic but as V b increases further (corresponding to logic level
1) V ce saturates at typically around 0.2 V, corresponding to logic level 0.

Figure 7.4 A NOR logic circuit with inputs A, B and C built according to resistor–transistor logic (RTL). Note how it is constructed from the basic inverter (Figure 7.3).
The relay has the input–output relationship shown in Figure 7.5. The earliest digital computers used electromechanical relays. They are large, slow,
expensive, energy-hungry (and hence expensive to run), and unreliable. Frequent operational errors during the execution of programs run with such
devices provided Hamming with the inspirational motivation for developing error-correcting codes. Thermionic valves (vacuum tubes) are faster
and more reliable, but even more expensive and energy-hungry. The first significant step towards miniaturization was taken with the replacement of
relays and valves by solid-state transistors (Figure 7.6). Provided the fabrication process does not pose new difficulties (a clock is usually cheaper
to make than a watch), miniaturization uses less material in fabrication and less energy in operation (cf. Section 7.1). At a stroke, the devices
became smaller, faster (the electrons carrying the information had less distance to travel), cheaper (not only because the volume of material
required was lower, but also because efficient massively parallel fabrication procedures were devised), used less energy, and like all solid-state
devices were more reliable (the thermionic valve was more reliable than the relay because it had no mechanical moving parts, but the vacuum
could leak and incandescent electron-emitting filaments could break). A major step in fabrication technology was the introduction of integration.
Miniaturization and, concomitantly, parallel fabrication now permits millions of integrated transistors to be fabricated on a single “chip” of silicon,
with additional gains in operation speed because the electrons have less far to travel, both within and between components.