Callister - An introduction - 8th edition

18.15 Semiconductor Devices • 753
Within this framework, numbers are represented by a series of two states (sometimes
designated 0 and 1). Now, transistors and diodes within a digital circuit operate
as switches that also have two states—on and off, or conducting and nonconducting;
“off” corresponds to one binary number state, and “on” to the other. Thus, a
single number may be represented by a collection of circuit elements containing
transistors that are appropriately switched.
Flash (Solid-State Drive) Memory
A rapidly evolving information storage technology that uses semiconductor devices
is flash memory. As with computer storage, flash memory is programmed and
erased electronically, as described in the preceding paragraph. However, unlike
computer storage, the flash technology is nonvolatile—that is, no electrical power
is needed to retain the stored information.There are no moving parts (as with magnetic
hard drives and magnetic tapes, Section 20.11), which makes flash memory
especially attractive for general storage and transfer of data between portable devices—
such as digital cameras, laptop computers, mobile phones, PDAs, digital audio players,
and game consoles. Furthermore, flash technology is packaged in memory cards
and USB flash drives. Unlike magnetic memory, flash packages are extremely
durable and are capable of withstanding relatively wide temperature extremes as
well as immersion in water.
integrated circuit
Microelectronic Circuitry
During the past few years, the advent of microelectronic circuitry, where millions
of electronic components and circuits are incorporated into a very small space, has
revolutionized the field of electronics. This revolution was precipitated, in part, by
aerospace technology, which necessitated computers and electronic devices that
were small and had low power requirements. As a result of refinement in processing
and fabrication techniques, there has been an astonishing depreciation in the
cost of integrated circuitry. Consequently, at the time of this writing, personal computers
are affordable to large segments of the population in many countries. Also,
the use of integrated circuits has become infused into many other facets of our
lives—calculators, communications, watches, industrial production and control, and
all phases of the electronics industry.
Inexpensive microelectronic circuits are mass produced by using some very ingenious
fabrication techniques. The process begins with the growth of relatively large
cylindrical single crystals of high-purity silicon from which thin circular wafers are cut.
Many microelectronic or integrated circuits, sometimes called chips, are prepared on
a single wafer. A chip is rectangular, typically on the order of 6 mm 1 1 4 in.2 on a side,
and contains millions of circuit elements: diodes, transistors, resistors, and capacitors.
Enlarged photographs of and elemental maps of a microprocessor chip are presented
in Figure 18.27; these micrographs reveal the intricacy of integrated circuits. At this
time, microprocessor chips with densities approaching one billion transistors are being
produced, and this number doubles about every 18 months.
Microelectronic circuits consist of many layers that lie within or are stacked
on top of the silicon wafer in a precisely detailed pattern. Using photolithographic
techniques, for each layer, very small elements are masked in accordance with a
microscopic pattern. Circuit elements are constructed by the selective introduction
of specific materials [by diffusion (Section 5.6) or ion implantation] into unmasked
regions to create localized n-type, p-type, high-resistivity, or conductive areas. This
procedure is repeated layer by layer until the total integrated circuit has been
fabricated, as illustrated in the MOSFET schematic (Figure 18.26). Elements of