Semiconductors : The Silicon Chip


Silicon is the raw material most often used in integrated
circuit (IC) fabrication. It is the second most abundant
substance on the earth. It is extracted from rocks and common
beach sand and put through an exhaustive purification process.
In this form, silicon is the purist industrial substance that man
produces, with impurities comprising less than one part in a
billion. That is the equivalent of one tennis ball in a string
of golf balls stretching from the earth to the moon.
Semiconductors are usually materials which have energy-band
gaps smaller than 2eV. An important property of semiconductors
is the ability to change their resistivity over several orders of
magnitude by doping. Semiconductors have electrical
resistivities between 10-5 and 107 ohms. Semiconductors can be
crystalline or amorphous. Elemental semiconductors are simple-
element semiconductor materials such as silicon or germanium.
Silicon is the most common semiconductor material used
today. It is used for diodes, transistors, integrated circuits,
memories, infrared detection and lenses, light-emitting diodes
(LED), photosensors, strain gages, solar cells, charge transfer
devices, radiation detectors and a variety of other devices.
Silicon belongs to the group IV in the periodic table. It is a
grey brittle material with a diamond cubic structure. Silicon is
conventionally doped with Phosphorus, Arsenic and Antimony and
Boron, Aluminum, and Gallium acceptors. The energy gap of
silicon is 1.1 eV. This value permits the operation of silicon
semiconductors devices at higher temperatures than germanium.
Now I will give you some brief history of the evolution of
electronics which will help you understand more about
semiconductors and the silicon chip. In the early 1900's before
integrated circuits and silicon chips were invented, computers
and radios were made with vacuum tubes. The vacuum tube was
invented in 1906 by Dr.Lee DeForest. Throughout the first
half of the 20th century, vacuum tubes were used to conduct,
modulate and amplify electrical signals. They made possible a
variety of new products including the radio and the computer.
However vacuum tubes had some inherent problems. They were
bulky, delicate and expensive, consumed a great deal of power,
took time to warm up, got very hot, and eventually burned out.
The first digital computer contained 18,000 vacuum tubes,
weighed 50 tins, and required 140 kilowatts of power.
By the 1930's, researchers at the Bell Telephone
Laboratories were looking for a replacement for the vacuum tube.
They began studying the electrical properties of semiconductors
which are non-metallic substances, such as silicon, that are
neither conductors of electricity, like metal, nor insulators
like wood, but whose electrical properties lie between these
extremes. By 1947 the transistor was invented. The Bell Labs
research team sought a way of directly altering the electrical
properties of semiconductor material. They learned they could
change and control these properties by "doping" the
semiconductor, or infusing it with selected elements, heated to a
gaseous phase. When the semiconductor was also heated, atoms
from the gases would seep into it and modify its pure, crystal
structure by displacing some atoms. Because these dopant atoms
had different amount of electrons than the semiconductor atoms,
they formed conductive paths. If the dopant atoms had more
electrons than the semiconductor atoms, the doped regions were
called n-type to signify and excess of negative charge. Less
electrons, or an excess of positive charge, created p-type
regions. By allowing this dopant to take place in carefully
delineated areas on the surface of the semiconductor, p-type
regions could be created within n-type regions, and vice-versa.
The transistor was much smaller than the vacuum tube, did not get
very hot, and did not require a headed filament that would
eventually burn out.
Finally in 1958, integrated circuits were invented. By the
mid 1950's, the first commercial transistors were being shipped.
However research continued. The scientist began to think that if
one transistor could be built within one solid piece of
semiconductor material, why not multiple transistors or even an
entire circuit. With in a few years this speculation became one
solid piece of material. These integrated circuits(ICs) reduced
the number of electrical interconnections required in a piece of
electronic equipment, thus increasing reliability and speed. In
contrast, the first digital electronic computer built with 18,000
vacuum tubes and weighed 50 tons, cost about 1 million, required
140 kilowatts of power, and occupied an entire room. Today, a
complete computer, fabricated within a single piece of silicon
the size of a child's fingernail, cost only about $10.00.
Now I will tell you the method of how the integrated
circuits and the silicon chip is formed. Before the IC is
actually created a large scale drawing, about 400 times larger
than the actual size is created. It takes approximately one year
to create an integrated circuit.