Semiconductors

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. Then they have to make a mask.
Depending on the level of complexity, an IC will
require from 5 to 18 different glass masks, or
"work plates" to create the layers of circuit
patterns that must be transferred to