Silicon Toady

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