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               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

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