SEMICONDUCTORS 1INTRODUCTION lecture 1

The most important and perhaps most exciting electrical devices used today are built from semiconductive materials. Electronic devices, such as diodes, transistors, thyristors, thermistors, photovoltaic cells, phototransistors, photoresistors, lasers, and integrated circuits, are all made from semiconductive materials, or semiconductors.1 What Is a Semiconductor? Microchips Solar cells Materials are classified by their ability to conduct electricity. Substances that easily pass an electric current, such as silver and copper, are referred to as conductors. Materials that have a difficult time passing an electric current, such as rubber, glass, and Teflon, are called insulators. There is a third category of material whose conductivity lies between those of conductors and insulators. This third category of material is referred to as a semiconductor. Asemiconductor
has a kind of neutral conductivity when taken as a group. Technically speaking, semiconductors are defined as those materials that have a conductivity σ in the range between 10−7 and 103 mho/cm (see Fig. 4.2). Some semiconductors are pure-elemental structures (e.g., silicon, germanium), others are alloys (e.g., nichrome, brass), and still others are liquids.Silicon Silicon is the most important semiconductor used in building electrical devices. Other materials such as germanium and selenium are sometimes used, too, but they are less popular. In pure form, silicon has a unique atomic structure with very important properties useful in making electrical devices.Silicon is ranked second in the order of elements appearing in the earth’s crust, an average of 27 percent occurring in igneous rocks. It is estimated that a cubic mile of seawater contains about 15,000 tons of silicon. It is extremely rare to find silicon in its pure crystalline form in nature, and before it can be used in making electronic devices, it must be separated from its binding elements. After individuals—chemists, material scientists, etc.—perform the purification process, the silicon is melted and spun into a large “seed” crystal. This long crystal can then be cut up into slices or wafers that semiconductor-device designers use in making electrical contraption