CHAPTER 1

MATERIAL PROPERTIES

Although many elements and intermetallic compounds exhibit semiconducting properties, silicon is used almost exclusively in the fabrication of semiconductor devices and microcircuits. Of the many reasons for this choice, the most impor- tant are the following:
1. Silicon is an elemental semiconductor. Together with germanium, it can be subjected to a large variety of processing steps without the problems of decomposition that are ever present with compound semiconductors. For much the same reason, more is known today about the preparation and properties of extremely pure single crystal germanium and silicon than for any other element in the periodic table.
2. Silicon has a wider energy gap than germanium. Consequently it can be fabricated into microcircuits capable of operation at higher temperatures than their germanium counterparts. At the present time the upper operating ambient temperature for silicon microcircuits is between 125°C and 175°C, which is entirely acceptable for both commercial and military applications.
3. Unlike germanium, silicon lends itself readily to surface passivation treat- ments. This takes the form of a layer of thermally grown SiO, which provides a high degree of protection to the underlying device.
Although the fabrication of devices such as metal-oxide-semiconductor (MOS) transistors has emphasized that this oxide falls short of providing perfect control of surface phenomena, it is safe to say that the development of this technique provided a decisive advan- tage for silicon over germanium as the starting material in microcircuits [1].