The transistors are made in materials which are relatively poor conductors of electricity and are classified as semiconductors. Elements such as silicon (Si) or germanium (Ge) are semiconductors. Compounds such as gallium arsenide (GaAs) are also semiconductors. The first transistor was made using germanium. However, at present, most of the chips are made using silicon.  Some of the reasons for this is (a) the easy availability of silicon (sand is impure form of silicon dioxide) and (b) the formation of a good quality silicon-di-oxide (SiO₂) on top of silicon. The SiO₂ layer forms automatically when Si is exposed to air or water. It offers good protection from chemical attack and also serves as an excellent insulator. In this course, we focus our attention on the processes relevant for creating silicon based microelectronic chips.

In the making of an IC, a variety of processes are involved. The silicon is in the form of a thin wafer. A Si wafer may be of 200 mm diameter and 0.45 mm thick, or 300 mm diameter and 1 mm thick. Currently most manufacturers use 300 mm wafers, while a few still employ 200 mm wafer. Thus, a typical wafer will be of the size of a plate, but with less thickness. The material is fragile and hence should be handled carefully.  Although the wafer thickness is at least 0.45 mm, the transistors are made within the first micron or less. Thus, only 0.1% of the entire thickness of the wafer is used. The remaining is used mainly for mechanical strength.  If the wafer is made with less thickness, it is likely to break very easily during handling.

The transistors are made on the wafer by adding certain materials (such as boron or phosphorous). We will learn the details in the later chapters. Once the transistors are made, they have to be connected properly using conducting wires. The wires should also be insulated. Otherwise, there will be short circuits and the chip will not function. Previously aluminum was used as the conducting material, but now copper is used as the conductor. These are called “interconnects”. The insulator is used previously is silicon di oxide, but now new materials called “low-k” materials are being used in the advanced chips. These are usually organic materials, which have a low dielectric constant.