2.1 INTRODUCTION TO SOLIDS AND CRYSTALS

As the atoms or molecules in solids are attached to one another with strong attracting forces, the solid maintains a definite volume and shape. A separate subject named solid-state physics has been evolved for the study of the electrical, thermal and electronic properties as well as behaviour of solids. Solid-state physics can be defined as the branch of physics which deals with physical properties of solids, particularly crystals, and the behaviour of electrons in these solids. A subset of solid-state physics is solid-state electronics, which mainly focuses on the electrical and electronic behaviour of solids, especially semiconductors and the movement/transport of electrons in semiconductor materials and different semiconductor devices, basic and advanced, which have evolved over the last century.

Solids may be broadly classified into two categories depending upon the arrangement of the atoms and molecules:

Example: Glass, plastic, rubber, etc.

Crystalline solids may be further subdivided into Single Crystal Solid and Polycrystalline Solid. In single crystal solids, the periodicity of the atoms extends throughout the material, as in the case of diamond, quartz, mica, etc. A polycrystalline material is an aggregate of a number of small crystallites with random orientations separated by well-defined boundaries. Most of the metals and ceramics exhibit polycrystalline structure. Silicon (Si), the most widely used semiconductor material, is also a polycrystalline in its natural form.

2.2 CRYSTALLINE SOLID

Before describing the arrangement of atoms in a crystalline solid, it is always convenient to describe the arrangement of the imaginary points in space which have a definite positional/spatial relationship with the atoms of the crystal. This set of imaginary three-dimensional points forms a framework on which the actual crystal structure is based.