S EMICONDUCTORS On the basis of conductivity On the basis of the relative values of electrical conductivity ( : ) or resistivity ( ; = / : ), the solids are broadly classified as: (i) Metals: They possess very low resistivity (or high conductivity). ; ~ – – – < m : ~ – S m – (ii) Semiconductors: They have resistivity or conductivity intermediate to metals and insulators. ; ~ – – < m : ~ – – S m – (iii) Insulators: They have high resistivity (or low conductivity). ; ~ – < m : ~ – – – S m – The values of ; and : given above are indicative of magnitude and could well go outside the ranges as well.
Relative values of the resistivity are not the only criteria for distinguishing metals, insulators and semiconductors from each other. There are some other differences, which will become clear as we go along in this chapter. Our interest in this chapter is in the study of semiconductors which could be: (i) Elemental semiconductors : Si and Ge (ii) Compound semiconductors : Examples are: Inorganic: CdS, GaAs, CdSe, InP, etc. Organic: anthracene, doped pthalocyanines, etc.
Organic polymers: polypyrrole, polyaniline, polythiophene, etc. Most of the currently available semiconductor devices are based on elemental semiconductors Si or Ge and compound inorganic