INTRODUCTION TO QUANTUM THEORY OF SOLIDS By Sushama Baag
CHAPTER OBJECTIVE LECTURE-1 • This chapter provides the basic concepts and terminology for understanding • semiconductors. • Importance is given to the concepts of energy band , • 2 kinds of electrical charge carriers called electrons and holes • and how the carrier can be controlled with addition of dopants. • Another important fact is the Fermi distribution function and • concept of Fermi level. • The electron and hole concentration are closely linked to Fermi level. • Mastery of the terms , concepts and models presented here will prepare • you for understanding not only the many semiconductor devices that • are in existence today but also many more that will be invented in the future.
Before the “How” , Ask Yourself “Why” • Before we dive in, we’ll take a moment to explain exactly why we’re so interested in, and dependent on semiconductors in the first place. • Semiconductors are so important to our electronic devices simply b’coz their properties can be easily and substantially manipiulated.
FORMATION OF ENERGY BANDS • Electrons in individual atoms can exist only at exact specified energy levels. • But the Pauli Exclusion Principle says that two electrons cannot exist in the same space at the same time. • So when we bring together billions of atoms, the electrons move their orbits ever-so-slightly, so that each one is different. • When we combine all these orbits, each different by a miniscule amount, the result is the formation of a ‘band’ of energy levels, in which an electron can exist virtually anywhere
Characteristics of Energy Bands • Energy bands become wider as the distances between atoms become shorter. • The higher the energy level, the wider is the corresponding energy band that it splits into.
Energy Band In Silicon • The actual band splitting in a crystal is much more complicated than shown in the figures above. • Let us consider an isolated Si atom(At. No.=14). 1s2 2s2 2p6 3s2 3p2