Module III : Basics of Analog and Digital Electronics Semiconductor, PN junction diode, Zener diode, rectifier- Half wave, full wave and Bridge rectifier, Introduction to Number system, basic Boolean laws, reduction of Boolean expressions and implementation with logic gates. 1 Introduction • All the matters are composed of three fundamental particles. Fundamenta l particle Nature of charge Mass in kg Neutron Proton no charge positive charge negative charge 1.675 x 10-27 1.675 x 10-27 Electron 9.107 x 10-31 2 Cont., Every atom consists of a nucleus carrying a positive charge around which electrons move in orbits/shells at a certain distance from the nucleus. The maximum number of electrons in any orbit is 2(n)2. Atom structure of copper is, 3 Cont., Electrons that are in shells close to nucleus are tightly bound to the atom and have low energy. Electrons that are in shells away from the nucleus are lightly bound to the atom and have high energy. Electrons with highest energy level exists in the outermost orbit. The outer most orbit is called as valence shell and the electrons are known as valence electron. 4 Energy Band The high energy outermost shell electrons can be easily extracted and take part in chemical reactions. In solids, atoms are close together. The outermost electrons are shared by other atoms. It forms a bond known as ‘covalent band’. As they are shared by adjacent atoms, they are not free under normal condition. Due to the coupling between the valence electrons, the energy levels associated with the valence electrons merge into each other. The merging forms an ‘energy band’. 5 Cont., Similarly, the energy levels of various electrons in various orbit levels merge to form various energy bands. Out of all the energy bands, the most important are, 1. Valence band 2. Conduction band 3. Forbidden band 6 Cont., 1. Valence band : The energy band formed due to merging of energy levels associated with valence electrons. The electrons in this bond are not free under normal condition. When certain energy is imparted, the electrons become free. 7 Cont., 2. Conduction band : The energy band formed due to merging of energy levels associated with the free electrons. Under normal condition, conduction band does not have any electrons. When certain energy is imparted, the electrons jump from valence band to conduction band and become free. While jumping from valence band to conduction band, the electrons have to cross an energy gap. 8 Cont., 3. Forbidden band : The energy gap which is separating the conduction band and valence band. The electrons cannot exist in the forbidden gap. 9