Textbook: Electronic Devices and Circuit Theory Robert L.Boylested

1-Theory of Semiconductors 2-Diode Basics and Applications 3-Special Purpose Diodes 4-BJT Basics and Configurations 5-BJT Biasing and Applications 6-FET Basics and Configurations 7-FET Biasing and Applications

 An atom is made up of electrons, protons and zero or some neutrons  Protons and neutrons are contained inside the nucleus while electrons revolve around the nucleus  Electrons can only revolve in discrete orbits around the nucleus called shells with certain energy levels associated with each shell.  Each shell can accommodate a maximum of 2n 2 electrons where n=1,2,3,… are the shell numbers  Each shell has n subshells. The first subshell can have 2 electrons and every proceeding subshell can have 4 electrons more than the preceding subshell.

 The outermost shell is called valence shell and the electrons in that shell are called valence electrons  Valence electrons contribute to the electrical properties of the atom  The atoms with empty or partially filled valence shells are better conductors like metals in the periodic table, whereas the filled or almost filled valence shells made an atom more insulated

 Conductivity of semiconductors lies between conductors and insulators  Silicon and Germanium each belongs to IV A and are well known semiconductors.  Both need 4 electrons to complete their valence shell.  Each atom of mentioned semiconductors share their four electrons to form covalent bonds with neighboring atoms to form a crystal lattice

 When an electron gets a specific amount of energy it can jump to a higher shell. Conversely, an electron can jump from a higher shell to a lower shell and release energy.  Conduction shell are the band of energy levels where electrons are loosely attached to an atom and can move freely. The electrons in conduction shell are called free electrons.  The energy gap between conduction shell and valence shell is called forbidden gap.  Forbidden gap is very large in insulators, moderate in semiconductors and very small or none in metals  Conductivity in semiconductors are directly proportional to temperature.

 Current in conductors are due to free electrons in the condition shell  The free electrons are loosely attached to atoms and can move freely around in metal structure  When an electron in valence shell of a semi- conductor gets enough energy it breaks the covalent bond and jumps to conduction bond, leaving behind a deficiency of charge called a hole

 Current in semiconductors are due to free electrons as well as valence electrons(hole current)  The number of holes and electrons in intrinsic semiconductors are same i.e. n i = p i  When a potential is applied drift current takes place