SemiconductorSemiconductors are materials with conductivitythat can be controlled through methods such asdoping or changing the temperature.Conductivity can be increased through doping,creating either p-type semiconductors or n-typesemiconductors.
Atomic Theory• Atom is smallest piece of an element that keeps its chemical properties• Atom contains 3 basic particles – Protons – Neutrons - Form the nucleus – Electrons – orbit around nucleus
Bohr models• The major advantage of the Bohr model was that it worked. It explained several things:• Atomic spectra - discussed above• Periodic behavior of elements - elements with similar properties had similar atomic spectra.• Each electron orbit of the same size or energy (shell) could only hold so many electrons.• First shell = two electrons• Second shell = eight electrons• Third shell and higher = eight electrons• When one shell was filled, electrons were found at higher levels.• Chemical properties were based on the number of electrons in the outermost shell.• Elements with full outer shells do not react.• Other elements take or give up electrons to get a full outer shell.
Valence shell• Outermost shell for a given atom• Determines the conductivity of the atom• Contains up to 8 electron 1 electron in valence shell – nearly perfect conductor 8 electron in valence shell – complete insulator 4 electron in valence shell - semiconductor
Covalent Bonding• A method by which atoms complete their valence shells by sharing valence electron with other atoms• Covalent bond will result in a stronger bond between the valence electrons and their parent atom (insulator)• However, valence electrons still possible to absorb sufficient kinetic energy from natural causes to break the covalent bond and assume free state• Refer figure 1.7
Energy Level• There are discrete energy levels associated with each orbiting electron• The more distance the electron from the nucleus, the higher energy state
Insulators• Electrons tightly bound to host ion – need large amounts of energy to break free – very low numbers of free electrons low conductivity – electric currents do not pass easily e.g. paper, rubber, PVCConductors• Electrons very loosely bound to host ions – very easy to break free from ions – free to "wander" around crystal large numbers of free electrons – about one per atom high conductivity – movement of electrons produces current in opposite direction e.g. metals - Cu, Ag, Al etcSemiconductors• Electrons have moderate binding energies – at absolute zero, all electrons are tightly bound insulator – at very high temps, material can conduct conductor – usually moderate numbers of free electrons about one per million atoms e.g. Si, Ge, GaAs
Conduction in metals• Free electrons in metal have a wide range of energies & velocities – behave as a "cloud" of electrons – individual electrons wander through crystal & collide with ion cores – individual electrons may travel in many different directions• No net flow of current - flow in one direction balanced by flow in another• Electron cloud can be accelerated by applied external electric-field – p.d. across the ends – cloud moves in opposite direction to field with drift velocity vd – constitutes an electric current in direction of field• Can show that V = IR (OHMs LAW)
Electron cloud Electric Field Drift of electron cloud vd- + I Current flow l Metal bar of Cross-sectional area A Individual electrons may travel in random directions