Unit 4

• Semiconductor

  Material that can be conditioned to act as a good conductor, good insulator, or anything in between
• Electronic materials

  • Conductors
  • Insulators
  • Semiconductors
• Conductors

  • Have low resistance which allows electrical current flow
• Alloy conductors

  • Brass
  • Steel
• Liquid conductors

  • Salt water
• Valence electron

  Electron in the outer shell of a conductor atom that is easily stripped, producing current flow
• Most insulators are compounds of several elements
• The atoms in insulators are tightly bound so electrons are difficult to strip away for current flow
• Common semiconductor elements

  • Carbon
  • Silicon
  • Germanium
• Silicon is the best and most widely used semiconductor
• Semiconductor valence orbit

  • Main characteristic is that it has four electrons in its outer or valence orbit
• Covalent bonds

  Links between semiconductor atoms that share their outer electrons to form a crystal lattice structure
• Covalent solids

  3D collection of atoms bound by shared valence electrons, difficult to deform, high melting points, poor electrical conductors, opaque
• Extrinsic semiconductor

  Semiconductor doped with impurities
• Extrinsic semiconductors

  • Silicon doped with Arsenic
  • Germanium doped with Gallium
• Doping

  Adding impurities to a semiconductor to make it conduct electricity
• Pentavalent doping

  Adding an impurity with 5 valence electrons, leaving 1 electron available to conduct current flow
• Trivalent doping

  Adding an impurity with 3 valence electrons, leaving a "hole" that can attract electrons to conduct current flow
• 1. type semiconductor

  Intrinsic semiconductor doped with trivalent impurity, majority carriers are holes
• Valence band

  Band where most electrons remain bound to atoms
• Conduction band

  Band of high energy orbitals that are generally empty and accept electrons from the valence band
• Energy gap

  The "leap" required for electrons to enter the conduction band from the valence band
• Conductors

  • Electrons can move freely among orbitals within an energy band, valence band is empty, energy gap is nonexistent or small
• Insulators

  • Valence band is full, energy gap is relatively large
• Semiconductors

  • Valence band is full but energy gap is intermediate, only a small leap is required for electrons to enter conduction band
• Silicon is a very common element, the main element in sand & quartz
• Intrinsic semiconductor

  Semiconductor not contaminated with impurity atoms
• In an intrinsic semiconductor, at any temperature above absolute zero, there is a finite probability that an electron will be knocked loose from its position, leaving behind an electron deficiency called a "hole"
• Conduction mechanism in intrinsic semiconductor

  1. Electron knocked loose from lattice

  2. Hole left behind

  3. If voltage applied, both electron and hole can contribute to current flow
• Conduction mechanism in intrinsic semiconductor (Generation and recombination)

  1. Creation rate of hole-electron pairs = recombination rate at stable state

  2. Mean lifetime of electrons = mean lifetime of holes
• Intrinsic carrier concentration (ni)

  Number of electrons in conduction band (and holes in valence band) per unit volume in a completely pure semiconductor
• Ns

  The number per unit volume of effectively available states; its precise value depends on the material, but it is of order 10^19 cm-3 at room temperature and increases with temperature
• Eg

  The energy gap (between the bottom of the conduction band and the top of the valence band)
• T

  The absolute temperature in Kelvin; it is assumed that kBT ≤ Eg/5
• Intrinsic carrier concentration in semiconductors

  1. The probability of exciting an electron from the top of the valence band to the bottom of the conduction band is proportional to the Boltzmann factor exp(-Eg/kBT)

  2. This process leaves behind a hole in the valence band and is called electron-hole pair creation

  3. At thermal equilibrium, the creation of electron-hole pairs is balanced by their recombination
• For intrinsic silicon, n = ni = p
• The product of electron and hole concentrations is pn = ni^2
• Intrinsic refers to properties of pure materials
• The density of silicon atoms is na ≈ 5x10^22 cm-3
• At room temperature, one bond per about 10^13 is broken