PROPERTIES OF MATERIALS
strength, electrical conductivity, magnetic properties
semiconductors, microchips, transistors

properties depend on:
kind of atoms
how atoms are arranged
how atoms are bonded

• STRENGTH
  = ability to resist changes of shape
  directly related to type of chemical bonding
  Van der Waals forces weakest
  • compressive strength = ability to withstand crushing
  • tensile strength = ability to withstand pulling apart
  • shear strength = ability to withstand twisting
  the three kinds of strength often independent

• composite materials
  = combination of several materials, strength of one of the constituents offsets weakness of another
  e.g.
  • plywood: thin wood layers with alternating grain directions
  • reinforced steel: steel rods (tensile strength) in concrete (compressive strength)
  • fiberglass: cemented mat of glass fibers
  • carbon fiber composites: strong + light structural materials
  • automobile wind shields: layered to resist shattering
  • tires: rubber + steel

ELECTRICAL CONDUCTIVITY
in order of conductivity:
superconductors, conductors, semiconductors, insulators

-

conductors: material capable of carrying electric current, i.e. material which has ``mobile charge carriers'' (e.g. electrons, ions,..)
e.g. metals, liquids with ions (water, molten ionic compounds), plasma

-

insulators: materials with no or very few free charge carriers;
e.g. quartz, most covalent and ionic solids, plastics

-

semiconductors: materials with conductivity between that of conductors and insulators;
e.g. germanium Ge, silicon Si, GaAs, GaP, InP

-

superconductors: certain materials have zero resisistivity at very low temperature

ENERGY BANDS IN SOLIDS

• In solid materials, electron energy levels form bands of allowed energies,
  separated by forbidden bands
• valence band = outermost (highest) band filled with electrons (``filled'' = all states occupied)
• conduction band = next highest band to valence band (empty or partly filled)
• ``gap'' = energy difference between valence and conduction bands, = width of the forbidden band

• Note:
  electrons in a completely filled band cannot move, since all states occupied (Pauli principle); only way to move would be to ``jump'' into next higher band - needs energy;
  electrons in partly filled band can move, since there are free states to move to.
• Classification of solids into three types, according to their band structure:
  • insulators:
    gap = forbidden region between highest filled band (valence band) and lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV;
  • semiconductors:
    gap is small - about 0.1 to 1 eV;
  • conductors:
    valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it

INTRINSIC SEMICONDUCTORS

• semiconductor = material for which gap between valence band and conduction band is small;
  (gap width in Si is 1.1 eV, in Ge 0.7 eV).
• at T = 0, there are no electrons in the conduction band, and the semiconductor does not conduct (lack of free charge carriers);
• at T > 0, some fraction of electrons have sufficient thermal kinetic energy to overcome the gap and jump to the conduction band;
  fraction rises with temperature;
  e.g. at 20deg C (293 K), Si has conduction electrons per cubic centimeter; at 50deg C (323 K) there are .
• electrons moving to conduction band leave ``hole'' (covalent bond with missing electron) behind;
  under influence of applied electric field, neighboring electrons can jump into the hole, thus creating a new hole, etc.
  holes can move under the influence of an applied electric field, just like electrons;
  both contribute to conduction.
• in pure Si and Ge, there are equally many holes (``p-type charge carriers'') as there are conduction electrons (``n-type charge carriers'');
• pure semiconductors also called ``intrinsic semiconductors''.

DOPED SEMICONDUCTORS

• ``doped semiconductor'' (also ``impure'' or ``extrinsic'') = semiconductor with small admixture of trivalent or pentavalent atoms;
• donor (n-type) impurities:
  • dopant with 5 valence electrons (e.g. P, As, Sb)
  • 4 electrons used for covalent bonds with surrounding Si atoms, one electron ``left over''
  • left over electron is only loosely bound only small amount of energy needed to lift it into conduction band (0.05 eV in Si)
  • ``n-type semiconductor'', has conduction electrons, no holes (apart from the few intrinsic holes)
  • example: doping fraction of Sb in Si yields about conduction electrons per cubic centimeter at room temperature, i.e. gain of over intrinsic Si.
• acceptor (p-type) impurities:
  • dopant with 3 valence electrons (e.g. B, Al, Ga, In) only 3 of the 4 covalent bonds filled vacancy in the fourth covalent bond hole
  • ``p-type semiconductor'', has mobile holes, very few mobile electrons (only the intrinsic ones).
• advantages of doped semiconductors:
  • can ``tune'' conductivity by choice of doping fraction
  • can choose `` majority carrier'' (electron or hole)
  • can vary doping fraction and/or majority carrier within piece of semiconductor
  • can make ``p-n junctions'' (diodes) and ``transistors''