DIODES AND TRANSISTORS

p-n JUNCTION:

• p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ;
• ``diffusion'' = movement due to difference in concentration, from higher to lower concentration;
• in absence of electric field across the junction, holes ``diffuse'' towards and across boundary into n-type and capture electrons;
  electrons diffuse across boundary, fall into holes (``recombination of majority carriers'');
  formation of a ``depletion region''(= region without free charge carriers) around the boundary;
• charged ions are left behind (cannot move):
  negative ions left on p-side net negative charge on p-side of the junction;
  positive ions left on n-side net positive charge on n-side of the junction
  electric field across junction which prevents further diffusion.

DIODE

• diode = ``biased p-n junction'', i.e. p-n junction with voltage applied across it
• ``forward biased'': p-side more positive than n-side;
• ``reverse biased'': n-side more positive than p-side;

• forward biased diode: the direction of the electric field is from p-side towards n-side
  p-type charge carriers (positive holes) in p-side are pushed towards and across the p-n boundary,
  n-type carriers (negative electrons) in n-side are pushed towards and across n-p boundary
  current flows across p-n boundary
• reverse biased diode: applied voltage makes n-side more positive than p-side electric field direction is from n-side towards p-side pushes charge carriers away from the p-n boundary depletion region widens, and no current flows
• diode only conducts when positive voltage applied to p-side and negative voltage to n-side
• diodes used in ``rectifiers'', to convert ac voltage to dc.

TRANSISTORS

• (bipolar) transistor = combination of two diodes that share middle portion, called ``base'' of transistor; other two sections: ``emitter'' and ``collector'';
• usually, base is very thin and lightly doped.
• two kinds of bipolar transistors: pnp and npn transistors
• ``pnp'' means emitter is p-type, base is n-type, and collector is p-type material;
• ``normal operation'' of pnp transistor:
  • apply positive voltage to emitter, negative voltage to collector;
  • if emitter-base junction is forward biased, ``holes flow'' from battery into emitter, move into base;
  • some holes annihilate with electrons in n-type base,
  • but base thin and lightly doped most holes make it through base into collector,
  • holes move through collector into negative terminal of battery;
  • i.e. ``collector current'' flows whose size depends on how many holes have been captured by electrons in the base;
  • this depends on the number of n-type carriers in the base which can be controlled by the size of the current (the ``base current'') that is allowed to flow from the base to the emitter;
  • the base current is usually very small; small changes in the base current can cause a big difference in the collector current;
    transistor acts as amplifier of base current, since small changes in base current cause big changes in collector current.
• transistor as switch:
  if voltage applied to base is such that emitter-base junction is reverse-biased, no current flows through transistor
  -- transistor is ``off''
  therefore, a transistor can be used as a voltage-controlled switch;
  computers use transistors in this way.

• ``field-effect transistor'' (FET)
  in a pnp FET, current flowing through a thin channel of n-type material is controlled by the voltage (electric field) applied to two pieces of p-type material on either side of the channel (current depends on electric field).
  This is the kind of transistor most commonly used in computers.

SUPERCONDUCTORS

• mobile electrons in conducting material move through lattice of atoms or ions that vibrate (thermal motion)
  when conductor is cooled down less vibration ``easier'' for electrons to get through
  resistivity of conductors decreases (i.e. they become better conductors) when they are cooled down
• in some materials, resistivity goes to zero below a certain ``critical temperature''
  -- these materials called superconductors
  -- critical temperature different for different materials;
• no electrical resistance electric current, once started, flows forever!
• superconductivity first observed by Heike Kamerlingh Onnes (1911) in Hg (mercury) at temperatures below 4.12 K.
  many other superconductors with critical temperatures below about 20K found by 1970
• ``high superconductors'': (Karl Alex Müller and Johannes Georg Bednorz, 1986)
  certain ceramic oxides show superconductivity at much higher temperatures; since then many new superconductors discovered, with up to 125K.
• advantage of high superconductors:
  can cool with (common and cheap) liquid nitrogen rather than with (rare and expensive) liquid helium;
  much easier to reach and maintain LN temperatures (77 K) than liquid Helium temperatures (few K).

• PROPERTIES OF SUPERCONDUCTORS:
  • electrical resistivity is zero (currents flowing in superconductors without attenuation for more than a year)
  • there can be no magnetic field inside a superconductor (superconductors ``expell'' magnetic field)
  • transition to superconductivity is a phase transition (without latent heat).
  • about 25 elements and many hundreds of alloys and compounds have been found to be superconducting (examples: In, Sn, V, Mo, Nb-Zr, Nb-Ge alloys, )
• applications of superconductors
  • superconducting magnets:
    magnetic fileds stronger, the bigger the current - ``conventional'' magnets need lots of power and lots of water for cooling of the coils;
    s.c. magnets use much less power (no power needed to keep current flowing, power only needed for cooling)
    most common coil material is NbTi alloy; liquid He for cooling
    e.g. particle accelerator ``Tevatron'' at Fermi National Accelerator Laboratory (``Fermilab'') uses 990 superconducting magnets in a ring with circumference of 6 km, magnetic field is 4.5 Tesla.
  • magnetic resonance imaging (MRI): create images of human body to detect tumors, etc.
    need uniform magnetic field over area big enough to cover person; can be done with conventional magnets, but s.c. magnets better suited - hundreds in use
  • magnetic levitation - high speed trains??

• explanation of superconductivity:
  (John Bardeen, Leon N. Cooper, J. Robert Schrieffer, 1957)
  
  • due to interaction of the electrons with the lattice (ions) of the material, there is a small net effective attraction between the electrons; (presence of one electron leads to lattice distortion, second electron attracted by displaced ions)
  • this leads to formation of ``bound pairs '' of electrons (called Cooper pairs); (energy of pairing very weak - thermal agitation can throw them apart, but if temperature low enough, they stay paired)
  • electrons making up Cooper pair have momentum and spin opposite to each other; net spin = 0 behave like ``bosons''.
  • unlike electrons, bosons like to be in the same state; when there are many of them in a given state, others also go to the same state
  • nearly all of the pairs locked down in a new collective ground state
  • this ground state is separated from excited states by an energy gap;
  • consequence is that all pairs of electrons move together (collectively) in the same state; electron cannot be scattered out of the regular flow because of the tendency of Bose particles to go in the same state
    no resistance