Saturday, April 9, 2011

Semiconductor physics:

  • The charge, or quantity of negative electricity is 1.60*10-19 C.
  • The number of electrons per second represents current, I.
  • The charge of a positive ion is an integral multiple of the charge of the electron.
  • Hole is an effective charge carrier.
  • The force, f on a unit positive charge, q is an electric field is the electric field intensity, at that point (f = q ).
  • The potential V of point B at x with respect to point A at x1 is the work done against the field in taking a unit positive charge from A to B.

  • The electric field equals the negative gradient of the potential , ?= - dV/dx
  • Electron volt (eV) is the unit of work or energy(electric, mechanical, thermal, etc.,)
  • For germanium EG is 0.785 eV, for silicon 1.21 eV.
  • Semiconductors have property of negative temperature coefficient of resistance.
  • A semiconductor in which electrons and holes are solely created by thermal excitation is called a pure or intrinsic semiconductor.
  • In intrinsic semiconductor the number of holes is always equal to the number of electrons.
  • Hole may serve as a carrier of electricity comparable in effectiveness with free electron.
  • If impurities added to pure semiconductor then it is called extrinsic semiconductor. This process is called doping.
  • The current in a conductor is due to flow of electrons, whereas the current in a semiconductor results from the movement of both electrons and holes.
  • The transport of the charge in a crystal under influence of an electric filed result in drift current.
  • Diffusion current is the result of a non-uniform concentration gradient.
  • The drift velocity is proportional to applied electric filed intensity .
    = µ , where µ (square meters per volt second, m2/vs) is called mobility of the electrons.
  • The current desist J is the current per unit area of the conducting medium.
  • The above equations defines Ohm's law, the conduction current is proportional to the applied voltage.
  • Power density , power dissipated within the metal by electrons is J = ? 2
  • If the dopant has five valance electrons those will call donor or n-type impurities ex. Antimony, phosphorus and arsenic.
  • If intrinsic semiconductor doped with n type impurities, the electrons will increase, holes will decrease. This type material is called as n-type semiconductor.
  • If the dopant has three valance electrons those will call acceptor or p-type impurities ex. Boron, gallium and indium.
  • If intrinsic semiconductor doped with p type impurities, the holes will increase, electrons will decrease. This type material is called as p-type semiconductor.
  • The product of free negative and positive concentrations is a constant independent of the amount of donor and acceptor impurity doping. This is called mass-action law.
  • np = ni2
  • Metal is unipolar where as semiconductor is bipolar (two charge carrying particles).
  • If a specimen carrying a current I is placed in a transverse magnetic filed B, an electric filed is induced in the direction perpendicular to both I and B. This phenomenon, known as the Hall Effect.
  • Hall Effect is used to determine type of semiconductor (p or n) and carrier concentration.
  • By Hall Effect can measure mobility.
  • Hall Effect applications: magnetic field meter, Hall Effect multiplier.
  • Thermistor has a negative temperature coefficient of resistance.
  • Silicon and germanium not used as thermistors because their properties are too sensitive to impurities.
  • A heavily doped semiconductor can exhibit a positive temperature coefficient of resistance. Such device called sensistor.
  • If radiation falls upon a semiconductor, its conductivity increases.
  • In Hall Effect the output voltage produced across the crystal is due to movement of charge carriers toward one end.
  • The voltage measured between the two faces is called hall voltage.
  • Hall voltage is zero for intrinsic semiconductor.
  • Hall coefficient depends on the type of material.
  • Hall Effect probes are used for measurement of DC current in a wire.
  • Fermi level is a measure of probability of occupancy of electrons or holes in the allowed energy states.
  • In pure semiconductor the Fermi level is at the middle of the conduction and valence band.
  • In n type semiconductor the Fermi level is just below the conduction band.
  • In p type Fermi level is just above the valance band.
  • In a heavily doped n- type semiconductor the Fermi level is in the conduction band, similarly in a heavily doped p- type semiconductor the Fermi level is in the valance band.
  • Fermi Dirac function f(E) gives the probability that a quantum state with energy E is occupied by an electron.
  • Fermi level EF represents the energy state with 50% probability of being filled if no forbidden band exists.
  • If E >> EF then f(E) = 0, means there is zero probability of finding an occupied quantum state of energy greater than EF.
  • If E << EF then f(E) = 1, means all quantum levels with energy less than EF are occupied.
  • Silicon and germanium are called indirect band gap semiconductor where as gallium arsenide is called direct band gap semiconductor.

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