Introduction to Semiconductor

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Lecture 1

• Introduction to Semiconductor

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Semiconductor

• Semiconductors are materials with conductivity
  that can be controlled through methods such as
  doping or changing the temperature. 
•   Conductivity can be increased through doping,
  creating either p-type semiconductors or n-type
  semiconductors.

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Atomic Theory

• Atom is smallest piece of an element that keeps
  its chemical properties
• Atom contains 3 basic particles
• Protons
• Neutrons
• Electrons orbit around nucleus

- Form the nucleus
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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.

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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

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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

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Energy Level

• There are discrete energy levels associated with
  each orbiting electron
• The more distance the electron from the nucleus,
  the higher energy state

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• 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, PVC
• Conductors
• 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 etc
• Semiconductors
• Electrons have moderate binding energies
• at absolute zero, all electrons are tightly
  bound insulator
• at very high temps, material can conduct
  conductor

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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 (OHM's LAW)

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