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Semiconductors, how they work
 

Semiconductors, how they work

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  • Silicon
  • Arsenic
  • Arsenic
  • Aluminum
  • Silicon
  • Silicon
  • Silicon

Semiconductors, how they work Semiconductors, how they work Presentation Transcript

  • Silicon has 4 electrons in outer layerIt wants 8 Si
  • • Silicon atoms can share electrons with other atoms to get 8 electrons in outer layer.• Silicon atoms combining together forms a crystal.• Pure silicon crystals are not good electrical conductors, they are insulators.
  • all silicon atoms in a crystal
  • Doping• Doping means adding other kinds of atoms to the silicon crystal.• Phosphorus and Arsenic atoms have 5 outer electrons.• Aluminum and Gallium have 3 outer electrons.• Doping makes the crystal into a semiconductor.
  • As As AsAs As As As As AsAs
  • extra electronsin Arsenic atoms
  • Al Al Al holes (missing electrons) in Aluminum atoms Al AlAl Al Al
  • • N type material = semiconductor with extra negative charges (extra electrons)• P type material = semiconductor with extra positive charged holes (fewer electrons or missing negative charges)
  • PN Junction• What happens when P type material and N type material are placed together?
  • • Electrons from Arsenic atoms in N type material are attracted to holes (missing electron positions) in Aluminum atoms in P type material.• Electrons from N type material move and fill in holes in P type.
  • + -+ -+ -
  • • Atoms have the same number of electrons as protons.• When electrons leave Arsenic atoms, they now have more protons than electrons.• Arsenic atoms become positive ions.• When aluminum atoms pick up extra electrons, they now have more electrons than protons.• Aluminum atoms become negative ions.
  • N Type P Typeextra electrons holes (missing electrons)
  • N Type P Typeextra electrons holes (missing electrons)
  • N Type P Typeextra electrons holes (missing electrons) depletion zone
  • • Harder and harder to get the depletion zone to be wider because – electrons are repelled by negative aluminum ions – holes are repelled by positive arsenic ions
  • N Type P Typeextra electrons holes (missing electrons) depletion zone
  • N Type P Typeextra electrons holes (missing electrons) depletion zone stabilized
  • What happens when a voltage source is applied to the 2 ends of thediode?
  • • Electrons are pushed from the negative supply into the N side of the diode.• Holes are pushed (electrons are drained) from the P side of the diode into the positive side of the supply.
  • What happens when a voltage source is applied to the 2 ends of thediode?
  • As more and more charges build up, easier for charges to move across the depletion zoneCurrent flows as electrons flow from N typethrough holes of P type out to positive source
  • Forward Biased• To cross depletion zone, must apply enough voltage (pressure or push) to get electrons past negatively charged region of depletion zone.• Forward bias voltage about 0.7 V for silicon diodes and 0.3 V for germanium diodes.
  • What happens if the voltage polarity is reversed?
  • What happens if the voltage polarity is reversed? depletion zone widens
  • Reversed Bias• When diodes are reverse biased, depletion zone widens• Harder for charges to cross depletion zone• High enough voltage will force charges across depletion zone – Heat from large reverse current destroys diode – Breakdown voltage = voltage level that causes diodes to conduct when reverse biased