NEEP 541 Ionization in Semiconductors

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NEEP 541Ionization in Semiconductors

• Fall 2002
• Jake Blanchard

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Outline

• Ionization in Semiconductors
• Transients
• Semiconductor fundamentals
• Band theory
• Doping
• Junctions

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Ionization Effects

• Radiation ionizes target through collisions with
  electrons
• As electrons slow, they remain free electrons
• In semiconductors, we think of positive charges
  (holes) and negative charges (electrons) as free
  particles
• The key question concerns the average densities
  of electrons and holes in the solid and the
  subsequent transport of these particles

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Transient Effects

• The primary manifestation of ionization is a
  transient increase in the electrical conductivity
  and transient currents across the semiconductor
  junctions
• In optical materials, ionization changes the
  absorption coefficient and luminescence (well
  discuss this later)

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How a Semiconductor Works

• Si has four electrons in its outer shell
• These form bonds with four neighboring atoms
• A perfect Si crystal is an insulator because all
  these outer electrons are tied up with
  neighboring atoms
• By mixing in impurities, you can alter this
  behavior

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Doping

• Adding P or As makes N-type Si.
• These have 5 outer electrons, so in Si they have
  one free electron and thus permit conduction
• A small amount makes a big difference
• N-type Si is a good conductor

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Doping

• Adding B or Ga creates P-type Si
• These have 3 electrons in the outer shell, so
  they form holes
• A Si electron is left free
• P-type Si is also a good conductor

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P-N junctions

• Combine a layer of P-type with a layer of N-type
  Si
• The interface is a junction
• This forms a diode current can only flow in one
  direction

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P-N junction

• When diode is working, both holes and electrons
  flow towards junction
• They combine at interface
• Net current results

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P-N Junction
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P-N Junction
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IV Curve
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Band Theory

• Fermi energy is the highest energy state that
  would be occupied at 0 K
• In solids, only certain energy levels can be
  occupied by electrons
• The allowed levels smear into bands, due to
  periodicity of the lattice
• In metals, the Fermi energy lies within an
  allowed energy band
• Hence, electrons close to Fermi level can scatter
  into it (by electric fields) fairly easily and it
  will conduct at 0 K

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Band Theory

• In semiconductors, the electrons with the highest
  energies exactly fill one energy band at 0 K
• The next higher band is empty
• Resistivity is infinite
• Filled band is the valence band
• Higher band is the conduction band
• The energy separation between the bands is the
  band gap
• The Fermi energy lies in this gap

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Band Gap
EC
EF
EG
EV
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Real Materials

• Previous comments are for perfect crystals
• Boundaries and defects disrupt periodicity
• This creates allowed energy levels in gap
• In single crystal Si, defects are isolated, so
  the electrons in these levels are bound

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Semiconductors

• Intrinsic semiconductors have finite probability
  (above 0 K) that some electrons will reach
  conduction band
• Extrinsic semiconductors have some energy levels
  in the gap, due to defects and impurities
• These levels can capture holes and electrons

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Doping

• Most practical semiconductors rely on impurities
  for their properties
• Impurities can produce energy levels with any
  charge at just about any location within the gap
• Donor defects give up electrons to the conduction
  band
• Acceptor defects capture an electron from the
  valence band