SCEN103 Class 27

More Semiconductor Physics -- Doping

• Difference between metal and insulator is full vs. partially-full bands
• Difference between insulator and semiconductor is one of degree;
  -- that is, how large is the band gap?
• Smaller than 2eV is usually considered to be a semiconductor
  -- probably because of photoconductivity
  -- lowest energy visible photons are red (2eV)

• Can we move energy levels closer to the conduction band?
  -- Yes, by doping the semiconductor with impurities!
• Consider the following schematic of the covalent bonding of neighboring silicon atoms:
  (Please keep in mind the 3-D model from the previous class of the actual arrangement of silicon atoms in a semiconducting crystal.)

• What happens if we add a few atoms of arsenic?
  Recall the periodic table and electronic configurations:
  

  Element:	Configuration:
  Silicon	[neon] 3s2 3p2
  Gallium	[argon] 4s2 4p1
  Arsenic	[argon] 4s2 4p3

• Arsenic has one more valence electron than silicon
  -- when it is placed substitutionally in the lattice for a Si atom four of the valence electrons bind to neighboring Si atoms, the one left over becomes free for conduction

• In the band structure picture, impurity states are created much closer to the conduction band
• These states are known as donors, since they "donate" electrons to the conduction band.
• It is much easier to "liberate" the electron for conduction at lower thermal energy.
• A semiconductor doped with donors is known as an "n-type" semiconductor. (n for negative charge carrier)

• What happens if we add a few atoms of gallium?
• Gallium has one fewer valence electron than silicon
  -- when it is placed substitutionally in the lattice for a Si atom only three of the neighboring neighboring Si atoms are successfully bound,
  -- a hole remains, which also aids conduction since electrons may now move more freely through the lattice, "jumping" into a hole and leaving a hole behind to be filled.

• In the band structure picture, impurity states are created much closer to the valence band.
• These states are known as acceptors, since they can accept electrons from the valence band.
• Again, it is much easier to "liberate" the electron for conduction at lower thermal energy.
• A semiconductor doped with acceptors is known as a "p-type" semiconductor. (p for positive charge carrier)