Silicon, Circuits, and the Digital Revolution

Insulator or Semiconductor?

• Viewgraph of energy levels as a function of atom separation for Group 4 elements
  -- four valence electrons
• Model of crystal structure (Bring ball-and-stick model to class.)
  -- covalent bonding to four nearest-neighbors
  ----- sharing of electrons
• Interactive Unit Cell from Education Applets in Solid State Materials,
  Center for Active Learning of Microelctronics and Photonics at SUNY Buffalo
  

• Relative size of bandgap
  -- the energy separation between valence and conduction bands
  

  Element:	Band Gap:	Configuration:
  	(eV)	(at room temp.)
  (Diamond) Carbon	5.5	[helium] 2s2 2p2
  Silicon	1.1	[neon] 3s2 3p2
  Germanium	0.7	[argon] 4s2 4p2

  The Tech

  For comparison to photon energies, recall: Photon "color":   Wavelength:   Energy:   (nm) (eV) Blue 430 2.9 Yellow 530 2.3 Red 650 1.9

• Diamond is considered to be an insulator,
  whereas Se, Si, and Ge are considered to be semiconductors.
• Diamond is transparent to visible light, Si and Ge are not;
  can be explained with relative band gap energies.
  -- optical absorption; energy or photon is absorbed by electron and photon disappears.
  -- (Examine wafer of silicon.)

• In general, thermal electrons still cannot "jump" bandgap in pure semiconducting materials; however
• Photons of sufficent energy can "promote" conduction electrons.
  • Photoconductivity -- e.g. Selenium with band gap of 1.8eV
  • Slide Show of xerography
  • On-line lesson on photocopying

Comments, suggestions, or requests to ghw@udel.edu. "http://www.physics.udel.edu/~watson/scen103/colloq2000/band-semi.html" Last updated April 19, 2000. © George Watson, Univ. of Delaware, 2000.