EC-5.4 Conductors, Semiconductors and Insulators

Work through these

• The three classes, read off one number

  A conductor has bands that overlap so electrons are always free to move, an insulator has a gap far too large to cross, and a semiconductor has one small enough to cross with modest help. Nothing else distinguishes them.

  NPTEL: Fundamentals of Electronic Materials and Devices · Course

• Intrinsic conduction, and why a pure semiconductor conducts at all

  At any temperature above absolute zero some electrons have enough thermal energy to cross the gap, leaving holes behind. Both carry current, and in a pure crystal there are exactly as many of one as the other.

• Why heating helps a semiconductor and hurts a metal

  Heat frees more carriers in a semiconductor, so its resistance falls. A metal already has all the carriers it will get, so heat only makes the lattice vibrate more and scatter them, and its resistance rises.

  NPTEL: Physics of Materials · Course

• Mobility, and what actually limits how fast a carrier moves

  A carrier does not accelerate freely; it is scattered by lattice vibrations and by impurities, and mobility measures how well it does despite that. The two scattering mechanisms dominate at different temperatures, which is why mobility peaks and then falls.

• Why silicon rather than germanium or anything else

  Silicon has a workable gap, is abundant, and grows a stable insulating oxide on its own surface, which is the property that made integrated circuits practical. Germanium is a better conductor and lost on the oxide.

  NPTEL: Fundamentals of Electronic Materials and Devices · Course

• Compound semiconductors, and what they are for

  Combining elements from either side of silicon in the periodic table gives materials with different gaps, some of them direct. That is how light-emitting devices, high-frequency transistors and solar cells with better efficiency get made.