Semiconductor is a material with variable electrical conductivity.

Energy bands

Atoms have energy bands in which electrons can exist. There are:

  • valence band - highest energy band of electrons bound to crystal lattice
  • conduction band - electrons are shared by the whole crystal and can move freely under the influence of electric field as a carrier.

The bands are separated by a band gap \(\Delta E\). In semiconductors the band gap is <~3 eV or <~5 eV depending on source.

Conductance mechanism of intrinsic semiconductor

  1. Electron absorbs energy >\(\Delta E\) and jumps to conduction band
  2. It leaves a hole in valence band
  3. Free electrons and holes can carry a current before they recombine

Concentration of carriers is proportional to temperature.

Doping

Introduction of carefully chosen impurities allows for modulation of semiconductor’s properties.

Take a tetravalent semiconductor.

  • adding a pentavalent element (donor) makes one electron loosely bound to the lattice -> N type
  • adding a trivalent element (acceptor) makes one hole loosely bound to the lattice -> P type

p-n junction

p-n junction is a boundary between touching n and p semiconductors. On that boundary, carriers diffuse and create potential barrier.

Polarization

Reverse bias

Connecting p semiconductor to minus and n semiconductor to plus makes electrons from n go towards the plus and holes from p towards the minus widening the depletion region and raising the potential barrier. Current doesn’t flow.

Forward bias

Connecting the p semiconductor to plus and n semiconductor to minus narrows the depletion region and lowers the potential barrier allowing for the flow of current.