• Metals (aluminium, copper, silver, ... ), graphite
• Atoms arranged in a regular crystal array
• Large number of atoms
• High conductivity
• No forbidden gap
• Conduction and valence gap overlap

• Conductivity of a material
  defined as the current density, J,  divided by the applied electric field, E
  
• Current density
  product of the charge of the mobile carriers, q,  their density, n,  and velocity, v_e
  can be expressed as a function of the electric field using the mobility, _n:

  J = qnv_e = qn_nE

• Conductivity  s = J/E = qn_n
  
• Resistivity  r = 1/s = 1 / (q_nn)