In an intrinsic semiconductor (pure material like Si or Ge), conductivity is determined by the number of thermally generated charge carriers (electrons and holes).

• Band Gap ($E_g$): Carriers are generated when electrons gain enough energy to jump from the valence band to the conduction band. A smaller $E_g$ allows more carriers to jump.
• Temperature ($T$): The number of carriers $n_i$ increases exponentially with temperature: $n_i\propto T^{3/2}{e}^{-E_g/(2k_{B}T)}$. Unlike metals, the conductivity of semiconductors increases with temperature because the effect of increased carrier concentration outweighs the effect of lattice scattering.