Physics of Semiconductor Devices

Impact ionization

At high electric fields, electrons gain so much energy from the applied electric field that when electrons scatter off each other, they can be scattered from the valence band into the conduction band. To be able to scatter an electron into the conduction band, the kinetic energy of the electrons must be greater than the energy gap, E_g. The condition, E_g = ½m^*v_d², can be used to estimate the critical field where impact ionization begins.

A semiconductor has a band gap of 0.9 eV, an effective mass of 0.7 m₀, and a mobility of 1800 cm²/Vs. Use the condition above to estimate the electric field where impact ionization will take place.