This velocity saturation phenomenon results from a process called optical phonon scattering. At high fields, carriers are accelerated enough to gain sufficient kinetic energy between collisions to emit an optical phonon, and they do so very quickly, before being accelerated once again. The velocity that the electron reaches before emitting a phonon is: m ∗ ∗ v emit 2 2 ≈ ≈ ℏ ℏ ω ω phonon (opt.) where ω phonon(opt.) is the optical-phonon angular frequency and m* the carrier effective mass in the direction of the electric field. The value of E phonon (opt.) is 0.063 eV for Si and 0.034 eV for GaAs and Ge. The saturation velocity is only one-half of v emit, because the electron starts at zero velocity and accelerates up to v emit in each cycle. (This is a somewhat oversimplified description.)