▎ 摘　　要

We develop a theory for microwave third-harmonic generation (THG) due to mobile carriers in doped graphene by employing the Boltzmann equation techniques. We put forward two distinct mechanisms to explain the microwave THG observed experimentally. These mechanisms are: (i) the Kane-model-like conduction band nonparabolicity, which arises in the presence of a finite gap of any origin between the conduction and the valence band, and (ii) the energy dependence of the electron-momentum relaxation time. We derive an explicit analytical expression for the third-harmonic component in the current density and employ it to evaluate the magnitude of the generated third harmonic as a function of the graphene's doping level, which can be changed by electrostatic gating. The curve of third-harmonic intensity vs electron density exhibits a pronounced maximum, thus illustrating our principal conclusion that there is an optimal value of the electron density for which the microwave THG can be most efficient.