The paper deals with the study and numerical simulation of the nonlinear electrical response of a waveguide-type P.I.N. InGaAs photodiode under intense illumination conditions, including the harmonic generation rate calculation. Indeed, significant optical power absorption in a small-sized device (i.e. microwave waveguide-type P.I.N. photodiode) can perturb, by space-charge effects, the electric field which governs the carrier transport, and then induce a nonlinear electrical response. The device depolarization, due to the current flow in the load circuit, also causes a nonlinearity via the internal electric field perturbation. These nonlinearities induce harmonics, and we investigate especially the third-order harmonics creation, which is closely connected to the third-order intermodulation products term in any narrow-band multichannel network.