A harmonic solution of Boltzmann ’s Transport Equation (BTE)taking into account its first three moments together with Poisson ’s equation has been found for a piecewise periodic voltage excitation in a semiconductor.The electric field and potential,density of electrons,electron momentum (or velocity)and electron energy have been discretised in the space domain,and they have been expanded in Fourier series in the time domain.The resulting equation system is clearly non-linear and it is solved by means of a Waveform-Balance technique.The time step is determined only by the required maximum harmonic frequency simply by means of the Nyquist ’s sampling theory.In this way the relaxation times of the semiconductor is implicitly considered in the analysis.This approach allows for a longer time step than a space-and time-discretised standard solution for many cases of interest.