The general framework of Fourier analysis of complex impedance including both the amplitude and phase information is presented. For the single frequency sine/cosine input, Fast Fourier Transform can be used to obtain the amplitude and phase of the harmonics in the output signal. Complex nonlinear least squares fitting can then be applied to correlate the experimental and calculated results as in conventional impedance spectroscopy. General purpose p–n junction diodes were used as model systems to test the method as well as to compare it to other means of analysis.