An improved version of the proportional difference operator method is presented for the determination of key parameters of a MOSFET, including threshold voltage and carrier mobility, and results in the proportional difference output characteristics of a MOSFET, which manifests spectral property. The accurate relationship between key parameters and the peak of the proportional difference output characteristic is provided. Subsequently, a simple discussion of the practical application of this method under different bias conditions, i.e. substrate biases, is also given. Further, some approximate equations with enough accuracy are provided to simplify the calculation complexity.