  A Langevin approach to understand the noise of microwave devices is
presented.
  The device is represented by its equivalent circuit with the internal noise
sources included as stochastic processes. From the circuit network analysis, a
stochastic integral equation for the output voltage is derived and from its
power spectrum the noise figure as a function of the operating frequency is
obtained. The theoretical results have been compared with experimental data
obtained by the characterization of an HEMT transistor series (NE20283A, by
NEC) from 6 to 18 GHz at a low noise bias point. The reported procedure
exhibits good accuracy, within the typical uncertainty range of any
experimental determination. The approach allows to extract all the information
required for understanding the noise performance of the device without any
restriction on the statistics of the noise sources. The results show the
relevant noise phenomena from a new angle.
