On fT

Everyone knows that f_T is the frequency “at which the current gain equals 1.” What is not commonly recognized is that while f_T is a useful figure of merit for devices there is a significantly better way than this definition to conceptualize f_T. In an overlooked 1969 paper Hermann Gummel proposed a different interpretation of f_T for bipolar transistors. In this paper we generalize Gummel's analysis and propose a device independent, simple, and intuitive definition of f_T that is easy to both measure and simulate.