Fluctuation Noise and Device Structure

In the present paper a generalised formula is developed for the power spectrum of the short-circuit noise current of three-terminal devices. The power spectrum is shown to be determined by a factor 2-kTg/¿Ie which can never be greater than 2 and never smaller than 1. This factor corresponds to the various "smoothing" factors (F, Q, H...) normally used when device noise is described by means of the shot noise associated with the d.c. current flowing through the device or by the measured transconductance. The limiting values for these factors are probably not device geometry determined but rather depend on the fundamental mechanism responsible for the discrepancy between the measured and the theoretically highest possible value Ie/kT of the transconductance. kTg/¿Ie, the internal feedback factor, describes the reduction of transconductance owing to space charge or bulk resistance in the current path whilst ¿ is a structural constant of the device related to the efficiency of the gate voltage control. The utility of the generalised fluctuation noise formula is demonstrated for different devices at various temperatures and the measured noise power spectrum is used for the determination of ¿.