Extension of the characteristic potential method for noise calculation and its application to shot noise in semiconductor devices

Characteristic potential method (CPM) for noise calculation has been developed for multi-terminal semiconductor devices under the drift-diffusion scheme. Merit of the CPM is that clear cut definitions of the terminal thermal noise currents and the terminal excess noise currents can be made for unipolar devices and homogeneous resistors. We prove that the terminal thermal noise currents and the terminal excess noise currents are uncorrelated for unipolar devices even when they come from the same local noise sources. We also suggest a way to define thermal noise and excess noise in bipolar devices using the derived formulas from the CPM. As applications of the CPM, we show that the high frequency excess noise observed in homogenous semiconductor resistors is really shot noise whose noise generating mechanism is just the same as that of vacuum diodes. We also show that the dominant high frequency noise in long-channel MOSFETs is thermal noise in the linear region, but the excess noise is getting more significant as the drain bias increases, and is important in the saturation region. The excess noise in the saturation region of the long-channel MOSFETs is shown to be shot noise. Finally, we try to explain the shot noise-like behaviors observed in forward-biased pn junction diodes by the conventional corpuscular theory of shot noise even though the impedance field method confirms that the shot noise behaviors are caused by the local noise sources in the neutral regions, not in the depletion regions.