2D bipolar Monte Carlo calculation of current fluctuations at the onset of quasisaturation of a Si BJT

Abstract

We report a study of the noise properties of a n–p–n Si BJT in the common-emitter configuration. A two-dimensional microscopic Monte Carlo simulator was employed in order to fully take into account non-stationary carrier-lattice transport. The influence of the polarization of the base-emitter junction on internal physical magnitudes was determined and we explain the origin of the intermediate and high-frequency transistor noise in terms of the evolution of these magnitudes with bias. The study revealed that, in the forward active mode, device noise behavior is exclusively determined by base majority carriers and that the shot character of noise leads to low-noise levels. As the device enters the quasi-saturation mode, the base operates in the high-injection regime and, as a consequence of this, a progressive degradation of both DC-current gain and noise takes place as base-emitter bias increases. We identified the heating of electrons through the structure and electron and hole plasmas coupling in the base as being responsible for such degradation.