HIGH-FREQUENCY NOISE OF BIPOLAR-DEVICES IN CONSIDERATION OF CARRIER HEATING AND LOW-TEMPERATURE EFFECTS

We derive an expression for the high-frequency spectrum of thermal current noise in bipolar devices from quantum mechanical linear response theory. This expression relates the noise spectrum to local small-signal quantities. It is independent of geometry and holds for arbitrary space-dependent carrier temperatures including low ones. The general result is applied to pn-diodes and bipolar transistors. In the isothermal case and for not too low temperatures the expressions known from the literature can be reproduced. We apply our approach to Si1-xGex heterojunction bipolar transistors in order to calculate the minimum noise figure at high frequencies. The electron heating in the collector is shown to have a noticeable influence on the noise figure at very high frequencies.