INFLUENCE OF LOW DOPED EMITTER AND COLLECTOR REGIONS ON HIGH-FREQUENCY PERFORMANCE OF SIGE-BASE HBTS

The high-frequency behavior of a Si/Si1-xGex/Si-heterojunction bipolar transistor is investigated by means of a two-dimensional device simulation program. A simplified method based on the quasi-static approximation and the well-known formula f(max) = root f(T)/8 pi R(B)C(C) is compared with the more accurate a.c. small-signal analysis in order to estimate the limits of the first approach. For a given Gaussian boron distribution in the base an optimized Ge profile is derived exploiting its influence on the spatial distribution of the transit time. Our main attention is directed to the dependence of f(T) and f(max) on the vertical profile in the low doped regions of emitter and collector. The study of the tradeoff between the capacitance of the BC junction and the transit frequency shows that a small capacitance C-C at the expense of f(T) is favorable for obtaining high f(max) values.