Three-Dimensional Base Distributed Effects of Long Stripe BJT’s: AC Effects on Input Characteristics

The small-signal voltage and current distributed effects in the polysilicon and intrinsic base regions of long stripe bipolar junction transistors (BJT’s) at high frequencies are investigated, and simple analytic equations describing the voltage and current distribution in these regions are derived. It is shown that the frequency-dependent debiasing effects in the polysilicon contacts and intrinsic base region change the current behavior and modulate the input admittance. The current and voltage distributions in the polysilicon region are nonuniform and vary with frequency. Conventional two-dimensional (2-D) device simulations cannot accurately predict this three-dimensional (3-D) effect. A quasi-3D simulation scheme combining a 2-D device simulator and the distributed model is presented to properly and efficiently describe the input characteristics of the device at high frequencies. It is also shown the use of various polysilicon sheet resistances, geometry sizes, and layout structures changes the distributed characteristics and modulates device performance at high frequencies. The impact of the layout structure and geometry size on RF circuit design due to the distributed effects is also studied.