3-DIMENSIONAL BASE DISTRIBUTED EFFECTS OF LONG STRIPE BJTS - AC EFFECTS ON INPUT CHARACTERISTICS

The small-signal voltage and current distributed effects in the polysi licon and intrinsic base regions of long stripe bipolar junction trans istors (BJT's) at high frequencies are investigated, and simple analyt ic equations describing the voltage and current distribution in these regions are derived. It is shown that the frequency-dependent debiasin g effects in the polysilicon contacts and intrinsic base region change the current behavior and modulate the input admittance, The current a nd voltage distributions in the polysilicon region are nonuniform and vary with frequency, Conventional two-dimensional (2-D) device simulat ions cannot accurately predict this three-dimensional (3-D) effect. A quasi-3D simulation scheme combining a 2-D device simulator and the di stributed model is presented to properly and efficiently describe the input characteristics of the de,ice at high frequencies. It is also sh own the use of various polysilicon sheet resistances, geometry sizes, and layout structures changes the distributed characteristics and modu lates device performance at high frequencies. The impact of the layout structure and geometry size on RF circuit design due to the distribut ed effects is also studied.