Effect of reverse base current on bipolar and BiCMOS circuits

A detailed study on the effect of reverse base current (RBC) on the switching behavior of bipolar BiCMOS circuits utilizing advanced high-performance bipolar transistors is presented. It is shown that as the collector doping N_c is increased to overcome the Kirk effect (base stretching) during the switching transient, the avalanche-generated reverse base current in the collector-base junction may cause problems for bipolar output devices switching out of saturation. A basic bipolar inverter and various BiCMOS driver circuits were simulated based on measured avalanche multiplication factors from advanced bipolar transistors with various collector doping N _c. In the case of the basic bipolar inverter, the reverse base current may prevent the switching device from being shut off completely during the on-to-off transition and a self-sustained state may result which reduces the output voltage swing. For the common-emitter (CE) BiCMOS driver, a similar self-sustained state may also occur with the added adverse effect of excessive leakage in standby. Design and scaling considerations are discussed