Performance degradation due to extrinsic base encroachment in advanced narrow-emitter bipolar circuits—part II: Non-threshold logic circuits

Abstract

Detailed two-dimensional numerical simulation studies are carried out to assess the performance degradation caused by the extrinsic base encroachment on NTL circuits utilizing advanced narrow-emitter bipolar transistors. In the steady state, the extrinsic base encroachment is shown to force the current to flow through the center of the emitter, resulting in higher current density and more severe base stretching at the center of the emitter. During the switching transient, the current flows predominantly through the emitter edge, and the turn-on delay is governed mainly by the perimeter thick base at the emitter edge. The extrinsic base encroachment also limits the over-drive (and hence the speed) of the NTL circuit when a speed-up capacitor is used. The base stretching at the center of the emitter is shown to be enhanced by the speed-up capacitor. Because of the modification of the emitter Gummel number near the emitter edge by the extrinsic base encroachment and the transient edge conduction mechanism, significantly more holes are injected into the emitter during the switching transient than in the steady state. This back injection is shown to be further enhanced by the speed-up capacitor. The design considerations for scaled-down devices are discussed. © 1989 IEEE