Modeling I2L performance and operational limits

CONVENTIONAL integrated injection logicl~’ (12L) has the attractive features of high density, low power-delay product, and simple fabrication sequence. However, the NPN current gain falloff associated with narrow, high fanout 12L gates severely limits the maximum gate operational speed. Furthermore, high current effects reduce injector transport efficiency, thus, increasing the powerdelay product. Figure 1 shows a complete 12L macromodel which includes the NPN current gain falloff and injector high current effects. Lateral current transport between adjacent gates is included in the macromodel’. The macromodel is synthesized from standard circuit elements making it compatible with commonly used circuit simulators, and, except for the NPN base resistance, requires only readily measured electrical parameters for definition; Table I. Ebers-Moll equivalent circuits are used to model the NPN transistor action of each collector; Q 1 Qq in Figure 1 for a fanout of four gate. Modeling the high-current NPN current gain falloff characteristics requires proper partitioning of the NPN base recombination currents and inclusion of the base resistance. Figure 2(a) illustrates the partitioning of the NPN base region for a narrow, fanout of four gate with base contact nearest the injector end. Section A is the base contact region and sections B-E are the active regions. The fractional recombination current for each active section is modeled by the dc current gain of the NPN, whereas, for the base contact region, it is accounted for by the saturation current of diode DCN. Base current measurements are required for gates of differing fanout counts using a consistent set of layout rules and fabrication. Figure 2(b) shows the partitioned base region with injected current de current paths, assuming the injector to be located at the base contact end of the base region. Using the geometry of these shaded current paths and the base sheet resistance, the b a e resistance components may be determined. Base resistance components, RBI R B ~ , are included in the macromodel as -