Investigation of very fast and high-current transients in digital bipolar IC's using both a new compact model and a device simulator

The design and optimization of high-speed integrated bipolar circuits requires accurate and physical transistor models. For this, an improved version of the compact model HICUM was developed. It is an extension of the small-signal model recently described to the large-signal (transient) case. The model, which takes into account emitter periphery and non-quasi-static (NQS) effects, is semi-physical, allowing the calculation of its elements for arbitrary transistor geometries from specific electrical and technological data. This is an important precondition for transistor optimization in a circuit and for worst case analysis. The model was verified for basic building blocks of high-speed digital circuits like emitter follower and current switch. For this, mixed-mode device/circuit simulation is used instead of measurements, since the latter would give too large errors for the fast transients of interest. It is demonstrated that-in contrast to the obsolete but frequently used SPICE Gummel/Poon model-the new HICUM is well suited for modeling very-high-speed transistor operation also at high current densities. Moreover, it is shown that at very fast transients the influence of NQS effects can no longer be neglected. As a practical application example, a high-speed E/sup 2/CL circuit is simulated using the new model. The results show again that high-current models are very useful for designing IC's at maximum operating speed. This is because the optimum emitter size is often the minimum size, which is limited by high-current effects. Especially, in the case of current spikes (e.g., in emitter followers) it is difficult to find the optimum emitter size without having adequate transistor models.<
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