A model-based comparison of switching characteristics between collector-top and emitter-top HBT's

A model-based comparison is made between the collector-top GaAs/AlGaAs heterojunction bipolar transistor (HBT) and the emitter-top HBT, in regard to their switching performance. An equivalent-circuit transistor model that is derived from a one-dimensional numerical model is employed, together with our original circuit simulator that allows embedding of arbitrary device models. All the simulated results, obtained from various kinds of ring oscillators and CML divide-by-four circuits, indicate the superiority of the collector-top structure, especially for nonsaturation logic.