New bipolar figure of merit "fo"

Abstract

Various figures of merit have been introduced over the last few years that attempt to improve or more accurately describe the speed of a bipolar transistor over the traditional figures of merit. The best known are ft and fmax. More recently, the term fv has been proposed as a complement to the others and is defined as the input bandwidth of a common emitter transistor. It will be shown, for general transistor speed comparisons, the gain, as well as the bandwidth, should be included to more accurately compare the relative speed capability against various bipolar processes, and also (for a given process) the speed of a transistor versus gain. Although fmax and ft are gain-bandwidth expressions, the former excludes output capacitance (Co) and the latter excludes base resistance (Rb) and collector-substrate (Cjs). If for a process development engineer the task is optimizing a process for maximum intrinsic speed (given external constraints such as emitter-base (EB) breakdown, collector-base (CB) breakdown, metal parasitics, etc), then none of these merit equations (in themselves) provide an adequate single solution. A new figure of merit (fo) will be proposed which should be a better speed gauge for process/device optimization.