Abstract

This paper reviews the relevance of the widely used device metrics fT, fmax as well as the recently introduced device metrics fA and fcross for broadband circuit design. Usually, IC processes are benchmarked on the basis of their fT and fmax. For most circuit applications however, there is only an indirect relation between fT, fmax and circuit bandwidths. Since the differential pair amplifier is a key building block in broadband circuits, the metric fA provides a nearly direct relation to broadband circuit performance. This is demonstrated via the maximum operating frequency of a current-mode logic frequency divider, processed in 3 generations of a BiCMOS process. Metric fcross is valuable for the design of circuits employing a cross-coupled differential pair as active negative resistance, such as in LCVCOs. The metrics can be expressed in terms of transistor parameters (e.g., Rb, Cbc, …), allowing to derive a link between circuit performance and technology. Based on our experience, we evaluate IC processes on the basis of fT, fA and fcross rather than fT and fmax.