A multi-finger modeling approach to correctly predict the inherent stability of a custom active device

As opposed to traditional approaches for extracting small-signal equivalent circuits of active devices, the use of extensive electro-magnetic (EM) simulations has been more recently demonstrated by several Authors. This contribution outlines the extraction of two kinds of EM-based models for a common-source and a common-gate device in a 250 nm GaN HEMT technology, namely a compact model (representing the intrinsic as a three-terminal network) and a multi-finger one (splitting the intrinsic region into elementary units). The latter representation, which is unique of the EM-based approach, is shown to accurately predict the stability properties of the actual common-gate device, whereas the compact model, notwithstanding the apparent similarity as to S-parameter modeling, misses the observed instability.