A Simple Test to Check the Inherent-Stability Proviso on Field-Effect Transistors

Available techniques for stability analysis of DC solutions, such as the popular Ohtomo test, make the task quite a straightforward one. Such tests are based on the assumption that the active subcircuit is inherently stable, which is reasonably easy to enforce in practice since the active devices themselves are typically inherently stable. However, besides this empirical observation, there is an unfortunate lack of techniques to verify the proviso itself before proceeding to the stability test of the whole circuit. This contribution presents a quick-and-easy technique for identifying a single, technology-dependent parameter and choosing the test section appropriately. Examples are also given of active devices that, quite unusually, were found to be non-inherently stable. It is shown that, due to the nature of the instability in the considered cases, the proviso cannot be checked without accessing the intrinsic circuit on a single-finger basis, which, in turn, calls for the availability of a multi-finger model. The devices illustrated in this contribution are based on a 250-nm GaN HEMT technology provided by the UMS foundry and are featured by standard and customized geometries, only the latter presented unstable behavior.