When do we need Non-Quasistatic CMOS RF-Models?

This paper presents criteria for the onset of NQS effects derived from time transient device simulations and Sparameter measurements. For the first time it has been proved that e.g. a 10μm NMOS transistor can be described up to 27MHz and a 0.2μm device up to 46GHz by the quasistatic approach while the accuracy of the description of the inversion layer charge is still 99%. Introduction Due to continuous down-scaling CMOS-technologies are playing a more important role in RF systems. Therefore there is a strong demand for compact small signal models describing accurately also the high-frequency region. An often mentioned problem hereby seem to be non-quasistatic phenomena. In contrast to (1, 2, 3) this paper will show, however, that their influence is often over-estimated under small signal conditions. The aim of this paper is to determine the transition between quasistatic and non-quasistatic effects in terms of the frequency, the channel length and the applied voltages. Thus, it will be possible to estimate a theoretical limit up to which quasistatic MOSFET models are reasonable. Fig. 1 summarizes our proceeding in a flowchart. Quasistatic Operation Assumption The device behaves quasistatically as long as the terminal voltages are varied slowly enough for the charge to follow ”immediately” at any position in the device. Then, these charges can be assumed identical to those that would be found if DC voltages were used instead (4). This can be interpreted in the way that the NQS operation starts as soon as the inertia of the charge carriers can not be neglected any more. ⇒ S-Parameters AC-Measurements ⇒ Qinv = Qinv (t) L = 0.2 . . . 10μm Device Simulations Time Transient (Medici) f = 0, 0.1 . . . 200GHz (TSuprem) Technology Simulations DC-Measurements Extraction of the QS/NQS Transition Fig. 1: Proceeding of our investigations. According to AC considerations our work focuses on the small variation of the inversion layer charge ∆Qinv(t) (fig. 2). This contrasts to the normal approaches which investigate the turn-on and turn-off behavior since the large-signal channel build-up lasts much longer than its reaction on small signal variations.