An analysis of small-signal substrate resistance effect in deep-submicrometer RF MOSFETs

Two different explanations of the S/sub 22/ kink phenomenon in deep-submicrometer RF MOSFETs have been reported: Hjelmgren and Litwin (see IEEE Trans. Electron Devices, vol.48, no.2, p.397-399, 2001) attributed the phenomenon to the substrate resistance, while Lu et al. (see ibid., vol.49, no.2, p.333-340, 2001) concluded that it results from the transconductance, or simply speaking, the size of the transistor. In this paper, we extend the dual-feedback circuit methodology for the three-terminal FET model proposed by Lu et al. into a more general four-terminal model in order to account for the influence of the substrate resistance. Our results show that, for a given MOSFET, either substrate resistance or transconductance may cause a kink in S/sub 22/. In addition to the single kink, which results from the above two factors, the double kinks, which appear when the substrate resistance of a MOSFET is within a middle range (approximately 10/sup 2/ to 10/sup 4/ /spl Omega/), can also be accounted for by our extended model. Experimental data representative of 0.25 /spl mu/m gate MOSFETs are adopted to verify our theory. Excellent agreement between theoretical values and experimental data has been found, which indicates our theory can successfully explain the S/sub 22/ kink phenomenon in deep-submicrometer RF MOSFETs.