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

Two different explanations of the S₂₂ 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 S₂₂. 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 10² to 10⁴ Ω), can also be accounted for by our extended model. Experimental data representative of 0.25 μm 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 S₂₂ kink phenomenon in deep-submicrometer RF MOSFETs.