Investigation of Four-Port Characteristic Dependence on Gate Length for MOSFETs in the Breakdown Region

Abstract

In this article, gate length-dependent RF four-port characteristics of MOSFETs are investigated in the avalanche regime by using the double feedback and chain rule methods for the first time. The obvious decline of the imaginary part of source input impedance at high drain voltage for short-channel MOSFETs is investigated through the double feedback instead of only the series feedback for the origin of the inductive source impedance. The dominant terms causing the reduction of inductive source impedance due to parallel feedback are determined by this chain rule method. When compared with the conventional analysis due to short-channel effects on the direct-current (DC) characteristics of threshold voltage and substrate current, short-channel MOSFETs resulting in the aspect of RF characteristics are further analyzed by using different gate length MOSFETs. Besides, as gate length reduces, drain-to-source transmission raises significantly in the avalanche regime due to the RF avalanche network according to this analysis. This mechanism results in lower isolation at the breakdown for the shorter gate length MOSFETs. Good agreement of the four-port scattering parameter (S-parameter) between simulation and measurement is obtained for the MOSFETs operating in the both breakdown and saturation regions. The methods can be utilized for other advanced technology nodes and are helpful for the design of RF CMOS amplifiers in the impact ionization region.