Systematic Experimental fT and fmax Comparison of 40-nm Bulk CMOS versus 45-nm SOI Technology

Abstract

The unity current gain frequency (fT) and the maximum oscillation frequency (fmax) are key parameters used to characterize the highest achievable speed of a semiconductor technology. However, there is typically a high uncertainty level concerning evaluation of these values, related to several factors. First, these metrics are obtained from small-signal transistor measurements at low gigahertz frequencies and extrapolated far to a range of several hundreds of gigahertz. Hence, a large deviation of obtained values is possible, depending on the point at which the extrapolation is taken. Second, depending on metallization layer down to which the transistor interconnect parasitics are de-embedded, the value of fmax may vary significantly. Therefore, it is a challenge to compare technologies in a fair and consistent way by means of reported fT and fmaxvalues, since these values could have been obtained for strongly differencing conditions. This paper presents a systematic comparison of fT and fmax values for two technologies: 40 nm bulk CMOS and 45 nm RF silicon-on-insulator (RF-SOI) CMOS. The values are obtained experimentally from S-parameter measurements under rigorously similar conditions for both technologies. We use the same extrapolation frequencies and de-embed the results down to the same metal level using the same de-embedding technique and similar structures. Finally, we discuss the results and provide additional insights.