Experimental Considerations on Accurate fT and fmax Extraction for MOS Transistors Measured Up to 110 GHz

The unity current gain frequency (f_T) and the maximum oscillation frequency (f_max) are key parameters used to characterize the highest achievable speed of a semiconductor technology. However, these values are usually evaluated based on transistor measurements performed at low gigahertz frequencies and extrapolated far to a range of several hundreds of gigahertz. Hence, a large deviation of obtained results is possible, depending on the point at which the extrapolation is taken. Additionally, depending on the decision down to which metallization layer the transistor interconnect parasitics are de-embedded, the value of f_max may vary significantly. This paper presents an experimental study on methodology how to extract accurately f_T and f_max from S-parameter measurements. We evaluate systematically various extrapolation frequencies and discuss the results. As an example, we characterize MOS devices of a 40 nm bulk CMOS technology by S-parameter measurements up to 110 GHz. We use OPEN-SHORT de-embedding technique and discuss its applicability.