Maximum oscillation frequency ( fMAX ) of mm-wave transistors is one of the key figures of merit (FOMs) for evaluating the HF-performance of a given technology. However, accurate measurements of fMAX are very difficult. Determination of fMAX is significantly affected by the measurement uncertainties in the admittance ( y ) parameters. In order to get rid of the random measurement error and to obtain a reliable and stable fMAX value, the frequency dependent y -parameters are described by rational functions formulated from the small-signal hybrid π -model of the transistor under investigation. The parameters of these functions are determined following a least square error technique that minimizes the functional error with the measured data. The approach is especially useful for a fast and reliable evaluation of fMAX value. Devices from two different SiGe and an FDSOI (Fully Depleted Silicon On Insulator) MOS technology are measured and stable fMAX values are estimated following this approach. © 1980-2012 IEEE.