The latest evolution in complementary metal-oxide-semiconductor technology has made the metal-oxide-semiconductor field effect transistor (MOSFET) a viable choice for rf applications, especially for frequencies in the low GHz region. However, hot-carrier effects should also be considered carefully when the devices are operating in the GHz regime. Here, we studied the effects of dc hot-carrier stress on lightly doped drain (LDD) n-type MOSFET (NMOSFET) high-frequency performance by measuring and simulating its s parameters. This is the first time, to the authors’ best knowledge, that such experiments are reported. We demonstrated clearly the effects of hot-carrier stressing on LDD NMOSFETs by giving representative s-parameter and noise measurement results from a 0.8 μm long device. We showed that hot-carrier stress can significantly degrade both s parameters and noise of NMOSFETs, and thus can have considerable consequences for circuit designers. Therefore, these effects should be carefully considered when using MOSFETs in high-frequency analog circuits. Unfortunately, both MEDICI® and SPICE simulation could not satisfactorily model the LDD MOS structure after hot-carrier stress. Various results indicated that the current MEDICI platform is not very consistent for ac simulation, although dc simulation is very good. SPICE simulation showed very promising results when modeling the changes in S12 and S21 due to hot-carrier stress, yet the results for S11 and S22 were not very good. This deficiency implies that a better small-signal model for LDD MOS structures would be necessary for SPICE to be useful in modeling hot-carrier effects on MOSFET high-frequency performance.