This paper investigates the characterization of charge trapping and its modelling on hafnia-based ferroelectric field effect transistors (FeFETs). Defect characterization on MOSFETs can be done by studying threshold voltage shifts (∆Vth) as a function of charging and relaxation times. At positive gate voltages one expects electron trapping in the gate oxide to result in a positive shift of the threshold voltage (Vth). However, on a FeFET those conditions will induce polarization changes in the gate oxide leading to a negative Vth-shift, complicating the characterization of defect levels. We aim to alleviate these difficulties by modelling the polarization and trapping in FeFETs over a wide range of timescales, suitable for defect characterization. We demonstrate quantitative agreement on long timescales with a static polarization model, while a time-dependent polarization model can be used for qualitative agreement over a wide range of times from 30 ms to 2 ks.