The steep-slope ferroelectric tunnel-FET (SS-FeTFET), consisting of an InGaAs TFET with a sub-60 mV/dec subthreshold swing (SS) at room temperature and an externally connected high-quality single-crystalline PZT capacitor, displays improved SS compared to the standalone TFET. In this article, we describe the measurement procedure and measurement results of this SS-FeTFET in great detail. To quantitatively extract the ferroelectric (FE) polarization during voltage sweeps, device simulations of the TFET are combined with the SS-FeTFET measurement results. Finally, qualitative insight in some peculiarities of the experimental observations is given, such as the apparent coercive voltage that is larger in the SS-FeTFET than in the standalone FE, the shape of the polarization during voltage sweeps, and the small polarization hysteresis loop at voltages close to the apparent coercive voltage.