In this paper, quantitative analysis was performed focusing on the structural effect on the ferroelectric switching of ferroelectric thin-film transistors (FeTFTs). FeTFTs and ferroelectric capacitor (FeCap) test element groups (TEGs) were designed and fabricated, and positive-up-negative-down (PUND) measurements were performed to analyze the switching characteristics of ferroelectric films in various structures constituting an FeTFT. It was verified that TiN/HZO/a-IGZO/Mo (MFSM, FeTFT source/drain contact) mostly contributed to the memory operation of an FeTFT, while TiN/HZO/a-IGZO (MFS, FeTFT channel) exhibits one-time memory operation with irreversible polarization switching. In addition, the switching characteristics of MFSM and MFS were different from those of MFM, especially after a few cycles, related to the oxygen vacancy migration between a-IGZO channels and HZO films. The extracted 2Pr values for MFS, MFSM and TiN/HZO/Mo (MFM, FeTFT source/drain parasitic capacitor) were 38, 28 and 20 [μC cm-2], respectively. Based on the operation differences according to the device structure, it was found that irreversible switching in the MFS region (channel) causes a rapid decrease in the memory window after the first switching in an FeTFT and degradation of a-IGZO and HZO films in the MFSM region (contact) including oxygen vacancy exchange and related defect generation causes subthreshold slope increases and negative threshold voltage shifts as cycling stress was applied.