Proposal of huge-charge-controlled field-effect transistor using ferroelectric gate insulator and its application to next-generation integrated circuits

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The objective of this research project is to realize new switching device using ferroelectric materials which can control huge charge density of 10-50 μC/cm^2. In this work, we used indium-tin-oxide (ITO) as a channel material instead of Si with ferroelectric gate insulator. First, we fabricated and characterized ferroelectric thin films, Pb(Zr,Ti)O_3 (PZT) and (Bi,La)_4Ti_3O_<12> (BLT), and ITO thin films. Next, we fabricated ferroelectric-gate thin film transistors (TFTs) using ITO/PZT and ITO/BLT structures. Drain current - drain voltage characteristics shows normal n-channel transistor operation with clear drain current saturation. In particular, a large on-current of 0.1 mA/μm was obtained for ITO/BLT TFTs with a channel length of 5 μm. This value is as large as the on-current of Si-MOSFET with same channel length. From this on-current, the charge density. used in the fabricated device is as large 20 μC/cm^2, which proved the operation principle proposed in this work. Furthermore, we examined the details of device parameter dependence of the electrical properties. We also demonstrated that the off-current of the device can be drastically reduced by mechanically polishing the surface of the ferroelectric film. The ITIO/BLT structure ferroelectric-gate TFT with an on-current of 10^<-3> A and an off-current of 10^<-11> A (on/off ratio is more than 10^7) was demonstrated. In addition, transparent ferroelectric-gate TFTs were also demonstrated on quartz substrates.