Thirty-Day-Long Data Retention in Ferroelectric-Gate Field-Effect Transistors with HfO₂ Buffer Layers

Metal–ferroelectric–insulator–semiconductor (MFIS) diodes and p-channel MFIS field-effect transistors (FETs) were fabricated and their electrical properties were characterized. These MFIS structures were formed using HfO₂ as an insulating buffer layer, and SrBi₂Ta₂O₉ (SBT) and (Bi,La)₄Ti₃O₁₂ (BLT) as ferroelectric films. HfO₂ buffer layers of about 8 nm physical thickness were deposited by ultrahigh-vacuum (UHV) electron-beam evaporation, then ferroelectric films of about 400 nm thickness were deposited by sol–gel spin coating. The fabricated p-channel MFIS-FETs with the SBT/HfO₂ gate structure exhibited a drain current on/off ratio larger than 10³ even after 30 days had elapsed. It was also found that the degradation of ferroelectricity was not pronounced even after applying 2.2×10¹¹ bipolar pulses.