Variable-area capacitors controlled by HfO₂-based ferroelectric-gate field-effect transistors

We propose a three-terminal variable-area capacitor integrated with a ferroelectric-gate field-effect transistor (FeFET) and a paraelectric thin-film capacitor. Owing to the large charge controllability and nonvolatile memory function of the ferroelectric Hf0.86Ce0.14O2 (14% Ce-HfO2) gate insulator, the variable-area capacitors exhibited high capacitance tuning ratios of up to 184 with steep modulation to gate voltage swing (440 mV/decade), a capacitance-switching speed of less than 10 ms, and capacitance-memory-retention characteristics of up to 105 s. An FeFET with an indium tin oxide (ITO) channel and a ferroelectric 14% Ce-HfO2 gate insulator was fabricated as a switching FET via chemical solution deposition, followed by stacking a sputtered Al2O3 film as a variable capacitor. The fabricated FeFETs exhibited a high on-current of ∼0.15 A/mm, a large on/off current ratio of 107, a field-effect mobility of 15.6 cm2/V s, and a memory window of ∼5 V. The high on-current, i.e., low on-resistance (84 Ω), and the large on/off current ratio allow the ITO channel to act as a wiring that connects and disconnects two capacitors with different electrode areas; therefore, the proposed variable capacitor can achieve wide and steep modulation by ferroelectric gating. This study provides prospects for the future integration of passive and active components for advanced high-efficiency and miniaturized electronics such as power devices and communication equipment.