Three-dimensional integrated metal-oxide transistors

The monolithic three-dimensional vertical integration of thin-film transistor (TFT) technologies could be used to create high-density, energy-efficient and low-cost integrated circuits. However, the development of scalable processes for integrating three-dimensional TFT devices is challenging. Here, we report the monolithic three-dimensional integration of indium oxide (In₂O₃) TFTs on a silicon/silicon dioxide (Si/SiO₂) substrate at room temperature. We use an approach that is compatible with complementary metal–oxide–semiconductor (CMOS) processes to stack ten n-channel In₂O₃ TFTs. Different architectures—including bottom-, top- and dual-gate TFTs—can be fabricated at different layers in the stack. Our dual-gate devices exhibit enhanced electrical performance with a maximum field-effect mobility of 15 cm² V⁻¹ s⁻¹, a subthreshold swing of 0.4 V dec⁻¹ and a current on/off ratio of 10⁸. By monolithically integrating dual-gate In₂O₃ TFTs at different locations in the stack, we created unipolar invertor circuits with a signal gain of around 50 and wide noise margins. The dual-gate devices also allow fine-tuning of the invertors to achieve symmetric voltage-transfer characteristics and optimal noise margins.