Enhancement-Mode Operation in Degenerate In2O3 Thin-Film Transistors via Local Carrier Suppression

Oxide semiconductors have recently emerged as promising back-end-of-line (BEOL) compatible channel materials for monolithic 3D integration, addressing limitations such as scalability and thermal constraints in conventional semiconductors. However, increasing the carrier concentration in oxide semiconductors to enhance mobility often leads to conductive behavior, hindering gate control. Here, a transistor structure was demonstrated that enabled a degenerate indium oxide (In2O3) channel thin-film transistor to operate in enhancement mode. By forming a p-n junction between a degenerate In2O3 and a heavily doped p-type Si (p++-Si) substrate, the local carrier concentration in the In2O3 was suppressed, lowering the Fermi level relative to the conduction band edge and establishing a homojunction potential barrier in the channel. This maintained the off-state at equilibrium and allowed enhancement-mode operation. With a 6-nm-thick degenerate In2O3 channel, the transistor achieved a field effect mobility (μFE) of 52.7 cm2/Vs and a threshold voltage (Vth) of +3.6 V. These findings demonstrated that local carrier suppression via electrostatic modulation was an effective strategy for achieving enhancement-mode transistors in high-mobility degenerate conductive metal oxides.