Polymer Doping Enables a Two-Dimensional Electron Gas for High-Performance Homojunction Oxide Thin-Film Transistors

High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In₂O₃) and polyethylenimine (PEI)-doped In₂O₃ (In₂O₃:x% PEI, x = 0.5–4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In₂O₃ and PEI-In₂O₃ via work function tuning of the In₂O₃:x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In₂O₃) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm² V⁻¹ s⁻¹ on a 300 nm SiO₂ gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In₂O₃ materials, which achieve a maximum mobility of ≈4 cm² V⁻¹ s⁻¹. Furthermore, a mobility as high as 30 cm² V⁻¹ s⁻¹ is achieved on a high-k ZrO₂ dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material.