Amorphous oxide semiconductor transistors have been a mature technology in display panels for upward of a decade, and have recently been considered as promising back‐end‐of‐line compatible channel materials for monolithic 3D applications. However, achieving high‐mobility amorphous semiconductor materials with comparable performance to traditional crystalline semiconductors has been a long‐standing problem. Recently it has been found that greatly reducing the thickness of indium oxide, enabled by an atomic layer deposition (ALD) process, can tune its material properties to achieve high mobility, high drive current, high on/off ratio, and enhancement‐mode operation at the same time, beyond the capabilities of conventional oxide semiconductor materials. In this work, the history leading to the re‐emergence of indium oxide, its fundamental material properties, growth techniques with a focus on ALD, state‐of‐the‐art indium oxide device research, and the bias stability of the devices are reviewed.

In this letter, we report on scaled ultrathin (~3 nm) InGaO (IGO) thin film transistors (TFTs) by atomic layer deposition (ALD) under a low thermal budget of 250 °C. The ALD-derived IGO channels are In-rich, with In/Ga atomic ratio of ~86:14, providing a high electron mobility of ~28.6 cm $^{{2}} \cdot \text{V}^{-{1}}\,\,\cdot \text{s}^{-{1}}$ under a ultrathin thickness of 3 nm. The resulting IGO TFTs exhibit excellent scaling behaviors down to sub-100 nm channel length ( $\text{L}_{\text {ch}}$ ). The IGO TFTs with a $\text{L}_{\text {ch}}$ of 80 nm show well-behaved electrical characteristics including a high on/off current ratio ( $\text{I}_{\text {on}}/\text{I}_{\text {off}}$ ) of ${1.8}\times {10} ^{{10}}$ , a low subthreshold swing (SS) of 92 mV/dec under $\text{V}_{\text {DS}}$ of 0.8 V. The negative- and positive-gate-bias-stress stability (NBS and PBS) of IGO TFTs are studied in both $\text{N}_{{2}}$ and air ambient, where a remarkably high PBS stability can be observed. The negative $\text{V}_{\text {th}}$ shifts during PBS and NBS test in $\text{N}_{{2}}$ ambient could be explained by the generation of donor-like traps originating from ionized oxygen vacancy, in addition to electron (de)trapping mechanism. This work presents the first demonstration of high-performance IGO TFTs with a miniatured device dimension, showing the potential for back-end-of-line (BEOL)-compatible monolithic 3D integration.

In this work, we report on back-end-of-line (BEOL)-compatible InGaZnO indium gallium zinc oxide (IGZO) thin film transistors (TFTs) with extreme scaled device dimension including channel thickness ( $\textit{T}_{\text{ch}}\text{)}$ down to 1.5 nm and channel length ( $\textit{L}_{\text{ch}}\text{)}$ down to 60 nm. These IGZO channels with a high In atomic ratio of 92% were derived by atomic-layer-deposition (ALD), where the IGZO thickness could be precisely controlled by ALD cycles. These TFTs were subjected to a mild O $_{\text{2}}$ annealing at 250 $^{\circ}$ C, the effect of which is also systematically investigated. It is found that both $\textit{T}_{\text{ch}}$ and O $_{\text{2}}$ annealing have significant effects on TFT performance. By using optimized O $_{\text{2}}$ annealing conditions, the ALD IGZO TFTs with scaled $\textit{T}_{\text{ch}}$ of 1.5 nm and $\textit{L}_{\text{ch}}$ of 60 nm exhibit desirable electrical performance including a high ON/OFF ratio ( $\textit{I}_{\biosc{on}}$ / $\textit{I}_{\biosc{off}}\text{)}$ $\sim$ 10 $^{\text{11}}$ , a decent high Ion of 354 $\mu$ A/ $\mu$ m under $\textit{V}_{\text{DS}}$ of 1.2 V, a steep subthreshold swing (SS) of 68 mV/dec, a small drain-induced-barrier-lowering (DIBL) of 30 mV/V, and a normal-off operation, which is comparable to the state-of-art sputtered IGZO TFTs. Furthermore, the optimized TFTs also exhibit significantly resolved threshold voltage ( $\textit{V}_{\text{T}}\text{)}$ roll-off and a remarkably high degree of stability to the positive gate bias stress (PBS). A trap model with its possible microscopic origin is proposed, which explains well the dependence of electrical performance on both $\textit{T}_{\text{ch}}$ and O $_{\text{2}}$ annealing, thus providing a new insight into the reliability of IGZO TFTs.

Device surface properties are critical for its performance such as channel electron density, leakage current, subthreshold swing (SS), and noise in gallium nitride high-electron-mobility transistors (HEMTs). In this article, the improved surface property of InAlN/GaN HEMTs with forming gas (FG, 5% H ₂ , and 95% N ₂ ) annealing is demonstrated. X-ray photoelectron spectra (XPS) show that the number of Ga–O bonds decreases while that of the Ga–N bonds increases, an indication of the surface native oxide removal after FG annealing. Compared with N ₂ annealing, an increase of two-dimensional electron gas (2DEG) electron density with FG annealing is determined by both energy band simulation and capacitance–voltage measurement. Transmission line measurement (TLM) shows that N ₂ annealing offers a lower ohmic contact resistance ( ${R}_{\text {C}}$ ) while FG annealing features a lower sheet resistance ( ${R}_{\text {sheet}}$ ). Herein, a FG/N ₂ two-step ohmic contact annealing is developed to achieve a SS of 110 mV/dec, a transconductance ( ${g}_{\text {m}}$ ) peak of 415 mS/mm, a record low drain-induced barrier lowing (DIBL) of 65 mV/V, and a record high power gain cutoff frequency ( ${f}_{\text {max}}$ ) of 270 GHz on a 50-nm InAlN/GaN HEMT on Si.

In this Letter, we report the electrical performance improvement of indium oxide (In2O3) thin film transistors (TFTs) via a low-temperature CF4/N2O plasma treatment. It is found that the fluorination via CF4/N2O plasma can reduce the excessive electrons in the In2O3 channel more effectively compared to the oxidative annealing, providing the same low off-currents at a lower temperature of 200 °C, while the hydrogenation could not give rise to the off-current reduction. The fluorinated In2O3 TFTs with a channel thickness of 3.5 nm, a HfO2 dielectric thickness of 3.5 nm, and a channel length ranging from 80 nm to 1 μm demonstrate markedly improved electrical performances, including a high field effect mobility of 72.8 cm2/V s, a more positive threshold voltage, a higher on/off current ratio of ∼106, a smaller subthreshold swing below 200 mV/dec, and a higher stability to both negative and positive gate biases. X-ray photoelectron spectroscopy (XPS) confirms the fluorine incorporation in In2O3/HfO2 heterojunction upon CF4/N2O plasma, speculatively passivating the oxygen vacancies and explaining TFT performance enhancement. This study suggests that the anion doping such as fluorine incorporation could be an effective method to improve the performance of oxide semiconductor TFTs with ultrathin channel and dielectric.