Heterojunction channel engineering is discussed as a means of enhancing the performance and bias stress stability of In–Ga–Zn–O thin-film transistors (IGZO TFTs). A heterojunction channel was formed by depositing In-rich IGZO on IGZO111 with an atomic ratio of In:Ga:Zn=1:1:1, whose energy band lineup was type-II with a potential well of 0.4 eV for electrons. The thickness of the bottom IGZO111 layer strongly affected the field-effect mobility ( μ _FE ) of the TFT owing to electron confinement effect at the heterojunction interface. μ _FE was enhanced from 12.4 cm ² V ⁻¹ s ⁻¹ for homogeneous IGZO111 TFT to 24.7 cm ² V ⁻¹ s ⁻¹ for the heterojunction TFT. Device simulation results revealed that electron confinement in the heterojunction channel played an important role in determining μ _FE in the TFT. A channel engineering approach will provide a method of overcoming the trade-off between μ _FE and the positive-bias stress stability of oxide TFTs.

We have found a memristive characteristic of an α-GTO thin-film device. The α-GTO thin-film layer is deposited using radio-frequency (RF) magnetron sputtering at room temperature and sandwiched between the Al top and bottom electrodes. It is found that the hysteresis loop of the flowing current (I) and applied voltage (V) characteristic becomes larger and stable after the one hundredth cycle. The electrical resistances for the high-resistance state (HRS) and low-resistance state (LRS) are clearly different and relatively stable. Based on various analysis, it is suggested that the memristive characteristic is due to the chemical reaction between the SnO₂ and SnO blocked by AlO_x on the Al bottom electrode. It is marvelous that the memristive characteristic can be realized by such common materials, simple structures, and easy fabrication.

In this paper, we investigated the influence of both source gas and deposition temperature in plasma-enhanced chemical vapor deposition (PE-CVD) for a SiO ₂ passivation layer on the electrical properties and reliability of a bottom-gate In-Ga-Zn-O thin-film transistor (IGZO TFT). Two gas chemistries consisting of SiH ₄ -N ₂ O-N ₂ and tetraethoxysilane (TEOS)-O ₂ were utilized as the source gases for the PE-CVD SiO ₂ deposition, and the deposition temperature (TD) was adjusted from 180 °C to 380 °C. The TFT properties were basically identical for both gas chemistries at TD of 180 °C. When TD increased to 300 °C or higher, the TFTs with the SiO ₂ passivation deposited by SiH ₄ -N ₂ O-N ₂ gas chemistry (SiH ₄ -SiO ₂ ) drastically changed from the transistor to the conductor. In contrast, the TFT with TEOS-SiO ₂ passivation maintained its TFT characteristics even at TD of 380 °C, despite the degradation of subthreshold characteristics and a negative shift of turn-on voltage were observed due to an electron injection barrier lowering as TD increased to 310 °C or higher. We also revealed that a stacked SiO ₂ passivation that is deposited at a different TD is an effective technique to improve the performance and reliability of the IGZO TFT.

Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (ΔE_c) of ~0.4 eV. A depth profile of background charge density indicated that a steep ΔE_c is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (μ_FE) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm² V⁻¹ s⁻¹, although a channel/gate insulator interface was formed by an IGZO−111 (μ_FE = ~12 cm² V⁻¹ s⁻¹). Device simulation analysis revealed that the improvement of μ_FE in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ΔE_c. Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.

A marked improvement in the reliability of a high-mobility In–Ga–Zn–O (IGZO) thin-film transistor (TFT) is presented after 150 °C annealing by applying hydrogenated IGZO (IGZO:H) as a channel. To enhance field-effect mobility (μ _FE), the atomic ratio of In:Ga:Zn was chosen to be 6:2:1. The IGZO:H TFT exhibited a μ _FE of 18.9 cm² V⁻¹ s⁻¹ without hysteresis. Moreover, the reliability of the IGZO:H TFT significantly improved after 150 °C annealing as compared with that of a conventional IGZO TFT. Thus, the use of IGZO:H is an effective method of improving both the electrical properties and reliability of TFTs for flexible electronics.