Hai Q. Chiang

High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layer

Transparent thin-film transistors (TTFTs) with an amorphous zinc tin oxide channel layer formed via rf magnetron sputter deposition are demonstrated. Field-effect mobilities of 5–15 and 20–50cm2V−1s−1 are obtained for devices post-deposition annealed at 300 and 600°C, respectively. TTFTs processed at 300 and 600°C yield devices with turn-on voltage of 0–15 and −5–5V, respectively. Under both processing conditions, a drain current on-to-off ratio greater than 107 is obtained. Zinc tin oxide is one example of a new class of high performance TTFT channel materials involving amorphous oxides composed of heavy-metal cations with (n−1)d10ns0 (n⩾4) electronic configurations.

Transparent thin-film transistors with zinc indium oxide channel layer

High mobility, n-type transparent thin-film transistors (TTFTs) with a zinc indium oxide (ZIO) channel layer are reported. Such devices are highly transparent with ∼85% optical transmission in the visible portion of the electromagnetic spectrum. ZIO TTFTs annealed at 600 °C operate in depletion-mode with threshold voltages −20 to −10V and turn-on voltages ∼3V less than the threshold voltage. These devices have excellent drain current saturation, peak incremental channel mobilities of 45–55cm2V−1s−1, drain current on-to-off ratios of ∼106, and inverse subthreshold slopes of ∼0.8V∕decade. In contrast, ZIO TTFTs annealed at 300 °C typically operate in enhancement-mode with threshold voltages of 0–10V and turn-on voltages 1–2V less than the threshold voltage. These 300 °C devices exhibit excellent drain–current saturation, peak incremental channel mobilities of 10–30cm2V−1s−1, drain current on-to-off ratios of ∼106, and inverse subthreshold slopes of ∼0.3V∕decade. ZIO TTFTs with the channel layer deposited ne...

Constant-Voltage-Bias Stress Testing of a-IGZO Thin-Film Transistors

Constant-voltage-bias (V_DS = V_GS = 30 V) stress measurements are performed for a period of 10⁵ s on thin-film transistors (TFTs) with amorphous indium-gallium-zinc-oxide (IGZO) channel layers fabricated via RF sputtering using a postdeposition annealing temperature of 200degC, 250degC, or 300degC. Thermal silicon dioxide is employed as a TFT bottom-gate insulator. All SiO₂/IGZO TFTs tested exhibit the following: 1) a positive rigid log(I_D)- V_GS transfer curve shift; 2) a continuous drain-current decrease over the entire stress duration; and 3) recovery of the log(I_D)-V_GS transfer curve toward the prestressed state when the stressed TFT is left unbiased in the dark at room temperature for an extended period of time. The SiO₂/IGZO TFTs subjected to a higher postdeposition annealing temperature are more stable. A small (and typically negligible) amount of clockwise hysteresis is present in the log(I_D) -V_GS transfer curves of IGZO TFTs. These instability and hysteresis observations are consistent with a SiO₂/ IGZO TFT instability mechanism involving electron trapping within the IGZO channel layer.

Thin-film transistors with transparent amorphous zinc indium tin oxide channel layer

Thin-film transistors (TFTs) with transparent amorphous zinc indium tin oxide (ZITO) channel layer are demonstrated. Optical transmission of the channel layer is approximately 85% in the visible portion of the electromagnetic spectrum. The channel layer is formed via rf magnetron sputter deposition and then furnace annealed in air. Peak incremental mobilities of 5–19 cm2 V−1 s−1 and turn-on voltages of −4 to −17 V are obtained for devices annealed post-deposition at 100–300 °C, respectively. Current–voltage measurements indicate n-channel, depletion-mode transistor operation with excellent drain current saturation and a drain current on-to-off ratio greater than 106. ZITO is one example of an emerging class of high performance TFT channel materials involving transparent amorphous multicomponent oxides composed of heavy-metal cations with (n − 1)d10ns0 (n ≥ 4) electronic configuration.

Thin-film transistors with amorphous indium gallium oxide channel layers

Indium gallium oxide-based thin-film transistors (TFTs) are formed using rf magnetron sputtering of the channel layer. These TFTs exhibit qualitatively ideal characteristics, including excellent drain current saturation. Various deposition parameters, annealing treatments, and stoichiometries are explored. Varying the oxygen partial pressure is found to have a significant effect on device performance. Decreasing the oxygen partial pressure increases the incremental channel mobility μinc while decreasing (becomes more negative) the turn-on voltage Von. Increasing indium concentration of the channel material increases μinc, while decreasing Von. The maximum value of μinc, ∼27cm2V−1s−1, is obtained by annealing at 600°C, with corresponding Von and drain current on-to-off ratio values of approximately −14V and >106, respectively. Additionally, TFTs subjected to a 200°C postdeposition annealing exhibit μinc and Von of ∼19cm2V−1s−1 and 2V, respectively.

Electrical Modeling of Thin-Film Transistors

An overview of device physics-oriented electrical modeling of thin-film transistors (TFTs) is presented. Four specific models are considered: (i) square-law, (ii) 3-layer, (iii) comprehensive depletion-mode, and (iv) discrete trap. For each model, a functional assessment of model equations is undertaken in terms of independent and dependent variables, model parameters, physical operating parameters, and constraining inequalities in order to facilitate mapping of model equations into a corresponding equivalent circuit. Channel mobility and “subthreshold” current trends are elucidated. Finally, a conductance integral equation based on Shockleys gradual channel approximation is introduced and is employed in model development and device assessment.

Zinc tin oxide thin-film transistors via reactive sputtering using a metal target

Zinc tin oxide based thin-film transistors are fabricated via reactive magnetron sputtering using a metallic zinc/tin alloy target. The oxygen partial pressure and total sputtering pressure are explored. An oxygen partial pressure and total sputtering pressure of 0.8 and 30mTorr, respectively, are found to be optimal. Devices with a reactively sputtered zinc tin oxide channel layer and channel layer annealing of 500°C exhibit incremental mobilities of ∼32cm2V−1s−1, turn-on voltage of ∼−4V and drain current on-to-off ratios of ∼107. Both direct current and radio frequency magnetron sputtering are explored showing similar characteristics.