Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

https://iopscience.iop.org/article/10.1088/1468-6996/11/4/044305/pdf

Present status of amorphous In–Ga–Zn–O thin-film transistors

A low-temperature, solution-based preparation of amorphous, metal oxide semiconducting thin-films is reported. This ‘sol–gel on chip’ hydrolysis approach yields thin-film transistors with high field-effect mobilities, reproducible and stable turn-on voltages and high operational stability.

Low-temperature, high-performance solution-processed metal oxide thin-film transistors formed by a ‘sol–gel on chip’ process

Transparent amorphous oxide semiconductors have unique electron transport properties, such as large electron mobility (10–50 cm2/Vs) and the absence of a Hall voltage sign anomaly, that are not seen in conventional amorphous semiconductors. This class of materials has been attracting much attention as a channel layer in thin-film transistors (TFTs) utilizing the above features along with the processing advantage that thin films can be deposited at low temperatures by conventional sputtering methods. The primary driving force for this trend is a rapidly emerging demand for backplane TFTs that can drive the next generation of flat-panel displays. This article reviews the recent advances in fundamental science of these materials and their TFT applications. Emphasis is placed on the view that high ionicity in chemical bonding and large spherical spread of unoccupied metal s orbitals in p-block metal oxides lead to the realization of electronic structures that are advantageous for n-channel TFT applications. Amorphous oxide semiconductors are compared with conventional hydrogenated amorphous silicon, which is used widely as the channel material for backplane TFTs in current liquid-crystal displays.

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Material characteristics and applications of transparent amorphous oxide semiconductors

Solid-state electronics

We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Two-dimensional numerical simulation of radio frequency sputter amorphous In–Ga–Zn–O thin-film transistors

We studied both the wavelength and intensity dependent photo‐responses (photofield‐effect) in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors (TFTs). During the a‐IGZO TFT illumination with the wavelength range from 460∼660 nm (visible range), the off‐state drain current (IDS_off) only slightly increased while a large increase was observed for the wavelength below 400 nm. The observed results are consistent with the optical gap of ∼3.05eV extracted from the absorption measurement. The a‐IGZO TFT properties under monochromatic illumination (λ=420nm) with different intensity was also investigated and IDS_off was found to increase with the light intensity. Throughout the study, the field‐effect mobility (μeff) is almost unchanged. But due to photo‐generated charge trapping, a negative threshold voltage (Vth) shift is observed. The mathematical analysis of the photofield‐effect suggests that a highly efficient UV photocurrent conversion process in TFT off‐region takes place. Finally, a‐IGZO mid‐g...

https://www.tandfonline.com/doi/pdf/10.1080/15980316.2008.9652066

Photofield‐effect in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors

We investigated the low-frequency noise properties in the inverted-staggered amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with the silicon dioxide (SiO2) gate dielectric. The dependence of noise level on gate area indicates that the 1/f noise is the dominate source and the contribution from TFT parasitic resistances can be ignored in long channel devices. The gate voltage dependent noise data closely follow the mobility fluctuation (Δμ) model, and the Hooge’s parameter (αH) was extracted to be ∼1.52×10−3, which is much lower than the reported αH for a-Si:H TFTs. Finally, in the comparative study, the noise level in an unannealed a-IGZO TFT was found to be higher than that in an annealed device. The present results suggest that the 1/f noise in our a-IGZO TFT samples is sensitive to the active layer quality (i.e., concentration of conduction band-tail and/or deep gap states). In addition, the observed low noise in a-IGZO TFT can be associated with the s-orbital conduction in amorphous oxide semiconductor.

Low frequency noise in long channel amorphous In–Ga–Zn–O thin film transistors

In this paper, we analyze application of amorphous In–Ga–Zn–O thin film transistors (a-InGaZnO TFTs) to current-scaling pixel electrode circuit that could be used for 3-in. quarter video graphics array (QVGA) full color active-matrix organic light-emitting displays (AM-OLEDs). Simulation results, based on a-InGaZnO TFT and OLED experimental data, show that both device sizes and operational voltages can be reduced when compare to the same circuit using hydrogenated amorphous silicon (a-Si:H) TFTs. Moreover, the a-InGaZnO TFT pixel circuit can compensate for the drive TFT threshold voltage variation (� VT) within acceptable operating error range. # 2009 The Japan Society of Applied Physics

https://iopscience.iop.org/article/10.1143/JJAP.48.03B025/pdf

Amorphous In?Ga?Zn?O Thin Film Transistor Current-Scaling Pixel Electrode Circuit for Active-Matrix Organic Light-Emitting Displays

We investigated the electrical properties and stability of short-channel half-Corbino amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). In the linear region, the fabricated half-Corbino a-IGZO TFT with a channel length of 4.5 μm achieves a geometrical factor ( f g ) of ∼2.7, a threshold voltage ( V T ) of ∼2.4 V, a field-effect mobility ( μ eff ) of ∼15 cm 2 /Vs, a subthreshold swing ( SS ) of ∼320 mV/dec and an off-current ( I OFF )  −13  A. In the saturation region, asymmetric electrical characteristics such as drain current were observed under different drain bias conditions. The electrical properties asymmetry of half-Corbino a-IGZO TFTs was explained by various geometrical factors owing to the short-channel effect. The reduced V T and increased SS at V DS  = 15 V is explained by the drain-induced Schottky barrier lowering. In addition, the bias-temperature stress (BTS) was performed for half-Corbino a-IGZO TFTs with both amorphous silicon oxide (a-SiO x ) single layer and a-SiO x /amorphous silicon nitride (a-SiN x ) bilayer passivation (PV) structures. The device with bilayer PV shows a threshold voltage shift (Δ V T ) of +2.07 and −0.5 V under positive (PBTS = +15 V) and negative BTS (NBTS = −15 V) at 70 °C for 10 ks, respectively. The origins of Δ V T during PBTS and NBTS for half-Corbino a-IGZO TFTs with single and bilayer PV structures were studied. To improve the device electrical stability, the bilayer PV structure should be used.

Tze Ching Fung

Papers

Two-dimensional numerical simulation of radio frequency sputter amorphous In–Ga–Zn–O thin-film transistors

Photofield‐effect in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors

Low frequency noise in long channel amorphous In–Ga–Zn–O thin film transistors

Amorphous In?Ga?Zn?O Thin Film Transistor Current-Scaling Pixel Electrode Circuit for Active-Matrix Organic Light-Emitting Displays

Half-Corbino short-channel amorphous In–Ga–Zn–O thin-film transistors with a-SiOx or a-SiOx/a-SiNx passivation layers