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We investigated the threshold voltage (Vth) instability for various gate dielectrics (SiNx and SiOx) in amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs). The a-IGZO TFTs with SiNx 150 °C exhibited reasonable electrical performance (field-effect mobility of 8.1 cm2/V s and Ion/off ratio of >108) but showed huge Vth shift under positive gate bias. The TFTs with SiOx dielectrics exhibit smaller Vth instability than those of SiNx dielectrics. This behavior can be explained by using simple charge trapping into the gate insulators and the difference of Vth instability on various dielectrics may be originated from the hydrogen contents, providing high density of charge traps in gate dielectrics.

We have fabricated 6.5 in. flexible full-color top-emission active matrix organic light-emitting diode display on a polyimide (PI) substrate driven amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs). The a-IGZO TFTs exhibited field-effect mobility (μFE) of 15.1 cm2/V s, subthreshold slope of 0.25 V/dec, threshold voltage (VTH) of 0.9 V. The electrical characteristics of TFTs on PI substrate, including a bias-stress instability after 1 h long gate bias at 15 V, were indistinguishable from those on glass substrate and showed high degree of spatial uniformity. TFT samples on 10 μm thick PI substrate withstood bending down to R=3 mm under tension and compression without any performance degradation.

We investigated the effect of the high-k TiOx (k∼40) gate dielectric on the mobility (μFE) of indium-gallium-zinc oxide (IGZO) transistors. As the thickness of the TiOx layer at the interface of the IGZO channel and SiNx gate dielectric layer increased from 2 to 8 nm, the μFE value was monotonously reduced from 9.9 to 1.8 cm2/V s. The degradation of the mobility was attributed to the Coulomb scattering mechanism rather than the phonon scattering mechanism of the high-k TiOx layer based on the behavior of the temperature-dependent mobilities for all of the IGZO transistors.

We investigated the impact of the passivation layer on the stability of indium-gallium-zinc oxide (IGZO) thin film transistors. While the device without any passivation layer showed a huge threshold voltage (Vth) shift under positive gate voltage stress, the suitably passivated device did not exhibit any Vth shift. The charge trapping model, which has been believed to be a plausible mechanism, cannot by itself explain this behavior. Instead, the Vth instability was attributed to the interaction between the exposed IGZO backsurface and oxygen and/or water in the ambient atmosphere during the gate voltage stress.

We investigated the feasibility of controlling the threshold voltage (Vth) by adjusting the thickness of the active layer (tactive) rather than by conventional chemical doping in indium-gallium-zinc oxide (IGZO) transistors with an inverted staggered structure. The value of Vth of the IGZO transistor was linearly modulated from −15.3±1.6to−0.1±0.21V by reducing tactive without any significant change in the field-effect mobility (μFE), subthreshold gate swing, or Ion∕off ratio. The free electron density extracted from the relationship between tactive and Vth was 1.9×1017cm−3, which was consistent with the value of 1.5×1017cm−3 obtained from the C-V measurement for the 30-nm-thick IGZO films. The slight increase in the μFE with increasing tactive, which was in contradiction with the behavior of the corresponding amorphous Si transistor, was explained by the anomalous behavior of the source/drain contact resistance.

The effect of water exposure on amorphous indium-gallium-zinc oxide (a-IGZO) semiconductors was reported. It was found that water can diffuse in and out of the a-IGZO film, reversibly affecting the transistor properties. Two competing mechanisms depending on the thickness of the active channel were clarified. The electron donation effect caused by water adsorption dominated for the thicker a-IGZO films (⩾100nm), which was manifested in the large negative shift (>14V) of the threshold voltage. However, in the case of the thinner a-IGZO films (⩽70nm), the dominance of the water-induced acceptorlike trap behavior was observed. The direct evidence for this behavior was that the subthreshold swing was greatly deteriorated from 0.18V/decade (before water exposure) to 4.4V/decade (after water exposure) for the thinnest a-IGZO films (30nm). These results can be well explained by the screening effect of the intrinsic bulk traps of the a-IGZO semiconductor.

The authors report the fabrication of high performance thin film transistors (TFTs) with an amorphous indium gallium zinc oxide (a-IGZO) channel, which was deposited by cosputtering using a dual IGZO and indium zinc oxide (IZO) target. The effect of the indium content on the device performance of the a-IGZO TFTs was investigated. At a relatively low IZO power of 400W, the field-effect mobility (μFE) and subthreshold gate swing (S) of the a-IGZO TFTs were dramatically improved to 19.3cm2∕Vs and 0.35V/decade, respectively, compared to those (11.2cm2∕Vs and 1.11V/decade) for the TFTs with the a-IGZO channel (reference sample) prepared using only the IGZO target. The enhancement in the subthreshold IDS-VGS characteristics at an IZO power of 400W compared to those of the reference sample was attributed to the reduction of the interface trap density rather than the reduction of the bulk defects of the a-IGZO channel.

The effect of Ar plasma treatment on amorphous indium gallium zinc oxide (a-IGZO) thin films was investigated. The net electron carrier concentration (1020–1021cm−3) of the a-IGZO thin films dramatically increased upon their exposure to the Ar plasma compared to that (1014cm−3) of the as-deposited thin film. The authors attempted to reduce the contact resistance between the Pt∕Ti (source/drain electrode) and a-IGZO (channel) by using the Ar plasma treatment. Without the treatment, the a-IGZO thin film transistors (TFTs) with W∕L=50∕4μm exhibited a moderate field-effect mobility (μFE) of 3.3cm2∕Vs, subthreshold gate swing (S) of 0.25V∕decade, and Ion∕off ratio of 4×107. The device performance of the a-IGZO TFTs was significantly improved by the Ar plasma treatment. As a result, an excellent S value of 0.19V∕decade and high Ion∕off ratio of 1×108, as well as a high μFE of 9.1cm2∕Vs, were achieved for the treated a-IGZO TFTs.

The authors report on the fabrication of thin film transistors (TFTs), which use an amorphous indium gallium zinc oxide (a-IGZO) channel, by rf sputtering at room temperature and for which the channel length and width are patterned by photolithography and dry etching. To prevent plasma damage to the active channel, a 100-nm-thick SiOx layer deposited by plasma enhanced chemical vapor deposition was adopted as an etch stopper structure. The a-IGZO TFT (W∕L=10μm∕50μm) fabricated on glass exhibited a high field-effect mobility of 35.8cm2∕Vs, a subthreshold gate swing value of 0.59V∕decade, a thrseshold voltage of 5.9V, and an Ion∕off ratio of 4.9×106, which is acceptable for use as the switching transistor of an active-matrix TFT backplane.