Circuits implemented with high-performance amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs) are realized on polyimide/polyethylene-terephthalate plastic substrates. The TFTs on plastic exhibit a saturation mobility of 19 cm²/V·s and a gate voltage swing of ~0.14 V/dec. For an input of 20 V, an 11-stage ring oscillator operates at 94.8 kHz with a propagation delay time of 0.48 μs. A shift register, consisting of ten TFTs and one capacitor, operates well with good bias stability. AC driving of pull-down TFTs gives the gate driver an improved lifetime of over ten years.

We present here an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) in which the accumulation layer is not only confined to the a-IGZO/gate-insulator interface, but extends the entire depth of the a-IGZO. This bulk accumulation TFT is achieved by the use of top- and bottom-gate, that are electrically tied together, resulting in drain current that is over seven times higher than that of a single-gate device, for an a-IGZO thickness of 10 nm. Thus, high drive current is achieved for a relatively small channel width due to bulk accumulation. Furthermore, being independent of carrier scattering at the interface and owing to the bulk accumulation/depletion, the subthreshold swing is always small and turn-on voltage around zero volts with device-to-device uniformity that is much better than that of single-gate TFTs.

Radiating amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with deep ultraviolet light (λ = 175 nm) is found to induce rigid negative threshold-voltage shift, as well as a subthreshold hump and an increase in subthreshold-voltage slope. These changes are attributed to the photo creation and ionization of oxygen vacancy states (V_O), which are confined mainly to the top surface of the a-IGZO film (backchannel). Photoionization of these states generates free electrons and the transition from the neutral to the ionized V_O is accompanied by lattice relaxation, which raises the energy of the ionized V_O. This and the possibility of atomic exchange with weakly bonded hydrogen leads to metastability of the ionized V_O, consistent with the rigid threshold-voltage shift and increase in subthreshold-voltage slope. The hump is thus a manifestation of the highly conductive backchannel and its formation can be suppressed by reduction of the a-IGZO film thickness or application of a back bias after radiation. These results support photo creation and ionization of V_O as the main cause of light instability in a-IGZO TFTs and provide some insights on how to minimize the effect.

Advancements in thin-film transistor (TFT) technology have extended to electronics that can withstand extreme bending or even folding. Although the use of ultrathin plastic substrates has achieved considerable advancement towards this end, free-standing ultrathin plastics inevitably suffer from mechanical instability and are very difficult to handle during TFT fabrication. Here, in addition to the use of a 1.5 μm-thick polyimide (PI) substrate, a 1.5 μm-thick PI film is also deposited on top of the TFT devices to ensure that the devices are located at the neutral plane of the two PI films for high folding stability. For mechanical support during TFT fabrication up to the deposition of the top PI film, the PI substrate is spin coated on top of a carrier glass that is coated with a mixture of carbon nanotubes (CNTs) and graphene oxide (GO). The mixture of CNT and GO facilitates mechanical detachment of the neutral plane (NP) TFTs from the carrier glass before they are transferred to a polydimethylsiloxane (PDMS) substrate as islands. Being located in the neutral bending plane, the NP TFT can be transferred to the PDMS without performance degradation and exhibit excellent mechanical stability after stretching the PDMS substrate up to a 25% elastic elongation.

A high-performance inverter implemented with single-gated driving and dual-gated load amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs) is demonstrated. The threshold voltage of the load TFT shifts to the negative gate voltage direction when a constant positive bias is applied on the top gate while sweeping the bottom gate. Using a positive top gate bias, the load TFT can be operated in the depletion mode to realize inverters with excellent switching characteristics, such as a wider swing range and a higher noise margin.