Metal oxide thin‐film transistors have been continuously researched and mass‐produced in the display industry. However, their phototransistors are still in their infancy. In particular, utilizing metal oxide semiconductors as phototransistors is difficult because of the limited light absorption wavelength range and persistent photocurrent (PPC) phenomenon. Numerous studies have attempted to improve the detectable light wavelength range and the PPC phenomenon. Here, recent studies on metal oxide phototransistors are reviewed, which have improved the range of light wavelengths and the PPC phenomenon by introducing an absorption layer of oxide or non‐oxide hybrid structure. The materials of the absorption layer applied to absorb long‐wavelength light are classified into oxides, chalcogenides, organic materials, perovskites, and nanodots. Finally, next‐generation convergence studies combined with other research fields are introduced and future research directions are detailed.

Visible light can be detected using an indium–gallium–zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 103 A/W, photosensitivity of 4.39 × 109, detectivity of 3.44 × 1013 Jones, and external quantum efficiency of 3.52 × 103% when illuminated by green light (532 nm). Ultraviolet–visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p–n junction with IGZO so that photogenerated electron–hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light. Furthermore, the SCL/IGZO phototransistor exhibited excellent durability and reversibility owing to the constant light and dark current and the time-dependent photoresponse characteristics over 8000 s when a red light (635 nm) source was turned on and off at a frequency of 0.1 Hz.

Resistive random access memory (RRAM) devices are fabricated through a simple solution process using glucose, which is a natural biomaterial for the switching layer of RRAM. The fabricated glucose-based RRAM device shows nonvolatile bipolar resistive switching behavior, with a switching window of 10 . In addition, the endurance and data retention capability of glucose-based RRAM exhibit stable characteristics up to 100 consecutive cycles and 10 s under constant voltage stress at 0.3 V. The interface between the top electrode and the glucose film is carefully investigated to demonstrate the bipolar switching mechanism of the glucose-based RRAM device. The glucose based-RRAM is also evaluated on a polyimide film to verify the possibility of a flexible platform. Additionally, a cross-bar array structure with a magnesium electrode is prepared on various substrates to assess the degradability and biocompatibility for the implantable bioelectronic devices, which are harmless and nontoxic to the human body. It is expected that this research can provide meaningful insights for developing the future bioelectronic devices.

We investigated a facile fabrication method, which is an insertion of a carrier-induced interlayer (CII) between the oxygenrich a-IGZO channel and the gate insulator to improve the electrical characteristics and stability of amorphous indium−gallium−zinc− oxide thin-film transistors (a-IGZO TFTs). The a-IGZO channel is deposited with additional oxygen gas flow during a-IGZO channel deposition to improve the stability of the a-IGZO TFTs. The CII is a less than 10 nm thick deposited thin film that acts to absorb the oxygen from the a-IGZO front channel through oxidation. Through oxidation of the CII, the oxygen concentration of the a-IGZO front channel is decreased compared to that of the oxygen-rich back channel, which forms a vertically graded oxygen deficiency (VGO) in the a-IGZO channel. Therefore, the electrical characteristics of the VGO TFTs are improved by increasing the carrier concentration of the front channel as the oxygen vacancy concentration in the front channel is increased through the oxidation of the CII. At the same time, the stability of the VGO TFTs is improved by maintaining a high oxygen concentration in the back channel even after oxidation of the CII. The field-effect mobility (μ FET ) of the VGO TFTs improved compared to that of the a-IGZO TFTs from 7.16 ± 0.6 to 12.0 ± 0.7 cm 2 /V•s. The threshold voltage (V th ) shifts under positive bias temperature stress and negative bias temperature illumination stress decreased from 6.00 to 2.95 V and −15.58 to −8.99 V, respectively.

In this study, the oxygen scavenger layer (OSL) is proposed as a back channel in the bilayer channel to enhance both the electrical characteristics and stability of an amorphous indium–gallium–zinc oxide thin-film transistor (a-IGZO TFT) and also to enable its fabrication at low temperature. The OSL is a hafnium (Hf)-doped a-IGZO channel layer deposited by radio-frequency magnetron cosputtering. Amorphous IGZO TFTs with the OSL, even if annealed at a low temperature (200 °C), exhibited improved electrical characteristics and stability under positive bias temperature stress (PBTS) compared to those without the OSL, specifically in terms of field-effect mobility (31.08 vs 9.25 cm2/V s), on/off current ratio (1.73 × 1010 vs 8.68 × 108), and subthreshold swing (0.32 vs 0.43 V/decade). The threshold voltage shift under PBTS at 50 °C for 10,000 s decreased from 9.22 to 2.31 V. These enhancements are attributed to Hf in the OSL, which absorbs oxygen ions from the a-IGZO front channel near the interface between a-IGZO and the OSL.

We fabricated wire-type indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) using a self-formed cracked template based on a lift-off process. The electrical characteristics of wire-type IGZO TFTs could be controlled by changing the width and density of IGZO wires through varying the coating conditions of template solution or multi-stacking additional layers. The fabricated wire-type devices were applied to sensors after functionalizing the surface. The wire-type pH sensor showed a sensitivity of 45.4 mV/pH, and this value was an improved sensitivity compared with that of the film-type device (27.6 mV/pH). Similarly, when the wire-type device was used as a glucose sensor, it showed more variation in electrical characteristics than the film-type device. The improved sensing properties resulted from the large surface area of the wire-type device compared with that of the film-type device. In addition, we fabricated wire-type IGZO TFTs on flexible substrates and confirmed that such structures were very resistant to mechanical stresses at a bending radius of 10 mm.

Amorphous indium-gallium-zinc oxide (a-IGZO) films, which are widely regarded as a promising material for the channel layer in thin-film transistors (TFTs), require a relatively high thermal annealing temperature to achieve switching characteristics through the formation of metal–oxygen (M–O) bonding (i.e., the activation process). The activation process is usually carried out at a temperature above 300 °C; however, achieving activation at lower temperatures is essential for realizing flexible display technologies. Here, a facile, low-cost, and novel technique using cellophane tape for the activation of a-IGZO films at a low annealing temperature is reported. In terms of mechanochemistry, mechanical pulling of the cellophane tape induces reactive radicals on the a-IGZO film surface, which can give rise to improvements in the properties of the a-IGZO films, leading to an increase in the number of M–O bonds and the carrier concentration via radical reactions, even at 200 °C. As a result, the a-IGZO TFTs, compared to conventionally annealed a-IGZO TFTs, exhibited improved electrical performances, such as mobility, on/off current ratio, and threshold voltage shift (under positive bias temperature and negative bias temperature stress for 10,000 s at 50 °C) from 8.25 to 12.81 cm2/(V·s), 2.85 × 107 to 1.21 × 108, 6.81 to 3.24 V, and −6.68 to −4.93 V, respectively.

IGZO TFTs with an instant glue passivation layer exhibit improved electrical performance and stability due to the atomic diffusion effect.