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

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.     

Fabrication of Flexible MoS2 Thin‐Film Transistor Arrays for Practical Gas‐Sensing Applications

By combining two kinds of solution‐processable two‐dimensional materials, a flexible transistor array is fabricated in which MoS₂ thin film is used as the active channel and reduced graphene oxide (rGO) film is used as the drain and source electrodes. The simple device configuration and the 1.5 mm‐long MoS₂ channel ensure highly reproducible device fabrication and operation. This flexible transistor array can be used as a highly sensitive gas sensor with excellent reproducibility. Compared to using rGO thin film as the active channel, this new gas sensor exhibits much higher sensitivity. Moreover, functionalization of the MoS₂ thin film with Pt nanoparticles further increases the sensitivity by up to ～3 times. The successful incorporation of a MoS₂ thin‐film into the electronic sensor promises its potential application in various electronic devices.     

Thin‐Film Transistors

Thin‐film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin‐film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.     

Recent Progress in n‐Channel Organic Thin‐Film Transistors

Particular attention has been focused on n‐channel organic thin‐film transistors (OTFTs) during the last few years, and the potentially cost‐effective circuitry‐based applications in flexible electronics, such as flexible radiofrequency identity tags, smart labels, and simple displays, will benefit from this fast development. This article reviews recent progress in performance and molecular design of n‐channel semiconductors in the past five years, and limitations and practicable solutions for n‐channel OTFTs are dealt with from the viewpoint of OTFT constitution and geometry, molecular design, and thin‐film growth conditions. Strategy methodology is especially highlighted with an aim to investigate basic issues in this field.     

Corrugated Heterojunction Metal‐Oxide Thin‐Film Transistors with High Electron Mobility via Vertical Interface Manipulation

A new strategy is reported to achieve high‐mobility, low‐off‐current, and operationally stable solution‐processable metal‐oxide thin‐film transistors (TFTs) using a corrugated heterojunction channel structure. The corrugated heterojunction channel, having alternating thin‐indium‐tin‐zinc‐oxide (ITZO)/indium‐gallium‐zinc‐oxide (IGZO) and thick‐ITZO/IGZO film regions, enables the accumulated electron concentration to be tuned in the TFT off‐ and on‐states via charge modulation at the vertical regions of the heterojunction. The ITZO/IGZO TFTs with optimized corrugated structure exhibit a maximum field‐effect mobility >50 cm² V^?1 s^?1 with an on/off current ratio of >10⁸ and good operational stability (threshold voltage shift <1 V for a positive‐gate‐bias stress of 10 ks, without passivation). To exploit the underlying conduction mechanism of the corrugated heterojunction TFTs, a physical model is implemented by using a variety of chemical, structural, and electrical characterization tools and Technology Computer‐Aided Design simulations. The physical model reveals that efficient charge manipulation is possible via the corrugated structure, by inducing an extremely high carrier concentration at the nanoscale vertical channel regions, enabling low off‐currents and high on‐currents depending on the applied gate bias.     

Improved Morphology and Performance of Solution‐Processed Metal‐Oxide Thin‐Film Transistors Due to a Polymer Based Interface Modifier

The influence of a polymer interface modifier on the performance of solution‐processed indium‐based metal‐oxide (MO) thin‐film transistors (TFTs) is investigated. We use the polymer ethoxylated polyethylenimine (PEIE). Compared to a reference sample this modification enhances the mobility by a factor of four, clearly reduces the contact and the sheet resistance, and decreases the charge carrier activation energy by about 20%. The improved electrical performance originates from both a reduced contact and a reduced sheet resistance of the TFTs. The molecular dipole of PEIE reduces the work function of the electrodes. Adversely the dipole enhances the off current and the trap density at the semiconductor/dielectric interface for bottom‐contact transistors with small channel length. The substrate becomes highly polar with a PEIE‐treatment. Accordingly, topographical studies of bottom‐contact TFTs show a very similar MO film morphology on the electrodes and in the channel for modified TFTs, whereas in the untreated samples the film has a higher roughness on the electrodes than in the channel. TFTs in top‐contact configuration with the polymer interface layer at the dielectric/semiconductor interface also show higher mobility compared to the reference MOTFTs which displays that the improved performance is due to the improved morphology of the MO film.