Heterojunction-engineered carrier transport in elevated-metal metal-oxide thin-film transistors

This study investigates the carrier transport of heterojunction channel in oxide semiconductor thin-film transistor(TFT)using the elevated-metal metal-oxide(EMMO)architecture and indium−zinc oxide(InZnO).The heterojunction band diagram of InZnO bilayer was modified by the cation composition to form the two-dimensional electron gas(2DEG)at the interface quantum well,as verified using a metal−insulator−semiconductor(MIS)device.Although the 2DEG indeed contributes to a higher mobility than the monolayer channel,the competition and cooperation between the gate field and the built-in field strongly affect such mobility-boosting effect,originating from the carrier inelastic collision at the heterojunction interface and the gate field-induced suppression of quantum well.Benefited from the proper energy-band engineering,a high mobility of 84.3 cm2·V^(−1)·s^(−1),a decent threshold voltage(V_(th))of−6.5 V,and a steep subthreshold swing(SS)of 0.29 V/dec were obtained in InZnO-based heterojunction TFT.

Fluorination-mitigated high-current degradation of amorphous InGaZnO thin-film transistors

As growing applications demand higher driving currents of oxide semiconductor thin-film transistors(TFTs),severe instabilities and even hard breakdown under high-current stress(HCS)become critical challenges.In this work,the triggering voltage of HCS-induced self-heating(SH)degradation is defined in the output characteristics of amorphous indium-galliumzinc oxide(a-IGZO)TFTs,and used to quantitatively evaluate the thermal generation process of channel donor defects.The fluorinated a-IGZO(a-IGZO:F)was adopted to effectively retard the triggering of the self-heating(SH)effect,and was supposed to originate from the less population of initial deep-state defects and a slower rate of thermal defect transition in a-IGZO:F.The proposed scheme noticeably enhances the high-current applications of oxide TFTs.

Advances in mobility enhancement of ITZO thin-film transistors:a review

Indium-tin-zinc oxide(ITZO)thin-film transistor(TFT)technology holds promise for achieving high mobility and offers significant opportunities for commercialization.This paper provides a review of progress made in improving the mobility of ITZO TFTs.This paper begins by describing the development and current status of metal-oxide TFTs,and then goes on to explain the advantages of selecting ITZO as the TFT channel layer.The evaluation criteria for TFTs are subsequently introduced,and the reasons and significance of enhancing mobility are clarified.This paper then explores the development of high-mobility ITZO TFTs from five perspectives:active layer optimization,gate dielectric optimization,electrode optimization,interface optimization,and device structure optimization.Finally,a summary and outlook of the research field are presented.

Indium-gallium-zinc-oxide thin-film transistors:Materials,devices,and applications

Since the invention of amorphous indium-gallium-zinc-oxide(IGZO)based thin-film transistors(TFTs)by Hideo Hosono in 2004,investigations on the topic of IGZO TFTs have been rapidly expanded thanks to their high electrical performance,large-area uniformity,and low processing temperature.This article reviews the recent progress and major trends in the field of IGZO-based TFTs.After a brief introduction of the history of IGZO and the main advantages of IGZO-based TFTs,an overview of IGZO materials and IGZO-based TFTs is given.In this part,IGZO material electron travelling orbitals and deposition methods are introduced,and the specific device structures and electrical performance are also presented.Afterwards,the recent advances of IGZO-based TFT applications are summarized,including flat panel display drivers,novel sensors,and emerging neuromorphic systems.In particular,the realization of flexible electronic systems is discussed.The last part of this review consists of the conclusions and gives an outlook over the field with a prediction for the future.

Temperature-dependent bias-stress-induced electrical instability of amorphous indium-gallium-zinc-oxide thin-film transistors

The time and temperature dependence of threshold voltage shift under positive-bias stress（PBS） and the following recovery process are investigated in amorphous indium-gallium-zinc-oxide（a-IGZO） thin-film transistors. It is found that the time dependence of threshold voltage shift can be well described by a stretched exponential equation in which the time constant τ is found to be temperature dependent. Based on Arrhenius plots, an average effective energy barrier Eτ stress= 0.72 eV for the PBS process and an average effective energy barrier Eτ recovery= 0.58 eV for the recovery process are extracted respectively. A charge trapping/detrapping model is used to explain the threshold voltage shift in both the PBS and the recovery process. The influence of gate bias stress on transistor performance is one of the most critical issues for practical device development.

Al_(2)O_(3)/HfO_(2) Nanolaminate Dielectric Boosting IGZO-Based Flexible Thin-Film Transistors

Flexible thin-film transistors(TFTs)have attracted wide interest in the development of flexible and wearable displays or sensors.However,the conventional high processing temperatures hinder the preparation of stable and reliable dielectric materials on flexible substrates.Here,we develop a stable laminated Al_(2)O_(3)/HfO_(2) insulator by atomic layer deposition at a relatively lower temperature of 150℃.A sputtered amorphous indium-gallium-zinc oxide(IGZO)with the stoichiometry of In_(0.37)Ga_(0.20)Zn_(0.18)O_(0.25) is used as the active channel material.The flexible TFTs with bottom-gate top-contacted configuration are further fabricated on a flexible polyimide substrate with the Al_(2)O_(3)/HfO_(2) nanolaminates.Benefited from the unique structural and compositional configuration in the nanolaminates consisting of amorphous Al_(2)O_(3),crystallized HfO_(2),and the aluminate Al-Hf-O phase,the as-prepared TFTs present the carrier mobilities of 9.7 cm^(2) V^(−1) s^(−1),ON/OFF ratio of-1.3×10^(6),subthreshold voltage of 0.1 V,saturated current up to 0.83 mA,and subthreshold swing of 0.256 V dec^(−1),signifying a high-performance flexible TFT,which simultaneously able to withstand the bending radius of 40 mm.The TFTs with nanolaminate insulator possess satisfactory humidity stability and hysteresis behavior in a relative humidity of 60-70%,a temperature of 25-30℃ environment.The yield of IGZO-based TFTs with the nanolaminate insulator reaches 95%.

Concise Modeling of Amorphous Dual-Gate In-Ga-Zn-O Thin-Film Transistors for Integrated Circuit Designs

An analytical model for current-voltage behavior of amorphous In-Ga-Zn-O thin-film transistors（a-IGZO TFTs）with dual-gate structures is developed.The unified expressions for synchronous and asynchronous operating modes are derived on the basis of channel charges,which are controlled by gate voltage.It is proven that the threshold voltage of asynchronous dual-gate IGZO TFTs is adjusted in proportion to the ratio of top insulating capacitance to the bottom insulating capacitance（C_（TI）/C_（BI））.Incorporating the proposed model with Verilog-A,a touch-sensing circuit using dual-gate structure is investigated by SPICE simulations.Comparison shows that the touch sensitivity is increased by the dual-gate IGZO TFT structure.

Review of flexible and transparent thin-film transistors based on zinc oxide and related materials

Flexible and transparent electronics enters into a new era of electronic technologies.Ubiquitous applications involve wearable electronics,biosensors,flexible transparent displays,radio-frequency identifications（RFIDs）,etc.Zinc oxide（ZnO） and relevant materials are the most commonly used inorganic semiconductors in flexible and transparent devices,owing to their high electrical performances,together with low processing temperatures and good optical transparencies.In this paper,we review recent advances in flexible and transparent thin-film transistors（TFTs） based on ZnO and relevant materials.After a brief introduction,the main progress of the preparation of each component（substrate,electrodes,channel and dielectrics） is summarized and discussed.Then,the effect of mechanical bending on electrical performance is highlighted.Finally,we suggest the challenges and opportunities in future investigations.

Influence of Post-Annealing on Electrical Characteristics of Thin-Film Transistors with Atomic-Layer-Deposited ZnO-Channel/Al_2O_3-Dielectric

High-performance thin-film transistors （TFTs） with a low thermal budget are highly desired for flexible electronic applications. In this work, the TFTs with atomic layer deposited ZnO-channel/Al2O3-dielectric are fabricated under the maximum process temperature of 200℃. First, we investigate the effect of post-annealing environment such as N2, H2-N2 （4%） and O2 on the device performance, revealing that o2 annealing can greatly enhance the device performance. Further, we compare the influences of annealing temperature and time on the device performance. It is found that long anneMing at 200℃is equivalent to and even outperforms short annealing at 300℃. Excellent electrical characteristics of the TFTs are demonstrated after 02 anneMing at 200℃ for 35 rain, including a low off-current of 2.3 × 10-13 A, a small sub-threshold swing of 245 m V/dec, a large on/off current ratio of 7.6×10s, and a high electron effective mobility of 22.1cm2/V.s. Under negative gate bias stress at -10 V, the above devices show better electrical stabilities than those post-annealed at 300℃. Thus the fabricated high-performance ZnO TFT with a low thermal budget is very promising for flexible electronic applications.

Degradation and its fast recovery in a-IGZO thin-film transistors under negative gate bias stress

A new type of degradation phenomena featured with increased subthreshold swing and threshold voltage after negative gate bias stress(NBS)is observed for amorphous InGaZnO(a-IGZO)thin-film transistors(TFTs),which can recover in a short time.After comparing with the degradation phenomena under negative bias illumination stress(NBIS),positive bias stress(PBS),and positive bias illumination stress(PBIS),degradation mechanisms under NBS is proposed to be the generation of singly charged oxygen vacancies(V_(o)^(+))in addition to the commonly reported doubly charged oxygen vacancies(V_(o)^(2+)).Furthermore,the NBS degradation phenomena can only be observed when the transfer curves after NBS are measured from the negative gate bias to the positive gate bias direction due to the fast recovery of V_(o)^(+)under positive gate bias.The proposed degradation mechanisms are verified by TCAD simulation.

A systematic study of light dependency of persistentphotoconductivity in a-InGaZnO thin-film transistors

Persistent photoconductivity(PPC)effect and its light-intensity dependence of both enhancement and depletion(E-/D-)mode amorphous InGaZnO(a-IGZO)thin-film transistors(TFTs)are systematically investigated.Density of oxygen vacancy(V O)defects of E-mode TFTs is relatively small,in which formation of the photo-induced metastable defects is thermally activated,and the activation energy(E a)decreases continuously with increasing light-intensity.Density of V O defects of D-mode TFTs is much larger,in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated.Furthermore,for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects.Under low light-exposure below the threshold,only a low PPC barrier is formed and the PPC recovery is fast.With increasing the light-exposure to the threshold,the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies(V O^2+)is fully developed,and the PPC barrier increases to∼0.25 eV,which remains basically unchanged under higher light-exposure.Based on the density of V O defects in the channel and the condition of light illumination,a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations.

Water-based processed and alkoxide-based processed indium oxide thin-film transistors at different annealing temperatures

In this study,indium oxide（In2O3） thin-film transistors（TFTs） are fabricated by two kinds of low temperature solution-processed technologies（Ta ≤ 300℃）,i.e.,water-based（DIW-based） process and alkoxide-based（2-ME-based）process.The thickness values,crystallization properties,chemical structures,surface roughness values,and optical properties of In2O3 thin-films and the electrical characteristics of In2O3 TFTs are studied at different annealing temperatures.Thermal annealing at higher temperature leads to an increase in the saturation mobility（μsat） and a negative shift in the threshold voltage（VTH）.The DIW-based processed In2O3-TFT annealed at 300℃ exhibits excellent device performance,and one annealed at 200℃ exhibits an acceptable μsat of 0.86 cm^2/V·s comparable to that of a-Si：H TFTs,whereas the 2-ME-based TFT annealed at 300℃ exhibits an abundant μsat of 1.65 cm^2/Vs and one annealed at 200℃ is inactive.The results are attributed to the fact that the DIW-based process induces a higher degree of oxidation and less defect states than the 2-ME-based process at the same temperature.The DIW-based process for fabricating the In2O3 TFT opens the way for the development of nontoxic,low-cost,and low-temperature oxide electronics.

Performance enhancement of solution-processed InZnO thin-film transistors by Al doping and surface passivation

Solution-processed oxide semiconductors have been considered as a potential alternative to vacuum-based ones in printable electronics.However,despite spincoated InZnO(IZO)thin-film transistors(TFTs)have shown a relatively high mobil-ity,the lack of carrier suppressor and the high sensitivity to oxygen and water molecules in ambient air make them potentially suffer issues of poor stability.In this work,Al is used as the third cation doping element to study the effects on the electrical,optoelectronic,and physical properties of IZO TFTs.A hydrophobic self-assembled monolayer called octadecyltrimethoxysilane is introduced as the surface passivation layer,aiming to reduce the effects from air and understand the importance of top surface conditions in solution-processed,ultra-thin oxide TFTs.Owing to the reduced trap states within the film and at the top surface enabled by the doping and passivation,the optimized TFTs show an increased current on/off ratio,a reduced drain current hysteresis,and a significantly enhanced bias stress stability,compared with the untreated ones.By combining with high-capacitance AlO_(x),TFTs with a low operating voltage of 1.5 V,a current on/off ratio of>10^(4) and a mobility of 4.6 cm^(2)/(V·s)are demonstrated,suggesting the promising features for future low-cost,low-power electronics.

Degradation mechanisms for a-InGaZnO thin-film transistors functioning under simultaneous DC gate and drain biases

Degradation of a-InGaZnO thin-film transistors working under simultaneous DC gate and drain bias stress is investigated,and the corresponding degradation mechanism is proposed and verified.The maximum degradation occurs under the bias stress condition that makes the electric field and electron concentration relatively high at the same time.Trapping of hot electrons in the etching-stop layer under the extended drain electrode is proven to be the underlying mechanism.The observed degradation phenomena,including distortion in the transfer curve on a logarithmic scale and two-slope dependence on gate bias on a linear scale,current crowding in the output curve,and smaller degradation in transfer curves measured under large drain bias,can all be well explained with the proposed degradation mechanism.

Contact resistance asymmetry of amorphous indium–gallium–zinc–oxide thin-film transistors by scanning Kelvin probe microscopy

In this work, a method based on scanning Kelvin probe microscopy is proposed to separately extract source/drain（S/D） series resistance in operating amorphous indium–gallium–zinc–oxide（a-IGZO） thin-film transistors. The asymmetry behavior of S/D contact resistance is deduced and the underlying physics is discussed. The present results suggest that the asymmetry of S/D contact resistance is caused by the difference in bias conditions of the Schottky-like junction at the contact interface induced by the parasitic reaction between contact metal and a-IGZO. The overall contact resistance should be determined by both the bulk channel resistance of the contact region and the interface properties of the metalsemiconductor junction.

Lateral-Coupled Junctionless IZO-Based Electric-Double-Layer Thin-Film Transistors Gated by Solid-State Phosphorosilicate Glass Electrolyte

We describe the lateral-coupled junctionless indium-zinc-oxide （IZO） thin-film transistors （TFTs） in which there are no junctions between channel and source/drain electrodes and with solid-state phosphorosilieate glass electrolyte （PSG） gating. Due to the three-dimensional high proton conduction and lateral coupled electric-double- layer （EDL） capacitance （〉 1 #Flora2） of the PSG, the low voltage （2.0 V） junctionless IZO TFTs and the dual eoplanar gate devices are obtained. An AND logic function is demonstrated on the basis of the junctionless EDL-TFTs. Such devices are promising for applications in pH sensors, humidity sensors, biosensors, and neuron network simulation.

Frequency Performance of Ring Oscillators Based on a-IGZO Thin-Film Transistors

Ring oscillators based on indium gallium zinc oxide thin film transistors are fabricated on glass substrates. The oscillator circuit consists of seven delay stages and an output buffer inverter. The element inverter exhibits a voltage gain higher than -6 V/V and a wide output swing close to 85% of the full swing range. The dynamic performance of the ring oscillators is evaluated as a function of supply voltage and at different gate lengths. A maximum oscillation frequency of 0.88MHz is obtained for a supply voltage of 50V, corresponding to a propagation delay of less than 85 ns/stage.

Effects of active layer thickness on performance and stability of dual-active-layer amorphous InGaZnO thin-film transistors

Dual-active-layer(DAL)amorphous InGaZnO(IGZO)thin-film transistors(TFTs)are fabricated at low temperature without post-annealing.A bottom low-resistance(low-R)IGZO layer and a top high-resistance(high-R)IGZO layer constitute the DAL homojunction with smooth and high-quality interface by in situ modulation of oxygen composition.The performance of the DAL TFT is significantly improved when compared to that of a single-active-layer TFT.A detailed investigation was carried out regarding the effects of the thickness of both layers on the electrical properties and gate bias stress stabilities.It is found that the low-R layer improves the mobility,ON/OFF ratio,threshold voltage and hysteresis voltage by passivating the defects and providing a smooth interface.The high-R IGZO layer has a great impact on the hysteresis,which changes from clockwise to counterclockwise.The best TFT shows a mobility of 5.41 cm^2/V·s,a subthreshold swing of 95.0 mV/dec,an ON/OFF ratio of 6.70×10^7,a threshold voltage of 0.24 V,and a hysteresis voltage of 0.13 V.The value of threshold voltage shifts under positive gate bias stress decreases when increasing the thickness of both layers.

High-photosensitivity polymer thin-film transistors based on poly(3-hexylthiophene)

Polymer thin-film transistors （PTFTs） based on poly（3-hexylthiophene） are fabricated by the spin-coating process, and their photo-sensing characteristics are investigated under steady-state visible-light illumination. The photosensitivity of the device is strongly modulated by gate voltage under various illuminations. When the device is in the subthreshold operating mode, a significant increase in its drain current is observed with a maximum photosensitivity of 1.7×10^3 at an illumination intensity of 1200 lx, and even with a relatively high photosensitivity of 611 at a low illumination intensity of 100 lx. However, when the device is in the on-state operating mode, the photosensitivity is very low： only 1.88 at an illumination intensity of 1200 lx for a gate voltage of -20 V and a drain voltage of -20 V. The results indicate that the devices could be used as photo-detectors or sensors in the range of visible light. The modulation mechanism of the photosensitivity in the PTFT is discussed in detail.

Charge transport in monolayer poly(3-hexylthiophene) thin-film transistors

It is found that ultrathin poly（3-hexylthiophene） （P3HT） film with a 2.5 nm-thick layer exhibits a higher mobility of 5.0× 10-2 cm2/V-s than its bulk counterpart. The crystalline structure of the as-fabricated ultrathin P3HT layer is verified by atomic force microscopy as well as grazing incidence X-ray diffraction. Transient measurements of the as-fabricated transistors reveal the influence of the interface traps on charge transport. These results are explained by the trap energy level distribution at the interface manipulated by layers of polymer film.