Ultrathin Transistors Research Articles

Toward Ultrathin: Advances in Solution-Processed Organic Semiconductor Transistors.

In recent years, organic semiconductor (OSC) ultrathin films and their solution-processed organic field-effect transistors (OFETs) have garnered attention for their high flexibility, light weight, solution processability, and tunable optoelectronic properties. These features make them promising candidates for next-generation optoelectronic applications. An ultrathin film typically refers to a film thickness of less than 10 nm, i.e., several molecular layers, which poses challenges for OSC materials and solution-processed methods. In this paper, first we introduce the carrier-transport regulation mechanism under ultrathin limits. Second, we summarize various solution-processed techniques for OSC ultrathin films and elucidate advances in their OFETs performance, such as enhanced or maintained mobilities, improved switching ratios, reduced threshold voltages, and minimized contact resistance. The relationship between the ultrathin-film thickness, microstructure of various OSCs (small molecules and polymers), and device performance is discussed. Third, we explore the recent application of OSC ultrathin-film-based OFETs, such as gas sensors, biosensors, photodetectors, and ferroelectric OFETs (Fe-OFETs). Finally, the conclusion is drawn, and the challenges and prospects of ultrathin OSC transistors are presented. Nowadays, research on ultrathin films is still in its early stages; further experience in precise film deposition control is crucial to advancing research and broadening the scope of applications for OSC ultrathin devices.

Exploring the Impact of Channel Thickness Scaling on PBTI and Low-Frequency Noise in Ultrathin IGZO Transistors

Exploring the Impact of Channel Thickness Scaling on PBTI and Low-Frequency Noise in Ultrathin IGZO Transistors

Remarkable Bias‐Stress Stability of Ultrathin Atomic‐Layer‐Deposited Indium Oxide Thin‐Film Transistors Enabled by Plasma Fluorination

AbstractA low‐thermal‐budget fabrication approach is developed to realize high‐performance fluorine‐doped indium oxide (In2O3:F) thin‐film transistors (TFTs) with remarkable bias‐stress stability. The ultrathin transistor channel layer is prepared by a re‐developed atomic layer deposition (ALD) process of using cyclopentadienyl indium(I) (InCp) and O2 plasma to deposit a crystalline In2O3 film, followed by a new fluorine doping strategy to use CF4 plasma to afford the In2O3:F layer. As revealed by the density functional theory (DFT) analysis, the fluorine doping can stabilize the lattice oxygen and electrically passivate the problematic VO defects in In2O3 by forming the FOFi spectator defects. Therefore, the fabricated In2O3:F TFTs show simultaneously excellent electrical performance and remarkable bias‐stress stability, with high µFE of 35.9 cm2 V−1 s−1, positive Vth of 0.36 V, steep SS of 94.3 mV dec−1, small hysteresis of 33 mV, and small ΔVth of −111 and 49 mV under NBS and PBS, respectively. This work demonstrates the high promise of the fluorinated ALD In2O3:F TFTs for the CMOS back‐end‐of‐line (BEOL) compatible technologies toward advanced monolithic 3D integration.

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure.

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

BEOL Compatible Ultra-Thin ITO Transistor With Performance Recoverable Capability by in Situ Electrothermal Annealing

BEOL Compatible Ultra-Thin ITO Transistor With Performance Recoverable Capability by in Situ Electrothermal Annealing

Sub-5 nm Ultrathin In2O3 Transistors for High-Performance and Low-Power Electronic Applications.

Ultrathin oxide semiconductors are promising candidates for back-end-of-line (BEOL) compatible transistors and monolithic three-dimensional integration. Experimentally, ultrathin indium oxide (In2O3) field-effect transistors (FETs) with thicknesses down to 0.4 nm exhibit an extremely high drain current (104 μA/μm) and transconductance (4000 μS/μm). Here, we employ ab initio quantum transport simulation to investigate the performance limit of sub-5 nm gate length (Lg) ultrathin In2O3 FETs. Based on the International Technology Roadmap for Semiconductors (ITRS) criteria for high-performance (HP) devices, the scaling limit of ultrathin In2O3 FETs can reach 2 nm in terms of on-state current, delay time, and power dissipation. The wide bandgap nature of ultrathin In2O3 (3.0 eV) renders it a suitable candidate for ITRS low-power (LP) electronics with Lg down to 3 nm. Notably, both the HP and LP ultrathin In2O3 FETs exhibit superior energy-delay products as compared to those of other common 2D semiconductors such as monolayer MoS2 and MoTe2. These findings unveil the potential of ultrathin In2O3 in HP and LP nanoelectronic device applications.

Optimizing Ultrathin 2D Transistors for Monolithic 3D Integration: A Study on Directly Grown Nanocrystalline Interconnects and Buried Contacts.

The potential of monolithic 3D integration technology is largely dependent on the enhancement of interconnect characteristics which can lead to thinner stacks, better heat dissipation, and reduced signal delays. Carbon materials such as graphene, characterized by sp2 hybridized carbons, are promising candidates for future interconnects due to their exceptional electrical, thermal conductivity and resistance to electromigration. However, a significant challenge lies in achieving low contact resistance between extremely thin semiconductor channels and graphitic materials. To address this issue, an innovative wafer-scale synthesis approach is proposed that enables low contact resistance between dry-transferred 2D semiconductors and the as-grown nanocrystalline graphitic interconnects. A hybrid graphitic interconnect with metal doping reduces the sheet resistance by 84% compared to an equivalent thickness metal film. Furthermore, the introduction of a buried graphitic contact results in a contact resistance that is 17 times lower than that of bulk metal contacts (>40nm). Transistors with this optimal structure are used to successfully demonstrate a simple logic function. The thickness of active layer is maintained within sub-7nm range, encompassing both channels and contacts. The ultrathin transistor and interconnect stack developed here, characterized by a readily etchable interlayer and low parasitic resistance, leads to heterogeneous integration of future 3D integrated circuits (ICs).

Liquid-metal-based three-dimensional microelectrode arrays integrated with implantable ultrathin retinal prosthesis for vision restoration

Electronic retinal prostheses for stimulating retinal neurons are promising for vision restoration. However, the rigid electrodes of conventional retinal implants can inflict damage on the soft retina tissue. They also have limited selectivity due to their poor proximity to target cells in the degenerative retina. Here we present a soft artificial retina (thickness, 10 μm) where flexible ultrathin photosensitive transistors are integrated with three-dimensional stimulation electrodes of eutectic gallium–indium alloy. Platinum nanoclusters locally coated only on the tip of these three-dimensional liquid-metal electrodes show advantages in reducing the impedance of the stimulation electrodes. These microelectrodes can enhance the proximity to the target retinal ganglion cells and provide effective charge injections (72.84 mC cm−2) to elicit neural responses in the retina. Their low Young’s modulus (234 kPa), owing to their liquid form, can minimize damage to the retina. Furthermore, we used an unsupervised machine learning approach to effectively identify the evoked spikes to grade neural activities within the retinal ganglion cells. Results from in vivo experiments on a retinal degeneration mouse model reveal that the spatiotemporal distribution of neural responses on their retina can be mapped under selective localized illumination areas of light, suggesting the restoration of their vision.

Wide-range and area-selective threshold voltage tunability in ultrathin indium oxide transistors

The scaling of transistors with thinner channel thicknesses has led to a surge in research on two-dimensional (2D) and quasi-2D semiconductors. However, modulating the threshold voltage (VT) in ultrathin transistors is challenging, as traditional doping methods are not readily applicable. In this work, we introduce a optical-thermal method, combining ultraviolet (UV) illumination and oxygen annealing, to achieve broad-range VT tunability in ultrathin In2O3. This method can achieve both positive and negative VT tuning and is reversible. The modulation of sheet carrier density, which corresponds to VT shift, is comparable to that obtained using other doping and capacitive charging techniques in other ultrathin transistors, including 2D semiconductors. With the controllability of VT, we successfully demonstrate the realization of depletion-load inverter and multi-state logic devices, as well as wafer-scale VT modulation via an automated laser system, showcasing its potential for low-power circuit design and non-von Neumann computing applications.

Performance optimization of nanometer-thin indium-tin oxide transistors by two-step air annealing

In this study, the influence of air annealing on nanometer-thin indium tin oxide (ITO) transistors was investigated. Compared with unannealed devices, the field-effect mobility and subthreshold swing of ultrathin ITO transistors with pre-annealing, post-annealing, and two-step annealing were greatly improved. Under a pre-annealing temperature at 200 °C and post-annealing temperature at 100 °C, the ultra-thin ITO transistors showed field-effect mobility of 34.2 cm2/Vs, steep subthreshold swing of 1.95 V/decade, and high Ion/Ioff ratio of 2.46 × 107. X-ray photoelectron spectroscopy revealed that the performance changes could be ascribed to the changes in the oxygen vacancies in the ITO channel under different annealing conditions. The dependence of subthreshold swing on the annealing process may be related to the absorption and release of water molecules from the ultrathin ITO surface. Overall, these results emphasize the importance of the annealing procedures for nanometer-thin ITO transistors.

Band gap engineering and optoelectronic properties of all-inorganic Ruddlesden-Popper halide perovskites Cs2B(X1-uYu)4 (B = Pb, Sn; X/Y = Cl, Br, I)

In this article two-dimensional Ruddlesden-Popper (2DRP) halide perovskites Cs2B(X1-uYu)4 where B = Pb, Sn; X/Y = Cl, Br, I; and u = 0, 0.25, 0.5, 0.75, 1 are investigated for potential applications in optoelectronic, photoemission and photovoltaic devices. In these compounds the corner sharing B-centered [BX2Y4]4- units build the 2D [BX2Y2]n2n− plane, where the halides occupy the in-plane bridging and out of plane apical site respectively, while the spacer cation (Cs+) balancing the charge and separate the octahedral layers. These compounds have direct band gaps and spin orbit (SOC) calculations split Pb/Sn p-orbital and reduce the band gaps. The wide (1.04–3.45) and direct band gaps enable these materials for futuristic applications in optoelectronics, high speed ultrathin transistors and photodetectors. The anti-bonding lone-pair Pb/Sn-s orbital at the valence band edge is excellent for light absorption and photovoltaic performance. These compounds are good dielectric and optically functional materials for photovoltaic and energy storage devices due to their small values of charge carrier effective masses and large exciton binding energies. Therefore, the optical coefficients like dielectric function (real and imaginary), absorption coefficient and index of refraction are calculated.

Structural and optoelectronic properties of Ge- and Si -based inorganic two dimensional Ruddlesden Popper halide perovskites

In the present work silicon (Si) and germanium (Ge) based two dimensional Ruddlesden-Popper halide perovskites (2DRP-HPs) Cs 2 BX 2 Y 2 (B = Ge/Si and X/Y= Cl/Cl, Br/Cl, I/Cl, Br/Br, I/Br and I/I) are studied for photovoltaic, visible emitter and other potential optoelectronic applications. The tolerance factor values of these compounds are greater than one and hence exist in the stable K 2 NiF 4 -type tetragonal structure. The direct band gaps (1.39-2.59 eV) make these compounds important for futuristic applications in optoelectronic devices operational in the visible region of the electromagnetic spectrum, photodetectors and high speed ultrathin transistors. The spin orbit coupling (SOC) effect increases from Cl to I and splits the p-orbital of halogen in the conduction band, due to which the band gap reduces. The small exciton binding energies, effective masses of charge carrier, enable these materials for energy storage and photovoltaic applications. At visible and ultraviolet energy range, these RP-HPs have excellent response to the incident photons, which confirms their commercialization in optoelectronic devices and photovoltaic cells.

Research progress in skin-like ultraflexible organic field-effect transistors

<p indent="0mm">Field-effect transistor is one of the most important basic components in modern microelectronic industry. Organic field-effect transistors (OFETs) possess the outstanding advantages in flexibility, light weight, solution processability and good biocompatibility. They have presented great application potential in the field of skin-like electronics, such as software robots, implant devices and imperceptible wearable devices. Skin-like ultraflexible OFETs can be imperceptibly conformable to the three-dimensional static/dynamic surfaces such as skin and prosthesis, maintain the stable electrical performance under deformation, and give the wearer the minimized burden and maximized comfort. Decreasing the thickness and Young’s modulus will effectively reduce the resilience of the device due to deformation, which is the main strategy to obtain the skin-like ultraflexible devices with three-dimensional conformability. In this paper, two types of skin-like flexible OFETs, including ultra-thin transistors and stretchable transistors, are summarized. The research process of the skin-like ultraflexible OFETs is discussed.

Performance Limit of Ultrathin GaAs Transistors.

High-electron-mobility group III-V compounds have been regarded as a promising successor to silicon in next-generation field-effect transistors (FETs). Gallium arsenide (GaAs) is an outstanding member of the III-V family due to its advantage of both good n- and p-type device performance. Monolayer (ML) GaAs is the limit form of ultrathin GaAs. Here, a hydrogenated ML GaAs (GaAsH2) FET is simulated by ab initio quantum-transport methods. The n- and p-type ML GaAsH2 metal-oxide-semiconductor FETs (MOSFETs) can well satisfy the on-state current, delay time, power dissipation, and energy-delay product requirements of the International Technology Roadmap for Semiconductors until the gate length is scaled down to 3/4 and 3/5 nm for the high-performance/low-power applications, respectively. Therefore, ultrathin GaAs is a prominent channel candidate for devices in the post-Moore era. The p-type ML GaAsH2 MOSFETs with a 2% uniaxially compressive strain and the unstrained n-type counterparts have symmetrical performance for the high-performance application, making ultrathin GaAs applicable for complementary MOS integrated circuits.

Atomically thin In2O3 field-effect transistors with 1017 current on/off ratio

Recent advances have enabled the demonstration of record high-performance atomically thin n-type indium oxide (In2O3) field-effect transistors with low thermal budget suitable for back-end-of-line logic or memory applications. By using ultra-thin layers of In2O3, its degenerate carrier density is suppressed so that it can be modulated by conventional dielectric gating. These devices have high on-currents due to its high mobility and low contact resistance; meanwhile they can have exceptionally low off-currents due to its wide bandgap. For both low-power logic and memory, the off-state performance should be understood in more detail, although they are constrained by the limits of traditional measurement techniques. In this Letter, we systematically probe the off-current of ultra-thin In2O3 transistors by adopting a wide channel geometry with temperature-dependent electrical measurements and demonstrate the potential for record high current on/off ratios of 1017 on In2O3 transistors.

Affinity driven ion exchange EG-OFET sensor for high selectivity and low limit of detection of cesium in seawater

The detection and quantification of Cs+ in aquatic media are an environmental safety and public health matter, so far limited by the lack of rapid, low cost, low limit of detection and selective analytical tools. Herein we demonstrate the efficient fabrication of a novel electrolyte-gated organic field-effect transistor sensor for the Cs+ detection in seawater based on the combination of two ultra-thin layers namely poly(3-hexylthiophene) as semiconductor and a single lipids monolayer as dielectric. The latter is end-capped with a specific Cs+ probe based on a calix[4]arene benzocrown ether to ensure the selectivity. Interestingly, we clearly evidence that by controlling affinity driven guest/host ion exchange, one can lift the general problem of selectivity encountered in all FET-based ion sensors, reaching a nearly perfect selectivity even in highly complex analyte solutions containing competitive ions, such as phosphate buffered saline solution or seawater. Such ultra-thin transistor structure exhibits, a limit of detection at the sub-femtomolar level which corresponds to a 5 folds’ magnitude lower than Inductively Coupled Plasma Mass Spectroscopy, the most commonly used technic today. These results pave the way to a generalized monitoring of Cs+ in complexed analytes.

Multi-functional logic circuits composed of ultra-thin electrolyte-gated transistors with wafer-scale integration

Wafer-scale integration of electrolyte gated transistors is demonstrated by using iCVD. A solid-state pEGDMA was used as a gate electrolyte, and it configures multi-functional logic circuits, such as inverter, NAND, and NOR with high performance.

Integrated contact lens sensor system based on multifunctional ultrathin MoS2 transistors

Smart contact lenses attract extensive interests due to their capability of directly monitoring physiological and ambient information. However, previous demonstrations usually lacked efficient sensor modalities, facile fabrication process, mechanical stability, or biocompatibility. Here, we demonstrate a flexible approach for fabrication of multifunctional smart contact lenses with an ultrathin MoS2 transistors-based serpentine mesh sensor system. The integrated sensor systems contain a photodetector for receiving optical information, a glucose sensor for monitoring glucose level directly from tear fluid, and a temperature sensor for diagnosing potential corneal disease. Unlike traditional sensors and circuit chips sandwiched in the lens substrate, this serpentine mesh sensor system can be directly mounted onto the lenses and maintain direct contact with tears, delivering high detection sensitivity, while being mechanically robust and not interfering with either blinking or vision. Furthermore, the in vitro cytotoxicity tests reveal good biocompatibility, thus holding promise as next-generation soft electronics for healthcare and medical applications.

Effect of Contact Resistance on the High-Field Characteristics of MoS2 Transistors

Layered transition metal dichalcogenides (TMDs) display a wide range of unique electronic properties and are potentially important for electronic device applications. Much interest in these materials led to in-depth research on the device properties of TMDs such as MoS2. Unlike graphene, this material has a relatively large band gap (1.3–1.9 eV) and exhibits stable on/off switching as a transistor. TMD materials represent ideal channel materials for device scalability as their few-atom-thick layers devoid of dangling bonds will be robust against short-channel effects for ultrathin channeled transistors. As MoS2 devices are scaled down, robust high-field operation is essential for the development of reliable electronic systems. In this work, we explore the high-field characteristics of MoS2-based transistors and investigate their relationship to the contact resistance. Importantly, we show that high-voltage characteristics, current drives, and breakdown voltages of MoS2 transistors can be significantly improved through the use of a metal-insulator-semiconductor contact at the metal/MoS2 interface.

Probing the Effect of Chemical Dopant Phase on Photoluminescence of Monolayer MoS2 Using in Situ Raman Microspectroscopy

Understanding the role of chemical dopants is crucial to modulating the optoelectronic properties of monolayer (ML) MoS2 and realizing its optoelectronic applications such as photodetectors, switching devices, and ultrathin transistors. Here, the effect of the dopant phase of the same dopants—liquid and gaseous—on the optical properties of ML MoS2 has been investigated using in situ Raman microspectroscopy with an environment-controlled reaction cell. The results show that the gaseous n-type dopant, that is, pyridine, completely quenched the PL intensity of ML MoS2, while liquid pyridine preserved 50% of the original PL intensity attributed to its less effective charge transfer to MoS2 than the gaseous counterpart. Photoemission of the quenched MoS2 could not be recovered with mild annealing in nitrogen and only partially recovered upon long-term exposure to air. The results show that the dopant phase strongly impacts the PL properties of ML MoS2.