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We report the comparison of the atomic layer deposited polycrystalline (p-)/amorphous (a-) HfO2 bilayer gate dielectric with p-HfO2/a-Al2O3 for enhanced $\beta $ -Ga2O3 metal oxide semiconductor capacitors (MOSCAPs). A discrepancy in the temperature-dependent hysteretic behaviors is observed, and thus, pulse capacitance–voltage ( ${C}$ – ${V}$ ) measurements are employed to investigate interface properties at the border between the amorphous (Al2O3 or HfO2) and polycrystalline (HfO2) oxide layers. The proposed p-HfO2 /a-HfO2 exhibits similar interface trap densities ( ${D}_{\text {it}}$ ) at the interface between the bilayer dielectric and $\beta \text {-Ga}_{{2}}\text {O}_{{3}}$ , and shows lower border trap density ( ${N}_{\text {bt}}$ ). An effective barrier height of 1.62 eV for the p-HfO2 /a-HfO2 with a breakdown field of 9.1 MV/cm, which is higher than 1.01 eV of p-HfO2 /a-Al2O3, is obtained from the current density-voltage ( ${J}$ – ${V}$ ) characteristics using the Folwer–Nordheim model. Moreover, as compared with a p-HfO2 single layer, the additional a-HfO2 layer on top of p-HfO2 is beneficial to improve hysteretic and leakage characteristics, while maintaining its high-quality interface. The p-HfO2/a-HfO2 bilayer with improved interface properties as well as a large positive flat-band voltage of 4.7 V is a promising candidate for the metal/high- ${k}$ stack of $\beta $ -Ga2O3 MOSCAPs.

Hydrogen fuel has attracted significant attention as an alternative clean energy for hydrogen vehicles and household fuel systems and has been expected to be pervasive. Thus, compact, low-power, and highly sensitive hydrogen sensors capable of being embedded in Internet-of-Things devices are in demand. In this work, an ultrahigh sensitive hydrogen sensor operating at room temperature based on the Pt-decorated graphene (Pt/Gr) gate AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) is presented. The proposed unique gate structure of island-like Pt on graphene not only modulates the density of 2-D electron gas (2DEG) but also provides efficient diffusion of hydrogen ions into the gate even at very low concentrations ( $\sim 1.6\times 10^{7}$ in a hydrogen concentration of 1000 ppm at room temperature. Upon exposure of at most 1 ppm hydrogen gas, the current increases significantly by a factor of 15.4, exhibiting an unprecedentedly high hydrogen response. Furthermore, the proposed AlGaN HEMT architecture has an outstanding hydrogen selectivity and insensitivity to NH3, H2S, and CO gases at the same time.

Deep-ultraviolet (DUV) photodetectors based on wide-band-gap semiconductors have attracted significant interest across a wide range of applications in the industrial, biological, environmental, and...

Germanium (Ge) has gained great attention not only for future nanoelectronics but for back-end of line (BEOL) compatible monolithic three-dimensional (M3D) integration recently. For high performance and low power devices, various high-k oxide/Ge gate stacks including ferroelectric oxides have been investigated. Here, we demonstrate atomic layer deposited (ALD) polycrystalline (p-) HfO2/GeOX/Ge stack with an amorphous (a-) HfO2 capping layer. The consecutively deposited a-HfO2 capping layer improves hysteretic behaviors ( ${\Delta } {V}$ ) and interface state density ( $\text{D}_{\mathrm{ it}}$ ) of the p-HfO2/GeOX/Ge stack. Furthermore, leakage current density ( ${J}$ ) is significantly reduced $(\times 100)$ by passivating leakage paths through grain boundaries of p-HfO2. The proposed HfO2 layer with the graded crystallinity suggests possible high-k/Ge stacks for further optimized Ge MOS structures.

Highly sensitive and system integrable gas sensors play a significant role in industry and daily life, and MoS2 has emerged as one of the most promising two-dimensional nanomaterials for gas sensor technology. In this study, we demonstrate a scalable and monolithically integrated active-matrix gas sensor array based on large-area bilayer MoS2 films synthesized via two-successive steps: radio-frequency magnetron sputtering and thermal sulfurization. The fabricated thin-film transistors exhibit consistent electrical performance over a few centimeters area and resulting gas sensors detect NO2 with ultra-high sensitivity across a wide detection range, from 1 to 256 ppm. This is due to the abundant grain boundaries of the sputtered MoS2 channel, which perform as active sites for absorption of NO2 gas molecules. The demonstrated active-matrix gas sensor arrays display good switching capabilities and are anticipated to be readily integrated with additional circuitry for different gas sensing and monitoring applications. Sensitive and scalable gas sensors are essential in daily life air-quality monitoring. Here, a monolithically integrated gas sensing circuit based on two-step-grown polycrystalline MoS2 films is fabricated, showing good switching and NO2 gas sensing response in a wide detection range of 1 to 256 ppm.

A Sentaurus TCAD 2-D model of β-Ga2O3 metal-oxide semiconductor field-effect transistors (MOSFETs) with a polycrystalline HfO2 gate-oxide deposited using atomic layer deposition (ALD), which has a semiconductor-on-insulator (SOI) structure, is developed. The results of model shows good agreement with the DC and the AC characteristics of the fabricated device by incorporating proper parameters for the materials, as well as the device models. We also investigate and compare electrical performance of the devices with modified HfO2 gate-oxide geometries. With a reduced HfO2 coverage over the channel, the transconductance (gm) is enhanced, the threshold voltage (Vth) shifts toward a positive voltage, both of which are advantageous for device applications. Moreover, radiation effects during transient operation of the β-Ga2O3 MOSFETs are evaluated and compared for the fabricated and the modified oxide geometries by incorporating carrier generation models with heavy-ions and alpha particles.

A transparent light emitting diode (LED) panel with a 16 × 16 multicolor LED array for a signage application is proposed and studied based on experimental and simulation results. As a transparent electrode, oxide/metal/oxide (OMO) trilayers with various Al interlayer thicknesses were fabricated and characterized using an established a simulation model. Using commercial ray tracing optical and SPICE simulation tools, we designed the LED panel and investigated in its optical and electrical properties. Moreover, in order to resolve the waveguide effect and voltage drop issue across the panel, we integrated additional optical structures and various OMO width into the design. The results show that the OMO electrode interconnects and proposed design considerations are pivotal aspects toward achieving transparent LED digital signage applications.

Ge/MoS 2 van der Waals heterostructure enables bias-dependent selective detection of visible and near infrared.

We fabricate top-gate β-Ga₂O₃ nanomembrane metal-semiconductor field-effect transistor (MESFET) using a mechanical exfoliation method, and investigate its electrical performance. The Schottky contact between top-gate metal and β-Ga₂O₃ (100) channel is evaluated by characterizing properties of Schottky barrier diode, exhibiting an on/off ratio of ~106, an ideality factor of 2.8 and a turn-on voltage of 1.1 V. The proposed top-gate β-Ga₂O₃ nanomembrane MESFET exhibits maximum transconductance of ~0.23 mS/mm, field-effect mobility of 1.2 cm²/V·s at VDS = 1 V and subthreshold slope (SS) of 180 mV/dec with high on/off ratio of >107. These results suggest that β-Ga₂O₃ nanomembrane MESFET could be a promising component toward β-Ga₂O₃-based high power device applications.

We demonstrate low subthreshold swing (SS) double-gate (DG) $\beta $ -Ga2O3 field-effect transistors (FETs) with polycrystalline hafnium oxide (HfO2) gate dielectrics. The atomic layer deposited HfO2 layer at an elevated temperature of 350 °C without post-deposition annealing has a polycrystalline structure. The threshold voltage is modulated from depletion- to enhancement-mode by independently controlled bottom-gate bias. The SS is reduced to as low as 64 mV/dec with mobility of 20 cm2/ $\text{V}\cdot \text{s}$ while maintaining a high ON/OFF ratio of $10^{{7}}$ and negligible hysteresis of 30 mV via double-gate operation. The device shows an ON/OFF ratio of $10^{{6}}$ and the SS of 320 mV/dec up to an operating temperature of 250 °C and the OFF-state breakdown voltage of 355 V. The demonstrated DG $\beta $ -Ga2O3 FETs with the polycrystalline HfO2 layer is a promising structure for energy efficient high temperature device and circuit applications.

Beta-gallium oxide (β-Ga2O3) is an emerging ultra-wide bandgap semiconductor material for high-power devices. However, one of the major drawbacks is the low thermal conductivity resulting in poor heat dissipation, and the so-called self-heating effect reduces carrier mobility and drain current degradation, and even causes a device reliability issue. Here, we propose a bottom-gate β-Ga2O3 field-effect transistor with a hexagonal boron-nitride (h-BN) gate-insulator and investigate the self-heating effect in comparison with an aluminum oxide (Al2O3) insulator using physics-based TCAD simulations. The h-BN with high thermal conductivity reduces the lattice temperature of the β-Ga2O3 channel and decreases drain current degradation. Furthermore, as the thickness of the insulator decreases below 50 nm and the channel length is scaled down to 5 μm, the reduced self-heating effect becomes more prominent. The results imply that the highly thermal-conductive h-BN insulator is promising for achieving high performance β-Ga2O3 metal insulator semiconductor field-effect transistor (MISFET) with the bottom-gate configuration.

Here, we report on heteroepitaxial α-Ga2O3 MOSFETs with a breakdown voltage (BV) of 2.3 kV at a specific on-resistance of 335 mΩ cm2. High-quality α-Ga2O3 layers were grown on a sapphire substrate via halide vapor-phase epitaxy (HVPE). A stack of Ti/Al/Ni/Au was used for the S/D electrode, exhibiting significantly reduced contact resistance as compared with a conventional Ti/Au stack. Consequently, the record BV and mobility of 20.4 cm2 V−1 s−1 were achieved. Moreover, a consistent critical field of 1 MV cm−1 was obtained for variable LGD. Our results are superior to recently reported heteroepitaxial α-/β-Ga2O3 MOSFETs, which is promising toward HVPE α-Ga2O3 based power devices.

Interface traps between a gate insulator and beta-gallium oxide (β-Ga2O3) channel are extensively studied because of the interface trap charge-induced instability and hysteresis. In this work, their effects on mobility degradation at low temperature and hysteresis at high temperature are investigated by characterizing electrical properties of the device in a temperature range of 20–300 K. As acceptor-like traps at the interface are frozen below 230 K, the hysteresis becomes negligible but simultaneously the channel mobility significantly degrades because the inactive neutral traps allow additional collisions of electrons at the interface. This is confirmed by the fact that a gate bias adversely affects the channel mobility. An activation energy of such traps is estimated as 170 meV. The activated trap charges’ trapping and de-trapping processes in response to the gate pulse bias reveal that the time constants for the slow and fast processes decrease due to additionally activated traps as the temperature increases.

MoS2/p-Ge/n-Ge bipolar heterojunction phototransistor is fabricated, which can detect from VIS to NIR. The responsivities are 35.21 and 133.56 A/W at 466 and 1550 nm, respectively. Photocurrent amplification of BHP, 6 times larger than photocurrent of p-Ge/n-Ge and p-Ge/MoS2 photodiodes.

As the technology node shrinks and shifts towards complex architectures, accurate control of automated semiconductor manufacturing processes, particularly plasma etching, is crucial in yield, cost, and semiconductor performance. However, current endpoint detection (EPD) methods relying on the experience of skilled engineers result in process variations and even errors. This paper proposes an enhanced optimal EPD in the plasma etching process based on a convolutional neural network (CNN). The proposed approach performs feature extraction on the spectral data obtained by optical emission spectroscopy (OES) and successfully predicts optimal EPD time. For the purpose of comparison, the support vector machine (SVM) classifier and the Adaboost Ensemble classifier are also investigated, the CNN-based model demonstrates better performance than the two models.

We report on the effect of Al2O3 surface passivation on electrical properties of beta-gallium oxide (β-Ga2O3) nanomembrane field-effect transistor (FET). The fabricated bottom-gate β -Ga2O3(100) FET exhibits enhanced channel conductance and reduced hysteresis after the conformal atomic layer deposited Al2O3 passivation investigated by high-resolution transmission electron microscope (HR-TEM) analysis. Moreover, abnormal positive threshold voltage (VTH) shifts under negative bias stress are turned into negative VTH shifts, and off-state breakdown characteristics is improved as well. A modeling work using physics-based TCAD shows reduced surface depletion effect after the surface passivation. The results demonstrate that high-quality ALD-Al2O3 surface passivation is an effective method to improve electrical properties of the bottom-gate β-Ga2O3 FET and its device applications.

Solution-processed high-k oxide layer, which is typically deposited using atomic layer deposition (ALD), has been proposed and recently demonstrated on molybdenum disulfide (MoS2) field-effect transistors (FETs). In this report, we statistically investigate electrical performance of multilayer MoS2 FETs fabricated on sol-gel AlOX gate-dielectric. More than 10 sample devices with different MoS2 thickness are characterized and compared. For electrical parameters extraction, Y -function method is adopted in order to minimize S/D electrode contact-induced variations. In spite of the relatively rougher surface of the sol-gel AlOX film, no significant difference of electrical performance is observed. The sol-gel prepared AlOX can be considered as a promising high-k gate dielectric for high-performance large-area transistion metal dichalcogenides (TMDs) devices fabrication.

So far many of research on transition metal dichalcogenides (TMDCs) are based on a bottomgate device structure due to difficulty with depositing a dielectric film on top of TMDs channel layer. In this work, we study different effects of various passivation layers on electrical properties of multilayer MoS2 transistors: spin-coated CYTOP, SU-8, and thermal evaporated MoOX. The SU-8 passivation layer alters device performance least significantly, and MoOX induces positive threshold voltage shift of ~8.0 V due to charge depletion at the interface, and the device with CYTOP layer exhibits decreased field-effect mobility by ~50% due to electric dipole field effect of C-F bonds in the end groups. Our results imply that electrical properties of the multilayer MoS2 transistors can be modulated using a passivation layer, and therefore a proper passivation layer should be considered for MoS2 device structures.

In this paper, we report on the electrical and thermal instability of β-Ga2O3 nanomembrane field-effect transistor with a bottom-gate configuration. The fabricated device exhibits high electrical performance of field-effect mobility of up to 60.9 cm2/V·s, on/off-current ratio of 109 and subthreshold slope of 210 mV/dec. However, we observe abnormal positive threshold voltage (VTH) shifts under negative bias-temperature stress at an elevated operating temperature of 80 °C as well as under temperature-dependent transfer characteristics up to 200 °C. This abnormal instability is significantly influenced by the surface depletion effect, and is discussed using energy band diagram. The opposite VTH shift was achieved by applying atomic-layer deposited Al2O3 passivation layer.

Molybdenum disulfide (MoS2) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS2 FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS2 surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS2 FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

We report a layer thickness-tunable direct synthesis growth method for bi- to few-layer crystalline molybdenum disulfide (MoS2) thin layers. For the first time, a facile, cost effective, and mass-scalable direct synthesis approach, based on a chemical bath deposition, is designed for quantum dot(QD)-based MoS2 layers using (NH4)6Mo7O24 and thiourea (CH4N2S) as precursors. Using this process, the uniformity of large area thin layer can be retained, and the applicability to various substrates can provide great opportunities in the fabrication of various atomically thin layered structures. The structural and optical properties of the MoS2 QD layers are systematically investigated. Raman, AFM and TEM analyses confirm the formation of continuous and crystalline bi-, tri- and few-layered MoS2. Their electrical properties are evaluated by bottom-gate FETs, and a field-effect mobility value of ~1.06 cm2 V−1 s−1 and a current on/off ratio in the order of ~ 105 are obtained. Particularly, MoS2 prepared as a thin film consisting QD structures of grains shows novel electrocatalytic property. MoS2 QDs on Au/Si are proven to be excellent electrocatalysts for hydrogen evolution reaction, featured by Tafel slope (94 mV decade−1), exchange current density (1.91×10-1 mA cm−2) and long-term durability for 20 h. Our approach opens new avenues for the design and synthesis of functional MoS2 layers for energy harvesting.

We fabricate top-gate

We propose nano-crystalline diamond (NCD) as a heteroepitaxial substrate for beta-gallium oxide (β-Ga2O3), and investigate self-heating effect in β-Ga2O3 MOSFET on the NCD compared with a native Ga2O3 and other alternative substrate (SiC) using physics-based TCAD simulation. The NCD substrate with high thermal conductivity reduces a lattice temperature of β-Ga2O3 and thus mitigates drain current degradation. Furthermore, the benefits become more pronounced with device scaling. These results suggest that the low-cost NCD can be a promising heteroepitaxial substrate for β-Ga2O3 devices applications.

In this work, we demonstrate the lateral metal–oxidesemiconductor field-effect transistors (MOSFETs) with β-Ga2O3 film (thickness = 150 nm) grown on a c-plane sapphire substrate by a hydride vapor phase epitaxy (HVPE) method using a CF4-based plasma treatment. By incorporating fluorine (F) the electrical characteristics of the surface and bulk regions of the resistive β-Ga2O3 film could be significantly improved. The analyses by secondary ion mass spectrometry and x-ray photoemission spectroscopy confirmed the formation of the substitutional fluorine, FO, with its peak concentration of 2.3 × 1018 cm−3 at 5 ~ 10 nm deep from the surface and about 1015 ~ 1016 cm−3 in the bulk region. While the argon-plasma treatment was insufficient for making a Ti/Au/Ti metal contact ohmic, the same contact formed on the CF4-treated surface showed an ohmic behavior with specific resistivity of 2.0 × 10−3 Ω·cm2 and contact resistance of 0.8 Ω·m without any post thermal annealing. With the improvement of the electrical properties in both the surface and bulk regions, the HVEP-grown hetero-epitaxial β-Ga2O3 MOSFET exhibited the field-effect mobility of up to 5.3 cm2/V⋅s and on/off ratio of 106.

Various approaches have been proposed for point-of-care diagnostics, and in particular, optical detection is preferred because it is relatively simple and fast. At the same time, field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection. In this work, we present highly sensitive, epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors, read-out circuits, and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA). Regardless of the physical forms or mechanical stress conditions, our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg·mL–1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM) and tapping-mode atomic force microscopy (tm-AFM). Furthermore, current modulation induced by the binding process was stably maintained for longer than 2–3 min. The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.

P-type semiconductors are indispensable for achieving complementary metal oxide semiconductor and integrated circuits based on two-dimensional (2D) semiconductors, and tungsten diselenide (WSe2) and molybdenum ditelluride (MoTe2) are the promising channel materials for PMOS. In this work, we report on the charge trapping effects on hysteretic behavior and field-effect mobility (μ FE) of the p-type WSe2 and MoTe2 FETs using fast pulsed current–voltage (I–V) measurements. The hysteresis is reduced by nearly 98% via ramped pulsed measurements, and μ FE is significantly enhanced via single pulse measurements by minimizing the charge trapping. Moreover, WSe2 FETs are found to be more susceptible to the charge trapping effects compared with MoTe2 FETs; WSe2 FETs exhibit more pronounced enhancement of μ FE and reduction of hysteresis. The intrinsic electrical characteristics of p-type 2D FETs under minimized charge trapping conditions can be investigated using the pulsed I–V characterizations.