Channel Layer Research Articles

Enhanced Electrical Performance and Stability of La‐Doped Indium Oxide‐Based Thin‐Film Transistors and Application Explorations

Metal‐oxide‐based thin‐film transistors (TFTs) fabricated by aqueous solution method are attracting more attention due to their potential applications in large‐scale display devices. Herein, solution‐processed TFTs with InLaO as channel layer are prepared as a function of lanthanum (La) doping molar ratio (La/In). Compared with In2O3 TFT, InLaO‐0.5 TFT demonstrates improved electrical performance, including a large μFE of 21.35 cm2 V−1s−1, a higher Ion/Ioff of ≈108, a smaller interfacial trap states (Dit) of 5.01 × 1011 cm−2, and a smaller subthreshold slope of 0.076 V decade−1. Low‐frequency noise tests are also carried out and conclude that the interfacial trap density of In2O3 TFTs can be modulated by La doping. Meanwhile, InLaO‐0.5 TFT also represents excellent bias stress stability, including threshold voltage shift (ΔVTH) of 0.13 and −0.03 V under positive and negative bias stress for 3600 s, respectively. Finally, a low‐voltage resistor‐loaded inverter has been built based on InLaO‐0.5 TFT, exhibiting full‐swing characteristics and a maximum gain of 10.2. All experimental results demonstrate the potential application of aqueous solution technology for future low‐cost, energy‐efficient, large‐scale, and high‐performance electronics.

Influence of back barrier layer thickness on device performance of AlGaN/GaN MOS-HEMT

In this work, we have performed the influence of back barrier layer thickness variation on AlGaN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOS-HEMT) device with 0.5 µm Schottky gate length. The AlGaN back barrier layer presented increases the conduction band with respect to GaN channel layer so that more no of electron confinement into the GaN channel layer and improve the high-frequency performance. The effect of the back-barrier layer thickness is performed by using 2-D TCAD Atlas Silvaco numerical simulation tool by taking Hydrodynamic mobility model. Due to a large amount of two-dimensional electron gas (2-DEG) density at the AlGaN/GaN heterointerface of the MOS-HEMT device higher drain current density is obtained. The 2-D simulation is carried out with a variation of back barrier layer thickness for various device parameter such as transfer characteristics (Id-Vg), drain current with a drain voltage (Id-Vd), transconductance (gm), drain induced barrier lowering (DIBL), conduction band energy and electron concentration into the channel. In this simulation, we have also performed the RF performance like a gate to source capacitance (Cgs) and current gain cut-off frequency of AlGaN/GaN MOS-HEMT device. The results obtained by variation of AlGaN back barrier layer thickness can be a better solution in future analog and RF device application. Copyright © 2018 VBRI Press.

Synaptic Transistors Exhibiting Gate-Pulse-Driven, Metal-Semiconductor Transition of Conduction.

Neuromorphic devices have been investigated extensively for technological breakthroughs that could eventually replace conventional semiconductor devices. In contrast to other neuromorphic devices, the device proposed in this paper utilizes deep trap interfaces between the channel layer and the charge-inducing dielectrics (CID). The device was fabricated using in-situ atomic layer deposition (ALD) for the sequential deposition of the CID and oxide semiconductors. Upon the application of a gate bias pulse, an abrupt change in conducting states was observed in the device from the semiconductor to the metal. Additionally, numerous intermediate states could be implemented based on the number of cycles. Furthermore, each state persisted for 10,000 s after the gate pulses were removed, demonstrating excellent synaptic properties of the long-term memory. Moreover, the variation of drain current with cycle number demonstrates the device’s excellent linearity and symmetry for excitatory and inhibitory behaviors when prepared on a glass substrate intended for transparent devices. The results, therefore, suggest that such unique synaptic devices with extremely stable and superior properties could replace conventional semiconducting devices in the future.

Record-Low Thermal Boundary Resistance between Diamond and GaN-on-SiC for Enabling Radiofrequency Device Cooling.

The implementation of 5G-and-beyond networks requires faster, high-performance, and power-efficient semiconductor devices, which are only possible with materials that can support higher frequencies. Gallium nitride (GaN) power amplifiers are essential for 5G-and-beyond technologies since they provide the desired combination of high frequency and high power. These applications along with terrestrial hub and backhaul communications at high power output can present severe heat removal challenges. The cooling of GaN devices with diamond as the heat spreader has gained significant momentum since device self-heating limits GaN's performance. However, one of the significant challenges in integrating polycrystalline diamond on GaN devices is maintaining the device performance while achieving a low diamond/GaN channel thermal boundary resistance. In this study, we achieved a record-low thermal boundary resistance of around 3.1 ± 0.7 m2 K/GW at the diamond/Si3N4/GaN interface, which is the closest to theoretical prediction to date. The diamond was integrated within ∼1 nm of the GaN channel layer without degrading the channel's electrical behavior. Furthermore, we successfully minimized the residual stress in the diamond layer, enabling more isotropic polycrystalline diamond growth on GaN with thicknesses >2 μm and a ∼1.9 μm lateral grain size. More isotropic grains can spread the heat in both vertical and lateral directions efficiently. Using transient thermoreflectance, the thermal conductivity of the grains was measured to be 638 ± 48 W/m K, which when combined with the record-low thermal boundary resistance makes it a leading-edge achievement.

An Empirical Modeling of Gate Voltage-Dependent Behaviors of Amorphous Oxide Semiconductor Thin-Film Transistors including Consideration of Contact Resistance and Disorder Effects at Room Temperature.

In this paper, we present an empirical modeling procedure to capture gate bias dependency of amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) while considering contact resistance and disorder effects at room temperature. From the measured transfer characteristics of a pair of TFTs where the channel layer is an amorphous In-Ga-Zn-O (IGZO) AOS, the gate voltage-dependent contact resistance is retrieved with a respective expression derived from the current–voltage relation, which follows a power law as a function of a gate voltage. This additionally allows the accurate extraction of intrinsic channel conductance, in which a disorder effect in the IGZO channel layer is embedded. From the intrinsic channel conductance, the characteristic energy of the band tail states, which represents the degree of channel disorder, can be deduced using the proposed modeling. Finally, the obtained results are also useful for development of an accurate compact TFT model, for which a gate bias-dependent contact resistance and disorder effects are essential.

Influence of Reduction in Effective Channel Length on Device Operations of In-Ga-Zn-O Thin-Film Transistors With Variations in Channel Compositions

Device scaling of oxide-channel thin-film transistors (TFTs) toward sub-micrometer regime has been an urgent issue to realize higher-resolution displays and 3-D structured electronic devices. The channel-shortening effect is a serious problem in implementing the oxide TFTs with short channel lengths. In this work, the short-channel effects (SCEs) of the In-Ga-Zn-O (IGZO) TFTs were investigated with the variations in channel composition when the channel length was varied from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{m}$ </tex-math></inline-formula> to 500 nm. Effective channel lengths were examined to decrease with increasing the In contents, in which the reduction of the effective channel length was estimated to be 240 and 720 nm when the In/Ga ratio increased 0.7 to 1.4. The drain bias-dependent roll-offs in turn-on voltages, which typically correspond to drain-induced barrier-lowering (DIBL), also showed significant channel composition dependence. When the In/Ga ratio within the channel increased from 0.2 to 1.4, the DIBL coefficients were estimated from 22 to 244 mV/V for the devices with a channel length of 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> . The channel composition-dependent variations in the effective channel length were suggested to result from the inter-diffusion and redox reactions at interfaces between the channel and electrode layers. Alternatively, the contact resistances were also examined to play major roles in determining the SCEs with scaling the devices, especially when the In contents decreased in the channel composition. Thus, properly controlling the channel composition can be concluded as such an important issue for compromising both technical strategies to reduce the channel resistance and to suppress the reduction of effective channel length.

(100) Plane β-Ga2O3 Flake Based Field Effect Transistor and Its Hydrogen Response

2-dimensional (100) plane β phase Ga2O3 (β-Ga2O3) flake based field effect transistor (FET) was fabricated, and its electrical characteristics was analyzed. The (100) plane β-Ga2O3 flake was mechanically exfoliated from the side wall of plane β-Ga2O3 bulk substrate. The minimum thickness of 57.3 nm was obtained for the very thin (100) plane β-Ga2O3 channel layer of the FET using inductively coupled plasma etching with BCl3/N2 chemistry. The current-voltage characteristics of the FET with various β-Ga2O3 channel thickness was investigated. The dependence of the channel thickness on the drain current density, threshold voltage, transconductance, and field effect mobility was studied. The hydrogen response of the (100) plane Ga2O3 flake based FET with catalytic Pt gate surface was measured in the range of 10–500 ppm at 400 °C, and modeled with a dissociative Langmuir isotherm. The device showed a reliable responsivity to the different concentration of hydrogen exposure, and the responsivity of 25.02% was observed for the 500 ppm hydrogen at 400 °C.

Self-Heating Stress-Induced Severe Humps in Transfer Characteristics of Amorphous InGaZnO Thin-Film Transistors

Under a self-heating stress (SHS), amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) would exhibit a severe hump in the transfer characteristics. A model based on state transformation of oxygen vacancies is proposed to explain this phenomenon. The channel region of TFT is considerably self-heated if a large current flows through due to the poor thermal conductivity of a-IGZO. The temperature in the channel region can be raised with the high-power SHS so high that the oxygen vacancies there perform a state transformation from deep-donors and/or traps to shallow-donors, making the carrier concentration increase remarkably in the a-IGZO channel layer. The temperature is highest in the center of channel region and thus the state transformation takes place there first, leading to the carrier concentration increasing there first. As a result, the TFT has a lower turn-on voltage in the central channel region than in the rest, bringing finally about the hump in the transfer curves. The model is well verified by annealing the a-IGZO, which shows that the carrier concentration surely increases greatly when the annealing temperature is over 300 °C.

Using Channel and Network Layer Pruning Based on Deep Learning for Real-Time Detection of Ginger Images

Consistent ginger shoot orientation helps to ensure consistent ginger emergence and meet shading requirements. YOLO v3 is used to recognize ginger images in response to the current ginger seeder’s difficulty in meeting the above agronomic problems. However, it is not suitable for direct application on edge computing devices due to its high computational cost. To make the network more compact and to address the problems of low detection accuracy and long inference time, this study proposes an improved YOLO v3 model, in which some redundant channels and network layers are pruned to achieve real-time determination of ginger shoots and seeds. The test results showed that the pruned model reduced its model size by 87.2% and improved the detection speed by 85%. Meanwhile, its mean average precision (mAP) reached 98.0% for ginger shoots and seeds, only 0.1% lower than the model before pruning. Moreover, after deploying the model to the Jetson Nano, the test results showed that its mAP was 97.94%, the recognition accuracy could reach 96.7%, and detection speed could reach 20 frames·s−1. The results showed that the proposed method was feasible for real-time and accurate detection of ginger images, providing a solid foundation for automatic and accurate ginger seeding.

3D-printed low-cost fabrication and facile integration of flexible epidermal microfluidics platform

Skin-mounted microfluidics devices have shown distinct advantages in sweat collection and analysis platform due to their multiple sweat treatment including sample collection, flow control, storage, analysis and expelling by special design in microfluidic networks. However, the fabrication of most reported wearable microfluidic devices with mechanical flexibility and elasticity mainly rely on soft-lithography technology whose implementation is limited by necessitating access to resource-intensive laboratory and costly facilities. Herein, we propose an accessible, low-cost fabrication and integration scheme, which renders flexible microfluidic architecture capable of performing reliable sweat collection and sensing. We utilize low-cost 3D-printing technology to rapidly yield positive mold of facile printable material (malt syrup) for personalized customization of microfluidic channels. An ingenious approach is also proposed to encapsulate microfluidic channel layer and integrate pH sweat sensor into the microfluidic device for dependable sweat collection and pH analysis. The proposed fabrication strategy of flexible and stretchable epidermal microfluidic sensor represents a low cost, reachable alternative to recently prevailing soft-lithography method, and hence significantly reduces the threshold of microfluidic development and manufacture.

Steep Subthreshold Swing and Enhanced Illumination Stability InGaZnO Thin-Film Transistor by Plasma Oxidation on Silicon Nitride Gate Dielectric.

In this paper, an InGaZnO thin-film transistor (TFT) based on plasma oxidation of silicon nitride (SiNx) gate dielectric with small subthreshold swing (SS) and enhanced stability under negative bias illumination stress (NBIS) have been investigated in detail. The mechanism of the high-performance InGaZnO TFT with plasma-oxidized SiNx gate dielectric was also explored. The X-ray photoelectron spectroscopy (XPS) results confirmed that an oxygen-rich layer formed on the surface of the SiNx layer and the amount of oxygen vacancy near the interface between SiNx and InGaZnO layer was suppressed via pre-implanted oxygen on SiNx gate dielectric before deposition of the InGaZnO channel layer. Moreover, the conductance method was employed to directly extract the density of the interface trap (Dit) in InGaZnO TFT to verify the reduction in oxygen vacancy after plasma oxidation. The proposed InGaZnO TFT with plasma oxidation exhibited a field-effect mobility of 16.46 cm2/V·s, threshold voltage (Vth) of −0.10 V, Ion/Ioff over 108, SS of 97 mV/decade, and Vth shift of −0.37 V after NBIS. The plasma oxidation on SiNx gate dielectric provides a novel approach for suppressing the interface trap for high-performance InGaZnO TFT.

Effective modulation of spin accumulation using a ferromagnetic/nonmagnetic bilayer spin channel

A lateral spin valve consisting of highly spin-polarized CoFeAl electrodes with a CoFeAl/Cu bilayer spin channel has been developed. Despite a large spin absorption into the CoFeAl capping channel layer, an efficient spin injection and detection using the CoFeAl electrodes enable us to observe a clear spin valve signal. We demonstrate that the nonlocal spin accumulation signal is significantly modulated depending on the relative angle of the magnetizations between the spin injector and absorber. The observed modulation phenomena is explained by the longitudinal and transverse spin absorption effects into the CoFeAl channel layer with the spin resistance model.

Microwave and furnace annealing in oxygen ambient for performance enhancement of p-type SnO thin-film transistors

In this study, we investigated the effect of microwave-irradiation annealing (MWA) and thermal furnace annealing (FA) in oxygen ambient on the active channel layer of p-type tin-oxide (SnO) thin-film transistors. At very low source-drain voltage of −0.1 V, the MWA at 1200 W and FA at 300 °C samples have exhibited significant improvement in the electrical characteristics such as subthreshold swing (SS) of 0.93 and 0.485 V dec−1, the I on/I off ratio of 1.65 × 104 and 3.07 × 104, the field-effect mobility (μ FE) of 0.16 and 0.26 cm2 V−1 s and ultra-low off-state current of 1.9 and 2.0 pA respectively. The observed performance enhancement was mainly attributed to the reduction of interface trap density (N t) by tuning the power of MWA and optimizing the temperature in FA. From the result, we observed the optical band gap (E g) increased by 6% in FA, and 12% in MWA, which confirms improved crystallinity and reduction of defect states. Additionally, a low thermal budget microwave anneal process has shown high transmittance of more than 86% in the visible region (380–700 nm). The physical characterization indicates the partial phase transformation of SnO to SnO2 with retaining p-type conductivity in both annealing processes. The results demonstrate that both the annealing process could be highly promising to be used in the complementary logic circuits of new generation flexible/transparent displays.

A quantitative approach for trap analysis between Al0.25Ga0.75N and GaN in high electron mobility transistors

The characteristics of traps between the Al0.25Ga0.75N barrier and the GaN channel layer in a high-electron-mobility-transistors (HEMTs) were investigated. The interface traps at the Al0.25Ga0.75N/GaN interface as well as the border traps were experimentally analyzed because the Al0.25Ga0.75N barrier layer functions as a dielectric owing to its high dielectric constant. The interface trap density Dit and border trap density Nbt were extracted from a long-channel field-effect transistor (FET), conventionally known as a FATFET structure, via frequency-dependent capacitance–voltage (C–V) and conductance–voltage (G–V) measurements. The minimum Dit value extracted by the conventional conductance method was 2.5 × 1012 cm−2·eV−1, which agreed well with the actual transistor subthreshold swing of around 142 mV·dec−1. The border trap density Nbt was also extracted from the frequency-dependent C–V characteristics using the distributed circuit model, and the extracted value was 1.5 × 1019 cm−3·eV−1. Low-frequency (1/f) noise measurement provided a clearer picture of the trapping–detrapping phenomena in the Al0.25Ga0.75N layer. The value of the border trap density extracted using the carrier-number-fluctuation (CNF) model was 1.3 × 1019 cm−3·eV−1, which is of a similar level to the extracted value from the distributed circuit model.

Microvalve with Trapezoid-Shaped Cross-Section for Deep Microchannels.

Simple microfluidic systems for handling large particles such as three-dimensional (3D) cultured cells, microcapsules, and animalcules have contributed to the advancement of biology. However, obtaining a highly integrated microfluidic device for handling large particles is difficult because there are no suitable microvalves for deep microchannels. Therefore, this study proposes a microvalve with a trapezoid-shaped cross-section to close a deep microchannel. The proposed microvalve can close a 350 m deep microchannel, which is suitable for handling hundreds of micrometer-scale particles. A double-inclined lithography process was used to fabricate the trapezoid-shaped cross-section. The microvalve was fabricated by bonding three polydimethylsiloxane layers: a trapezoid-shaped liquid channel layer, a membrane, and a pneumatic channel layer. The pneumatic balloon, consisting of the membrane and the pneumatic channel, was located beneath a trapezoid-shaped cross-section microchannel. The valve was operated by the application of pneumatic pressure to the pneumatic channel. We experimentally confirmed that the expansion of the pneumatic balloon could close the 350 m deep microchannel.

Ultrafast and Highly Sensitive Dual-Channel FET Photodetector Based on a Two-Dimensional MoS2 Homojunction.

Two-dimensional transition metal dichalcogenide-based phototransistors have been intensively studied in recent years due to their high detection rate and flexibility. However, the photogating effect, usually appearing in the devices, leads to a poor transient photoresponse, which slows down the imaging rate of the camera based on the devices. Here, we demonstrate a dual-channel two-dimensional field-effect phototransistor composed of a vertical molybdenum disulfide (MoS2) p-n homojunction as the sensitizing channel layer. Owing to the effective separation by the vertical built-in electric field and rapid migration of photoexcited electrons and holes in the separated channels, the fabricated dual-channel FET device simultaneously exhibits prominent responsivity and greatly improved time response in comparison to the pristine MoS2 FET detectors. Excellent device performance has been achieved, with a responsivity of 3.4 × 104 A/W at a source-drain voltage (VDS) of 1 V, corresponding to a detectivity (D*) of 1.9 × 1013 Jones@532 nm and a gain of more than 105 electrons per photon, an external quantum efficiency of 9.6%, and a response time of tens of milliseconds. Especially, the response time of the dual-channel FET device is 3 orders of magnitude faster than that of the pristine device. Our results provide a new way to overcome the inherent photogating drawback of two-dimensional FET optoelectronic devices and to develop a related high frame rate imaging system.

Mist chemical vapor deposition of crystalline MoS2 atomic layer films using sequential mist supply mode and its application in field-effect transistors

Molybdenum disulfide (MoS2) mono/bilayer have been systematically investigated using atmospheric-pressure mist chemical vapor deposition (mist CVD) from (NH4)2MoS4 dissolved in N-methyl-2-pyrrolidone as a precursor. Film deposition was performed by alternating MoS2 mist storage within a closed chamber and mist exhaust, i.e. sequential mist supply mode at different furnace temperatures, storage times of precursor, and repetition cycles of mist supply on thermally grown SiO2 (th-SiO2) and mist-CVD grown Al1−x Ti x O y (ATO) layers coated on p+-Si substrates. The average size of the MoS2 flake and their number of stack layers could be controlled by tuning the deposition parameters combined with substrate pretreatment. Field-effect transistors with MoS2 atomic mono/bilayer as a channel layer exhibited mobility up to 31–40 (43–55) cm2 V−1 s−1 with a threshold voltage of −1.6 (−0.5) V, subthreshold slope of 0.8 (0.11) V dec.−1, and on/off ratio of 3.2 × 104 (3.6 × 105) on th-SiO2 (ATO) layers as gate dielectric layers without mechanical exfoliation. These findings imply that mist CVD is available for the synthesis of metal transition metal dichalcogenide and metal oxide layers as channel and gate dielectric layers, respectively.

Study of InGaZnO Thin Film Transistors With Dual Treatment of Pre-Oxidation ZrO2 High-κ Dielectric and Post-Oxidation InGaZnO Channel by Neutral Beam System

With better field-effect mobility (&gt;10 cm2/V·S), smaller subthreshold swing (S.S), and other electrical characteristics, amorphous IGZO thin film transistors (a-IGZO TFTs) has been studied extensively for its promising applications, such as liquid crystal displays and flat-panel displays. In this investigation, TFT devices are experimented to enhance the electrical characteristics. Atmosphere Pressure-PECVD (AP-PECVD) is used to deposit a-IGZO, which is designed as device channel layer. In order to maintain enough gate control ability with thinner effective oxide thickness (EOT), high-κ material ZrO2 is used as dielectric, lowering the gate leaking current. With post dual neutral beams O2 plasma treated on both a-IGZO channel layer 400 W and ZrO2 dielectric 100 W, the best result for experimental devices show that the a-IGZO TFT has electrical characteristics of field-effect mobility of 22.22 cm2/V·S, threshold voltage (VT) of 2.86 V, S.S of 74 mV/decade, and Ion/Ioff ratio of 8.2×105. Keywor

Design of microfluidic channels to prevent negative filtration in implantable hemofiltration devices.

In order to improve the quality of life of dialysis patients, our group have been developing an implantable hemofiltration device (IHFD) composed of multiple layers of dialysis membranes and microfluidic channels. To improve the hemodialysis performance of IHFD, preventing the negative filtration, which is caused by the oncotic pressure of blood, is mandatory. In this study, we fabricated IHFDs with five different microchannel designs and experimentally investigated the performance of each device in in vitro experiment. In addition, the successful IHFD was further evaluated by ex vivo experiments with a beagle dog. The experiments verified the effectiveness of the microchannel design, which will be used for the IHFD for in vivo experiments with pigs in the future.

Analytical Impedance of Oxygen Transport in the Channel and Gas Diffusion Layer of a PEM Fuel Cell

Analytical model for impedance of oxygen transport in the gas–diffusion layer (GDL) and cathode channel of a PEM fuel cell is developed. The model is based on transient oxygen mass conservation equations coupled to the proton current conservation equation in the catalyst layer. Analytical formula for the “GDL+channel” impedance is derived assuming fast oxygen and proton transport in the cathode catalyst layer (CCL) In the Nyquist plot, the transport impedance consists of two arcs describing oxygen transport in the air channel (low–frequency arc) and in the GDL. The characteristic frequency of GDL arc depends on the CCL thickness: large CCL thickness strongly lowers this frequency. At small CCL thickness, the high–frequency feature on the arc shape forms. This effect is important for identification of peaks in distribution of relaxation times spectra of low–Pt PEMFCs.