Schottky Barrier Layer Research Articles

Lexible sensors for food monitoring. Part II: Applications

Monitoring and maintaining food quality, safety, and authenticity are the most important concerns in the food industry. The cutting-edge flexible sensors for food monitoring precisely satisfy the needs of acquiring information on multiple parameters in a small space, they provide for the more reasonable layout, get data on the mechanical deformations, and can be conformably attached to arbitrarily curved surfaces. The flexible sensing materials with a large area of specific surface, that ensure high mobility and density of the media, feature dense active sites, outstanding adjustability and high processing capacities, such as two-dimensional carbon nanomaterials, conductive polymers, and nano-hybrid materials; those materials have further improved the sensitivity, stability and selectivity of the flexible sensors’ perception. This article attempts to critically review the present state-of-arts developments in relation to the materials, manufacturing techniques and sensing mechanisms of the devices, as well as the applications of the electrically-transduced flexible sensors. Moreover, this article elaborates on the transduction mechanisms of the several typical transducers, with a focus on the physics behind, including the modulation of the doping level, Schottky barrier, and interfacial layer that typically cause changes in conductivity, functionality and permittivity. We also highlight the benefits and the technical challenges along with the appropriate solutions provided by the presented flexible sensors, and we also consider the potential strategies that allow overcoming limitations in power consumption, quantitatively assess the trade-offs in maintaining the quality and marketability, to optimize wireless communication and explore new sensing patterns.

Flexible sensors for food monitoring. Part I: Principle

Monitoring and maintaining food quality, safety, and authenticity are the most important concerns in the food industry. The cutting-edge flexible sensors for food monitoring precisely meet the needs of acquiring information on multiple parameters in small space and more reasonable layout, providing data on mechanical deformations, and conformably attaching to arbitrarily curved surfaces. Flexible sensing materials with a large specific surface area, high carrier mobility and carrier density, dense active sites, outstanding tunability, and processability, such as two-dimensional carbon nanomaterials, conductive polymers, and nanohybrid materials, have further improved the sensitivity, stability, and selectivity of flexible sensors. This article attempts to critically review state-of-the-art developments with respect to materials, fabrication techniques, and sensing mechanisms of devices, as well as the applications of the electrically-transduced flexible sensors. In addition, this review elaborates on the transduction mechanisms of several typical transducers, with a focus on the physics behind, including the modulation of doping level, Schottky barrier, and interfacial layer that typically lead to changes in conductivity, work function, and permittivity. We also highlight the benefits, technical challenges with corresponding solutions of current flexible sensors, and discuss potential strategies to overcome limitations in energy consumption, quantify the trade-offs in maintaining quality and marketability, optimize wireless communication, and explore new sensing patterns.

Highly Selective and Humidity-Resistant Triethylamine Sensors Based on Pt and Cr2O3 Nanoparticles

A triethylamine gas sensor with humidity resistance, high selectivity, and high response is designed. Chromium oxide particles were prepared by heating them in a water bath at 80 °C. Pt/Cr2O3 nanoparticles were obtained by sodium borohydride reduction. The structure morphology of nanoparticles was characterized by some methods. The gas response test results showed that the modification of Pt improved the response value and selectivity of Cr2O3 to triethylamine. It was found that Cr2O3–2Pt nanoparticles had the best performance, and at the optimum operating temperature of 160 °C, its response value reached 200, 68 times that of Cr2O3 (S = 2.95). In addition, the modification of Pt made Cr2O3–2Pt nanoparticles had a wide range of humidity applications. The mechanism of gas response is explained by using the Schottky barrier and hole accumulation layer model of Pt and Cr2O3 contact and energy band theory.

Tunneling of photon-generated carrier in the interface barrier induced resistive switching memory behaviour

The resistive switching effect is a great physical phenomenon that the resistance of a material can be reversibly changed by applying an electric pulse, which is useful to constructing the nonvolatile resistance random access memory (RRAM) in the next generation of memory system. In this work, a sandwich structure (ITO/WO3/AZO) was prepared by using WO3 film (∼300 nm) as the dielectric layer meanwhile indium tin oxide (ITO) as the top electrode and aluminium-doped zinc oxide (AZO) as the bottom electrode. An enhanced resistive switching memory behavior was observed in the sample processed by light-illumination. Furthermore, the set voltage (Vset) and reset voltage (Vreset) are increased but the HRS/LRS resistance ratio is decreased with the increasing of illumination time for 600 °C annealed sample. Through further analysis, a physical model on the tunneling of photon-generated carrier in the Schottky barrier layer driven by electric pulse is proposed to explain the enhanced resistive switching memory behavior. The suggested mechanism is highly pivotal for the resistive switching phenomenon to be properly applied in the nonvolatile RRAM device.

High Voltage Graphene Nanowall Trench MOS Barrier Schottky Diode Characterization for High Temperature Applications

Graphene’s superior electronic and thermal properties have gained extensive attention from research and industrial sectors to study and develop the material for various applications such as in sensors and diodes. In this paper, the characteristics and performance of carbon-based nanostructure applied on a Trench Metal Oxide Semiconductor MOS barrier Schottky (TMBS) diode were investigated for high temperature application. The structure used for this study was silicon substrate with a trench and filled trench with gate oxide and polysilicon gate. A graphene nanowall (GNW) or carbon nanowall (CNW), as a barrier layer, was grown using the plasma enhanced chemical vapor deposition (PECVD) method. The TMBS device was then tested to determine the leakage current at 60 V under various temperature settings and compared against a conventional metal-based TMBS device using TiSi2 as a Schottky barrier layer. Current-voltage (I-V) measurement data were analyzed to obtain the Schottky barrier height, ideality factor, and series resistance (Rs) values. From I-V measurement, leakage current measured at 60 V and at 423 K of the GNW-TMBS and TiSi2-TMBS diodes were 0.0685 mA and above 10 mA, respectively, indicating that the GNW-TMBS diode has high operating temperature advantages. The Schottky barrier height, ideality factor, and series resistance based on dV/dln(J) vs. J for the GNW were calculated to be 0.703 eV, 1.64, and 35 ohm respectively.

GaN HEMTs with quaternary In0.05Al0.75Ga0.2N Schottky barrier layer

A quaternary InAlGaN Schottky barrier layer is used in GaN high electron mobility transistors (HEMTs) in this study. For potential application to high speed electronic devices, GaN HEMTs with a quaternary InAlGaN Schottky barrier layer are fabricated and characterized. The fabricated device shows a maximum drain current density of 1.8 A mm−1, a maximum trans-conductance of 557 mS mm−1, on/off current ratio higher than 105, a subthreshold swing (S.S.) of 140 mV/decade, and a gate-diode leakage current of 1.2 mA mm−1. For the device with a 100-nm-long gate footprint, a current gain cut-off frequency (fT) of 102 GHz and a maximum oscillation frequency (fmax) of 130 GHz are extracted at the same bias condition (VDS = 8 V and VGS = −2.2 V) whose Lg · fT product is evaluated to be 10.2 GHz · µm.

Large lateral photovoltaic effect in µc-SiOx:H/a-Si:H/c-Si p–i–n structure

In this paper, we report on a large lateral photovoltaic effect (LPE) in a hydrogenated microcrystal silicon-oxygen (µc-SiOx:H)-based p–i–n structure. Compared with LPE in a hydrogenated amorphous silicon (a-Si:H)-based p–i–n structure, this structure showed an abnormal current–voltage (I–V) curve with a lower photoelectric conversion efficiency, but exhibited a much higher LPE with the highest position sensitivity of 64.3 mV/mm. We ascribe this to the enhancement of the lateral gradient of excess transmitted carriers induced by increasing both Schottky barrier and p-type layer body conductivity. Our results suggest that this µc-SiOx:H-based p–i–n structure may be a promising candidate for position-sensitive detectors (PSDs). Moreover, our results may also imply that solar cell devices with abnormal I–V curves (or low efficiency) could find their new applications in other aspects.

Influence of InGaP and AlGaAs Schottky Layers on ESD Robustness in GaAs pHEMTs

GaAs high-electron-mobility transistors (HEMTs) have been widely used for radio-frequency (RF) applications due to the excellent material properties. One of the essential elements of the HEMTs is the gate Schottky barrier layer. InGaP has been proposed and proven as a better Schottky barrier material for the RF performance of the GaAs HEMTs. This letter investigates the influence of the GaAs HEMTs with two different Schottky layers, which are InGaP and AlGaAs on device transient characteristics under electrostatic discharge (ESD) stress. Although InGaP presents significant advantages on improving RF performance of GaAs HEMTs, it shows inferiority in ESD robustness.

Double-Peak N-Shaped Negative Differential Resistance in a Quantum Dot Field Effect Transistor

Double-peak N-shaped negative differential resistance (NDR) with a high peak-to-valley ratio is observed in the output characteristics of a GaAs-based modulation-doped field effect transistor with InAs quantum dots in the barrier layer (QDFET). One NDR peak with a higher source-drain voltage VDS is explained as the real-space transfer (RST) of high-mobility electrons in the channel into the quantum dots layer, while the other with a lower VDS is caused by the high-mobility RST electrons in the channel into the modulation-doped AlGaAs barrier layer on the other side. We depict a point how a thinner Schottky barrier layer provides a stronger potential, opening a possibility of two-directional channel electron transfer when a much higher VG is applied. The result suggests that the QDFET can be an attractive candidate for high-speed logic application and memory devices.

Fabrication of InAs composite channel high electron mobility transistors by utilizing Ne-based atomic layer etching

High electron mobility transistors (HEMTs) with InAs∕InGaAs composite channel were fabricated by employing low damage, highly selective Ne-based atomic layer etching (ALET) for the dry gate recess process. Ne-based ALET exhibited very high etch selectivity of InP over InAlAs which is suitable for dry gate recess process removing InP etch stop layer on top of InAlAs Schottky barrier layer. The plasma induced damage on the exposed InAlAs surface due to the ALET was much lower than that due to the conventional Ar-based reactive ion etching (RIE), which was verified by atomic force microscopy and Hall measurements. For further comparison, dc characteristics were compared for the two types of 0.3μm HEMTs fabricated by utilizing ALET and conventional RIE during the dry gate recess processes.

Effects of Gate-Recess Structure on High-Frequency Characteristics of 0.1 μm Metamorphic HEMTs

We investigate effects of the gate-recess structure on the high-frequency characteristics of the depletion-mode metamorphic high electron mobility transistors (HEMTs). We characterize the dc and radio frequency performances of two different gate-recess structures and perform a comparative study using the hydrodynamic device model simulation and small-signal parameter analysis. The narrow gate-recess structure shows significantly higher dc performances than the wide gate-recess structure, and this phenomenon is due to the presence of the negatively charged surface states on the Schottky barrier layer surface in the wide gate-recess structure. Despite the superior dc characteristics, the narrow gate-recess structure shows more degraded maximum frequency of oscillation of than that of the wide gate-recess structure and almost the same cutoff frequency of . The degraded of the narrow gate-recess structure is attributed to a higher gate-to-drain capacitance . The significant increase of is caused by the reduction of effective gate-to-drain spacing due to the nonlinear electric potential distribution in the gate-to-drain region.

InGaP/InGaAs Pseudomorphic Heterodoped-Channel FETs With a Field Plate and a Reduced Gate Length by Splitting Gate Metal

Depositing gate metal across a step undercut between the Schottky barrier layer and the insulator-like layer is employed to obtain a reduced gate length of 0.4 mum with an additional 0.6-mum field plate from a 1-mum gate window. Most dc and ac characteristics including current density (IDSS=451mA/mm), transconductance (gm,max=225mS/mm), breakdown voltages (VBD(DS)/V BD(GD)=22/-25.5V), gate-voltage swing (GVS=2.24V), cutoff, and maximum oscillation frequencies (ft/fmax=17.2/32GHz) are improved as compared to those of a 1-mum gate device without field plate. At a VDS of 4.0 V, a maximum power added efficiency of 36% with an output power of 13.9 dBm and a power gain of 8.7 dB are obtained at a frequency of 1.8 GHz. The saturated output power and the linear power gain are 316 mW/mm and 13 dB, respectively

Correlating gate sinking and electrical performance of pseudomorphic high electron mobility transistors

Ti interdiffusion from the Ti/Pt/Au gate into the AlGaAs Schottky barrier layer (SBL) of 0.25-μm GaAs Pseudomorphic High Electron Mobility Transistors (PHEMTs) has been studied using the accelerated life testing technique. Based on measurements and modeling, analytical expressions for quantitative correlation between the positive pinch-off voltage ( V P) shift as well as the saturation drain current ( I Dsat) decrease and the physical damage occurring during gate sinking has been developed. It is suggested that the main cause for device failure is the growth of the TiAs phase leading to the decrease in the SBL thickness. Additionally, it is suggested that V P may be used as a better indicator for device degradation than I Dsat since it is linearly proportional to the degrading physical characteristic – the Schottky barrier layer thickness.

Influences of Gate Recess Structure on the DC Characteristics of 0.1 Micron Metamorphic HEMTs

Effects of the gate recess structures on the DC performances were investigated in 0.1-μm metamorphic high-electron-mobility transistors. Narrow gate recess structure showed significantly enhanced DC characteristics compared to wide gate recess structure in terms of drain-source saturation current increasing from 440 to 710 mA/mm and extrinsic transconductance increasing from 420 to 910 mS/mm. We propose that the observed variations in DC characteristics are due to the deep-level acceptor-type interface defects formed between the silicon-nitride passivation layer and the Schottky barrier layer. We performed hydrodynamic model simulation to verify the proposed mechanism, and the calculated result showed a good agreement with the experimental observation.

Physical Identification of Gate Metal Interdiffusion in GaAs PHEMTs

The Ti metal interdiffusion of Ti/Pt/Au gate metal stacks in 0.15-/spl mu/m GaAs PHEMTs subjected to high-temperature accelerated lifetest has been physically identified using scanning transmission electron microscopy. Further energy dispersive analysis with X-ray (EDX) analysis confirms the Ti diffusion into the AlGaAs Schottky barrier layer and the decrease of Schottky barrier height suggests the Ti-AlGaAs intermetallic formation, which is consistent with previous Rutherford backscattering spectroscopy/X-ray photoelectron spectroscopy studies. The Ti metal interdiffusion reduces the separation of the gate metal and InGaAs channel, thus leading to a slight Gm increase, positive shift in pinchoff voltage, and S21 increase during the preliminary portion of the lifetest. Accordingly, the Ti interdiffusion effect implies that the lifetime of GaAs PHEMTs subjected to high-temperature accelerated lifetest could be dependent upon the initial thickness of the Schottky layer underneath the gate metal.

MOCVD Growth of InAlAsSb Layer for High-Breakdown Voltage HEMT Applications

ABSTRACTThis paper studies the decomposition characteristic of group-III sources during InAlAsSb growth on InP substrates by metalorganic chemical vapor deposition (MOCVD) using trimethylindium (TMI), trimethylaluminum (TMA), trimethylantimony (TMSb) and arsine (AsH3). A composition analysis of InAlAsSb layers shows that the group-III compositions in the InAlAsSb layer change remarkably when the flow rate of the group-V source is varied. To clarify the reason for this phenomenon, the growth rates of InAsSb and AlAsSb component are examined. Their changes indicate that TMSb suppresses the decomposition of TMA while AsH3 enhances it. Moreover, the HEMT structure with InP/InAlAsSb Schottky barrier layer, whose InP layer acts as a recess-etch-stop layer, is fabricated for the first time. The I-V characteristics of a fabricated Schottky barrier diode indicate that the reverse leakage current of InP/InAlAsSb is about one order of magnitude smaller than that of commonly used InP/InAlAs.

Electrical properties of modulation-doped InxAl1−xAs/InyGa1−yAs structures on GaAs and InP substrates with 0.2⩽x, y⩽0.8

We have investigated the compositional dependence of surface morphology and electronic properties of modulation-doped InxAl1−xAs/InyGa1−yAs structures (0.2⩽x,y⩽0.8) on strain-relaxed buffers on GaAs and InP substrates. We have found that modulation-doped In0.70Al0.30As/In0.70Ga0.30As on InP shows a two-dimensional electron-gas mobility of 1.3×104 cm2/V s (300 K) and 9.3×104 cm2/V s (77 K) at a sheet carrier concentration of 2.0×1012 cm−2. We suggest that this material system has a potential for device applications if a high-quality Schottky barrier layer can be fabricated.

Pseudomorphic InP HEMTs with dry-etched source vias having 190 mW output power and 40% PAE at V-band

We report state-of-the-art V-band power performance of 0.15-/spl mu/m gate length InGaAs/InAlAs/InP HEMT's which have 15 /spl mu/m/spl times/23 /spl mu/m dry-etched through-substrate source vias (substrate thickness 50 /spl mu/m). The 500-/spl mu/m wide InP HEMT's were measured in fixture at 60 GHz and demonstrated an output power of 190 mW with 40% power-added efficiency (PAE) and 6.8 dB power gain at an input power of 16 dBm. These results represent the best combination of power and PAE reported to date at this frequency for any solid state device. The results are achieved through optimization of the InP-based heterostructure which incorporates a graded pseudomorphic InGaAs channel and a graded pseudomorphic InAlAs Schottky barrier layer, and the use of 15 /spl mu/m/spl times/23 /spl mu/m dry-etched through-substrate source vias.

Low-frequency gate current noise of InP based HEMTs

The low-frequency 1/ f noise in the gate current I G of InP based HEMTs is studied for a broad range of operation conditions. When the Schottky barrier is in reverse operation, the corresponding gate current noise spectral density S I G shows an I G n law, with n≈2 for not too large drain bias. For sufficiently large gate bias, the Schottky barrier becomes forward biased and the corresponding noise intensity tends to be proportional to the gate current. These results will be compared with existing theories in order to assess the responsible mechanisms. From the gate current dependence of the corresponding noise spectral density, it is derived that generation–recombination noise by traps in the Schottky barrier layer is responsible for the 1/ f noise.

Recent Development in InGaAs based Millimeter Wave HEMTs

Modulation doped structures involving GaAIAs-GaAs combination initiated a vigorous activity of materials growth and device fabrication of high electron mobility transistors (HEMTs) for microwave and millimeter wave applications. Choice of materials for higher carrier sheet concentration and enhanced mobility has ultimately zeroed on InGaAs as channel layer with a number of possible donor and Schottky barrier layer combinations. Use of pure InAs sandwiched between two thin layers of InGaAs has resulted in highest mobility measured so far. Recent developments have led to extremely low noise HEMT devices i.e., 0.8 and 1.2dB noise figures at 60 and 94GHz respectively. The present review covers some basic concepts of HEMTs operation, device design, fabrication and the required materials. Highlights of the methods involved in the epitaxial growth of various layers of HEMT structures are also included for comparing the capabilities of the existing techniques and evaluating the potentials of upcoming one i.e., atomic layer epitaxy.