Double Heterojunction Research Articles

One-pot preparation of double S-scheme Bi2S3/MoO3/C3N4 heterojunctions with enhanced photocatalytic activity originated from the effective charge pairs partition and migration

In this article, double S-scheme Bi2S3/MoO3/C3N4 heterojunctions with excellent photocatalytic performance were fabricated via a facile one-pot solid-state reaction. The structural and morphological analysis of the as-prepared heterojunctions was implemented to understand the crystal structure and surface properties. The results of photocatalytic experiments exhibit that Bi2S3/MoO3/C3N4 heterojunctions prepared by 11.5 g thiourea, 0.1333 g ammonium molybdate tetrahydrate and 0.02 g bismuth(III) citrate display the highest activity under solar light irradiation, the activity of which is seven times of that of C3N4, and nearly three times of that of MoO3/C3N4. The ameliorated performance originates from the construction of double S-scheme heterojunctions which possesses the effective charge pairs partition and migration. The cycling experiments display that Bi2S3/MoO3/C3N4 heterojunctions exhibit remarkable stability. In the light of the radical trapping experiments, nitroblue tetrazolium (NBT) experiments, electron spin resonance (ESR), and the match of energy band, double S-scheme mechanism was proposed. This work also provided a new facile solid-state reaction method to synthesize double S-scheme heterojunctions with high performance for efficient abatement of pollutants.

A Beyond-1-Tb/s Coherent Optical Transmitter Front-End Based on 110-GHz-Bandwidth 2:1 Analog Multiplexer in 250-nm InP DHBT

This article presents an optical transmitter front-end based on 2:1 analog multiplexer (AMUX) for beyond-1-Tb/s/carrier coherent optical communications systems. An AMUX is monolithically integrated with an optical modulator driver to realize an AMUX-Driver IC that can directly drive an optical modulator. The AMUX-Driver IC fabricated by using our in-house 250-nm InP double heterojunction bipolar transistors (DHBTs), which have a peak cutoff frequency ( $f_{\mathrm {T}}$ ) and maximum oscillation frequency ( $f_{\mathrm {max}}$ ) of 460 and 480 GHz, respectively, achieves a record bandwidth of 110 GHz and a 1.5- $\text{V}_{\mathrm {ppd}}$ linear differential output range with a <3% total harmonic distortion (THD). A coherent optical transmitter front-end module compactly integrating two AMUX-Driver ICs and an InP-based optical in-phase quadrature (IQ) modulator achieves an electro-optic (EO) bandwidth higher than 80 GHz at the 6-dB point. Beyond-1-Tb/s/carrier optical modulation based on 168-GBaud polarization-division-multiplexed (PDM) 16-ary quadrature-amplitude-modulation (16QAM) was successfully demonstrated by using this integrated module.

The role of silver nanoparticles in performance of multi-quantum well double heterojunction InGaN/GaN solar cells

In this paper, we have used an analytical method for modeling a nitride-based p-i-n solar cell and considered the role of implanting randomly distributed Silver nanoparticles into the active layer employing spectral density function based effective medium theory. It has been shown that an optimum condition for harvesting maximum photon absorption that depends on nanoparticle filling fraction. Besides, we have studied the effect of variation of the indium content percentage of the InGaN intrinsic layer in the performance of the device. The characteristic parameters of the devices in the presence of Ag nanoparticles in optimum condition represent an enhancement from 12.7% to 36.7% in the power conversion efficiency (PCE) depending on the indium molar percentage.

A 300-GHz High-Power High-Efficiency Voltage-Controlled Oscillator With Low Power Variation

This letter presents a 300-GHz voltage-controlled oscillator (VCO) with high output power and efficiency. The VCO core employs a differential Colpitts topology with inductive degeneration to facilitate a fundamental oscillation at 300 GHz. A common-base output buffer regulates the output power to minimize the power variation during the frequency tuning. The 300-GHz VCO is fabricated in a 130-nm InP double heterojunction bipolar transistor (DHBT) technology. The VCO exhibits a measured frequency tuning range of 9.9 GHz (294.9–304.8 GHz). The peak output power is measured to be 4.7 dBm at 297 GHz. The dc power consumption is 75.6 mW, leading to a high dc-to-RF conversion efficiency of 3.9%. The VCO output power is maintained nearly constant with only a 0.3-dB variation over the entire frequency tuning range. The phase noise is −86.6 dBc/Hz at 10-MHz offset frequency.

InGaP/GaAsSb/InGaAsSb/InP Double Heterojunction Bipolar Transistors With Record f t of 813 GHz

We fabricated InGaP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with an aggressive lateral and vertical scaling technology to improve the current gain cutoff frequency ( ${f}_{\text {t}}$ ) further. A 13-nm-thick GaAsSb/InGaAsSb base and a 40-nm-thick InP collector are used to reduce electron transit time. In addition, the width of the base electrode on each side of the emitter is reduced to about 0.05 $\mu \text{m}$ to suppress increases in parasitic collector capacitance. A fabricated DHBT with the emitter size of 0.24 $\mu \text{m}\,\,\times7.8\,\,\mu \text{m}$ exhibits maximum differential current gain of ~95 and collector-emitter breakdown voltage of 2.6 V. At a collector current density of 18 mA/ $\mu \text{m}^{{2}}$ , the DHBT exhibits ${f}_{\text {t}}$ of 813 GHz, which is the highest among all types of transistors measured at a room temperature.

Nongalvanic Generic Packaging Solution Demonstrated in a Fully Integrated D-Band Receiver

This article presents a packaging technique for monolithic microwave integrated circuits (MMIC) demonstrated in a fully integrated receiver (Rx) module at the D -band (110–170 GHz). The solution consists of an MMIC-to-waveguide transition realized using an on-chip probe mounted in the E -plane of a split-block waveguide module. An artificial magnetic conductor structure is implemented to suppress cavity modes and achieve better coupling from the waveguide to the probe. The transition's performance is experimentally verified using a back-to-back test chip, and measurement results show that the proposed packaging solution achieves a low insertion loss of only 0.7 dB and covers a very wide frequency range extending from 105 to 175 GHz. The proposed transition is also integrated with an in-phase/quadrature-phase (I/Q) Rx on the same chip. The Rx is realized in a 250-nm indium phosphide double heterojunction bipolar transistor technology and consists of a low-noise amplifier, an I/Q mixer, and a frequency tripler. Measurement results show that the Rx module achieves an average conversion gain of 23 dB across the frequency range of 110–145 GHz and has an average noise figure of 10.6 dB. The Rx MMIC has a dc power consumption of 440 mW and occupies an area of 1.6 × 1.6 mm2. This article addresses one of the main challenges in systems operating above 100 GHz and presents a fully integrated packaging solution that suits large integrated circuits and does not require any galvanic contacts nor impose any limitations on MMIC dimensions.

Z-scheme AgBr@Ag/CoAl layered double hydroxide heterojunction for superior photocatalytic Cr(VI) reduction under visible light

Construction of the Z-scheme heterojunction photocatalytic system is a promising strategy for improving the catalytic activity of photocatalysts. Herein, a novel Z-scheme AgBr@Ag/CoAl layered double hydroxide (LDH) photocatalyst was successfully prepared. The AgBr@Ag/LDH composites displayed superior performance for Cr(VI) reduction under visible light. The optimal composite with a 10 wt% silver content showed the highest photocatalytic activity, which was 12.2 and 12.8 times greater than that of LDH and AgBr, respectively. Furthermore, this composite also exhibited good photocatalytic stability towards Cr(VI) reduction without an obvious decrease after 5 cycles. The formation of a Z-scheme heterojunction in the AgBr@Ag/LDH composite effectively promoted the separation of photogenerated electron-hole pairs and kept the electrons with a high reducing capability in the conduction band of LDH, thus leading to its excellent photocatalytic activity towards the reduction of Cr(VI). This study may provide an insight into the design of the LDH-based Z-scheme photocatalytic systems for Cr(VI)-containing wastewater treatment.

Development of double heterojunction of Pr2Sn2O7@Bi2Sn2O7/TiO2 for hydrogen production

The double heterojunction of Pr2Sn2O7@Bi2Sn2O7/TiO2 composite was prepared by one-step hydrothermal method using TiO2 electrospun nanofibers as substrate. The structures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet (UV)-visible diffuse reflectance spectroscopy and photoluminescence. The performance of photocatalytic hydrogen production over the composite catalyst was studied under UV–visible light. The Pr2Sn2O7@Bi2Sn2O7/TiO2 composite nanofibers showed high photocatalytic activity for hydrogen production. The hydrogen production rate reached 587.1 μmol h−1 g−1, which was 2.7 and 7.1 times that of Bi2Sn2O7/TiO2 and TiO2, respectively. The main reasons for the high photocatalytic activity included broadened spectral range, 4f–4f transition from Pr3+ of Pr2Sn2O7 and increased separation of the photogenerated carriers by double heterojunction via direct Z-scheme transfer.

Construction of BPQDs/Ti3C2@TiO2 Composites with Favorable Charge Transfer Channels for Enhanced Photocatalytic Activity under Visible Light Irradiation.

Design and construction of double heterojunction is favorable to improve the separation and migration efficiency of photogenerated carriers, thus preferably solve the problems of environmental pollution and energy crisis. Herein, TiO2 nanoparticles (NPs) are in-situ grown on highly conductive Ti3C2 nanosheets via low-temperature hydrothermal strategy, and then black phosphorus quantum dots (BPQDs) are introduced on the surface of TiO2 NPs. Under hydrothermal temperature 120 °C, the BPQDs/Ti3C2@TiO2 photocatalyst exhibits remarkable enhanced photocatalytic degradation of methyl orange (MO) and hydrogen evolution reaction (HER) compared with BPQDs/Ti3C2 and Ti3C2@TiO2 composites. Enhanced photocatalytic activity can be attributed to (i) the BPQDs with tunable bandgaps are deposited on the TiO2 NPs to form intimate heterojunction, which facilitates the electrons transfer from the conduction band (CB) of BPQDs to the CB of TiO2; (ii) the electrons quickly migrate from CB of TiO2 NPs to the Ti3C2 nanosheets with excellent electronic conductivity via electron transfer channel, which is beneficial to prolong the lifetime of electrons and hinder the recombination of photogenerated carriers; (iii) the enhanced visible light absorption and enlarged specific surface area of BPQDs/Ti3C2@TiO2 further accelerate the photocatalytic reaction. This work emphasizes the essential role of quantum dots in the construction of double heterojunction and the potential application of Ti3C2 MXene for improving photocatalytic activity.

Heterojunctions for Photocatalytic Wastewater Treatment: Positive Holes, Hydroxyl Radicals and Activation Mechanism under UV and Visible Light

AbstractForming heterojunctions by coupling two or more semiconductors is an important strategy to develop stable and efficient photocatalysts able to operate both under near-UV and visible light. Five novel heterojunction systems were synthesized in the present study, using a modified sol-gel method: Bi2Mo3O12/TiO2, ZnFe2O4/TiO2, FeTiO3/TiO2, WO3(US)/TiO2 and WO3/TiO2. These heterojunction semiconductors were characterized by using XRD, SEM and EDX, UV–Vis diffuse reflectance spectroscopy and BET. Their photocatalytic activities were evaluated using methyl orange (MO) degradation under both near-UV and visible light. From the various heterojunctions developed, the WO3(US)/TiO2 photocatalyst was the one that showed the highest photocatalytic efficiency with this being assigned to the formation of a double heterojunction involving anatase, rutile and monoclinic WO3 phases. On this basis, a photocatalyst activation mechanism applicable to near-UV and visible light irradiation was proposed. This mechanism explains how the photogenerated electrons (e–) and positive holes (h+) can be transferred to the various phases. As a result, and given the reduced holes and electron recombination surface, hydroxyl radicals found were more abundant. To confirm this assumption, hole formation in the valence band was studied, using hole-scavenging reactions involving ion iodine (I–), while hydroxyl radical production used fluorescence spectroscopy.

GeSn/GeSiSn double-heterojunction short channel tunnel field-effect transistor design

The traditional GeSn/GeSiSn single-heterojunction (SH) n-channel tunneling field-effect transistor (NTFET) faces a serious off-state current (IOFF) breakdown and a subthreshold swing (SS) degradation when the channel length is scaled to 10 nm. In this work, we propose a double-heterojunction (DH) short channel NTFET by utilizing heterojunction engineering. A 60% lower IOFF value (4.83 × 10−10 A μm−1), a 6.2 times higher ION value (9.39 × 10−6 A μm−1) and an SS of 39.14 mV/dec are achieved in the DH NTFET at 0.3 V compared to the SH NTFET when the channel length is 10 nm.

Impact of thermal process on interfacial microstructures of Ti- and Pt-based electrodes on C-doped InGaAsSb

The authors investigate the impact of the thermal process on interfacial microstructures of Ti- and Pt-based electrodes deposited on C-doped InGaAsSb with the aim of improving base electrode formation in double heterojunction bipolar transistors (DBHTs). In a Ti-based electrode, as-deposited Ti reacts with oxygen originating from a native oxide of the C-doped InGaAsSb surface and forms a Ti–O transition layer at the Ti/InGaAsSb interface. After annealing at 300 °C, oxygen in the transition layer diffuses into the upper Ti layer, which causes a reduction of contact resistivity. Annealing at 400 °C causes significant interface degradation due to Ti-As reaction. In contrast, an Au/Pt/Ti/Pt electrode is more stable against annealing. However, Pt diffusion into the InGaAsSb to a depth of 20–30 nm is observed after annealing at 400 °C. To use Pt as the base electrode of ultrahigh-speed DHBTs with a thin InGaAsSb base, the thickness of the Pt layer must be carefully optimized to suppress the penetration of Pt into the base and collector regions.

Current Spreading in Back-Contacted GaInP/GaAs Light-Emitting Diodes

The recently proposed diffusion-driven charge transport (DDCT) method can allow a paradigm shift in the design of optoelectronic devices, by changing both the current injection principle and the device structure. The DDCT injection technique is based on the bipolar electron and hole diffusion currents that are used to electrically inject charge carriers into an active region (AR) located outside the p-n junction. In this article, we study an interdigitated back-contacted DDCT-light-emitting diode (LED) based on a GaInP/GaAs double heterojunction (DHJ) structure consisting of lateral heterojunctions (LHJs) located above a uniform AR. The structure uses single-sided electrical injection and is suitable for large-area applications and thin-film devices with near-surface ARs. Our analysis, based on charge transport simulations, suggests that the structure permits more efficient current spreading and lower surface recombination than conventional structures, leading to a very high internal quantum efficiency (IQE) and injection efficiency exceeding 99%. Particularly, we investigate the implications of using the new structure for improving the efficiency of LEDs, bringing them closer to the threshold of electroluminescent cooling (ELC). The results predict an above-unity internal power conversion efficiency for the DDCT-LEDs, substantially exceeding the efficiency of conventional reference devices, highlighting the new possibilities that DDCT devices offer especially for high-power ELC at room temperature.

Rational Band Engineering of an Organic Double Heterojunction for Artificial Synaptic Devices with Enhanced State Retention and Linear Update of Synaptic Weight.

Herein, we propose an organic double heterojunction to enable a nonvolatile step modulation of the conductance of an artificial synapse; the double heterojunction is composed of N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), copper phthalocyanine (CuPc), and para-sexiphenyl (p-6P). The carrier confinement in the CuPc region present in the double-heterojunction structure enabled the nonvolatile modulation of the postsynaptic current. The proposed organic synapse exhibited an excellent conductance change, characteristic with a nonlinearity (NL) value below 0.01 in the long-term potentiation (LTP) region. Furthermore, the NL value for long-term depression (LTD) could be reduced effectively from 45 to 3.5 by a pulse modulation technique. A simple artificial neural network (ANN) was theoretically designed using the LTP/LTD characteristic curves of such organic synapses, and then, learning and recognition tasks were performed using Modified National Institute of Standards and Technology digit images. A four-amplitude weight update method enabled considerable enhancement of the recognition rate from 53 to 70%. Although the designed ANN was based on a single-layer perceptron model, a high maximum accuracy of 75% was achieved. These newly studied techniques for synaptic devices are expected to open up new possibilities for the realization of artificial synapses based on organic double heterojunctions.

Effect of AlxGa1−xN buffer layer on the structural and electrical properties of AlGaN/GaN/AlxGa1−xN double heterojunction high electron mobility transistor structures

In this paper, the authors report the effect of the AlxGa1−xN buffer layer on the structural and electrical properties of an AlGaN/GaN/AlxGa1−xN double heterojunction high electron mobility transistor (HEMT). As the Al composition of the buffer layer increased, the two-dimensional electron gas (2DEG) confinement of the channel was shown to improve, which was confirmed by the simulation. The AlGaN buffer HEMT showed improved structural characteristics, such as the surface morphology, crystal quality, and interface roughness compared with the conventional HEMT with a C-doped GaN buffer. A slight decrease in 2DEG characteristics owing to the negative polarization charge was observed. However, in the breakdown voltage characteristics, comparable results were obtained as 652 V for the HEMT with C-doped GaN, 624 V for the HEMT with an Al0.044Ga0.956N buffer, and 642 V for the HEMT with an Al0.088Ga0.912N buffer, although the AlGaN buffers were not doped for semi-insulating.

High-Frequency Characteristics of InGaP/GaAs Double Heterojunction Bipolar Transistor Epitaxially Grown on 200 mm Ge/Si Wafers

N-p-n InGaP/GaAs double heterojunction bipolar transistor has been successfully grown on a 200 mm Ge/Si wafer using metalorganic chemical vapor deposition with low defect density of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Non-gold metals of Ni/Ge/Al and Ti/Al are used to form the ohmic contact for small pieces device fabrication. Both direct-current (dc) and high-frequency characteristics of the device were measured. The device with emitter area of 6 × 8 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> shows a dc gain of 55 at a collector current of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 4 mA, with high collector-emitter breakdown voltage of ~17 V. The high-frequency response with cutoff frequency (fT) of 23 GHz and maximum available frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) of 10 GHz can be achieved. These results demonstrate that InGaP/GaAs double heterojunction bipolar transistor grown on low defect density Ge/Si wafer has the potential for realizing III-V CMOS integrated platform for high-frequency applications.

Performance Analysis of Double Gate Hetero Junction Tunnel Fet

In this paper, a novel heterojunction tunnel field-effect transistor (HTFET) using Sentaurus technology computer-aided design (TCAD) simulations has been presented. The InAs/GaSb compound materials are used in both single gate heterojunction TFET (SG-HTFET) and Double gate heterojunction TFET (DG-HTFET) with SiO2 gate oxide layer to increase performance of the device.The implemented SG-HTFET and DG-HTFET device are increase the TFET's cross-sectional tunnel area. This result develops the subthreshold swing (SS) by 2.45 times, drive current (ION) is close to 10-6 A/µm, leakage current (IOFF) is close to 10-17 A/µm and also diminish the ambipolarity of the device compared to the TFET.

High-efficient separation of photoinduced carriers on double Z-scheme heterojunction for superior photocatalytic CO2 reduction

Developing heterojunction is one of promising approaches to acquire desired photocatalysts with high-efficient photocatalytic activity. In this work, sheet-like ternary ZnO/ZnWO4/g-C3N4 composite was synthesized via stepwise calcination treatment. The double interface electric fields built in the ZnO/ZnWO4/g-C3N4 heterojunction can promote efficient separation of photogenerated charge carriers in space. Moreover, in contrast with the individual ZnO, g-C3N4, ZnWO4 and their binary composites, this double Z-scheme heterojunction achieves more light harvesting, larger pore volume, stronger photoreduction capacity and CO2 adsorption capacity. Therefore, the sheet-like ZnO/ZnWO4/g-C3N4 heterojunction exhibits efficient conversion of the CO2 molecules into solar fuels under the light irradiation. The production yield of photocatalytic CO2 reduction over the double Z-scheme heterojunction is 13.19 μmol h−1 g−1 and the conversion rate of hydrocarbon fuel is highly up to 91.5%, which are much higher than that of other samples. This work offers a novel perspective to achieve high-efficiency heterojunction system for photoredox applications such as photocatalytic antibacterial, nitrogen fixation and degradation of pollutions.

Direct parameter extraction method for InP heterojunction bipolar transistors based on the combination of T- and π-models up to 110 GHz

A parameter-extraction approach for determination of the small-signal equivalent circuit model for InP/InGaAs double heterojunction bipolar transistors is presented in this paper. This method combines the advantages of conventional T- and π-type equivalent-circuit topologies. All the equivalent circuit elements are only extracted analytically from S-parameter data without any numerical optimization. The agreements between the measured and the model-calculated data are excellent in the frequency range of 0.1–110 GHz.

III-Nitride/Si Tandem Solar Cell for High Spectral Response: Key Attributes of Auto-tunneling Mechanisms

The key attributes of double hetero junction tandem solar cell based on III-Nitride alloys and silicon is investigated thoroughly. GaN/InGaN/GaN based tandem solar cells are predicting impressive efficiency, however due to its high cost; it is far from immediate implementation. Hence, an attempt is made to realize GaN/InGaN on crystalline silicon(c-Si) substrate that results in high spectral efficiency and cost effectiveness. The proposed multi-junction tandem solar cell consists of a GaN layer, intrinsic-InxGa1−xN (i-InGaN), tandem InxGa1−xN (t-InGaN) upon crystalline p-silicon and n-silicon layers. Energy band diagrams, current-voltage curve, power graph, electric field and potential graphs are explored using TCAD tool. Additionally, key parameters such as ’In’ content, thickness of layers, absorption coefficients, polarization charges are optimized. A remarkable conversion efficiency of 25.6% and 26.1% are achieved with & without polarization effect, respectively and became a potential candidate for next-generation photovoltaics applications.