Double Heterojunction Structure Research Articles

Construction of TiO2/WO3/TiO2 double heterojunction films for excellent electrochromic performance

Electrochromic devices are applied extensively to camouflages, smart windows, heat insulation layers, and automobile rearview mirrors, etc. The amorphous WO3 is a very attractive electrochromic material, whereas it suffers from degradation of optical modulation and reversibility on ion exchange owing to those deep trapped ions with irreversible reaction behavior. Herein, we designed and, by using magnetron sputtering, prepared a composite film with TiO2/WO3/TiO2 double heterojunctions, which is capable of eliminating the deep trapped ions by itself under ultraviolet light (UV) assistance. The electrochromic device based on this composite film, after being recovery by short-time UV irradiation, can maintain a high transmission modulation of 94.72% after 7000 cycles of the voltammetry measurement. This feature allows the device to maintain its initial electrochromic performance after prolonged use. Moreover, the double heterojunction structure can reduce colouring time and enormously improve the colouration efficiency (CE) of electrochromic devices. Experimental research shows that when the thickness of the bottom and upper TiO2 layer of the WO3 film was 145.5 nm and 97.0 nm, respectively, the CE of electrochromic devices reached a perfectly high value (479.3 cm2/C), being much higher than that of WO3 devices (69.5 cm2/C). Functions of the TiO2/WO3/TiO2 double heterojunction in electrochromic device were investigated by combining theoretical analysis and experiment validation, and these results provide a general framework for developing and designing superior electrochromic materials and devices.

Visible-light-induced degradation of rhodamine B by double Z-scheme magnetic ATP-g-C3N4/SnFe2O4/Bi2WO6 heterojunctions: Performance and mechanism

A magnetic ternary double Z-scheme ATP-g-C3N4/SnFe2O4/Bi2WO6 (AG/SFO/BWO) heterojunction was prepared by a simple and green hydrothermal process, which was successfully used in the dye degradation. AG/7.5 %SFO/BWO catalyst demonstrated outstanding visible-light-induced degradation, achieving an impressive 96.72 % degradation of rhodamine B (RhB) in 2 h. Compared with Bi2WO6 (BWO) and SnFe2O4/Bi2WO6 (SFO/BWO), the degradation efficacy was increased about 1.65 and 1.16 times, respectively. After being used for five batches, the degradation rate of RhB remained over 90 %. Moreover, AG/7.5 %SFO/BWO showcased remarkable efficacy in degrading methylene blue (MB), Congo red (CR), and tetracycline (TC). Bacterial toxicity test indicated the toxicity of dyes was rapidly decreased in 24 h. Physicochemical properties and morphological characteristics of the prepared photocatalysts were investigated by various methods, including XRD, FTIR, XPS, SEM, TEM, BET, VSM, UV–vis, and EIS. AG/7.5 %SFO/BWO, utilizing a dual-Z scheme pathway, efficiently absorbed broad light, yielding multiple active agents, primarily hole (h+) and superoxide radical •O2−. The double Z-scheme heterojunction structure enhanced the electron-hole separation and ensured a stable photochemical environment. UPLC-HRMS was used to identify intermediates during the RhB degradation. Its degradation pathway and catalytic mechanism were illustrated systematically. The bio-toxicity (acute toxicity, developmental toxicity, and mutagenicity) of the intermediates was also evaluated by T.E.S.T.

Gain and excess noise properties of 3-gain-stage InGaAs/InAlAs avalanche photodetector

—We constructed a model for a 3-stage InGaAs/InAlAs avalanche photodiode using numerical simulations based on the gain-noise mechanism. The contact layer, barrier layer, charge layer and multiplication layer of this model are optimized accordingly. The performance of the device can be improved by optimizing the thickness and doping concentration of the contact layer, increasing the thickness of the charge layer, and adding a barrier layer. At the same time, the addition of double heterojunction structure makes the performance of the device better, which enables a higher impact ionization rate of the multiplication layer. According to DSMT theory calculation results, the optimized 3-gain-stage device's gain is 90, and the excess noise factor is about 3.1. Compared with the 10-stage avalanche photodiode (with an excess noise factor of 5) studied at the same gain, this work shows higher performance at the lower stage. The optimized structure can provide design and optimization ideas for more stages of avalanche photodiode to increase gain and reduce excess noise.

Visible-blind near-infrared organic photodetectors

The presently available commercial photodetectors have a broadband photoresponse and require different external optical filters to block the undesired light outside the detection spectrum window, e.g., near-infrared (NIR) detection. The use of an NIR bandpass has technical limitations in curved or flexible large-area photodetectors, as an NIR bandpass depends critically on the difference in the interference of the optical path in the filter. This work reports the effort to develop a high-performance filter-free organic photodetector (OPD) with a double bulk heterojunction (BHJ) structure. The visible-blind NIR photodetection is realized by eliminating the photocurrent generated in the front visible light-absorbing BHJ optical depletion layer, due to the suppression of electron transport by incorporating a copper thiocyanate-based electron-blocking layer in the OPD. Only excitons generated in the rear NIR-absorbing BHJ contribute to photocurrent in the OPD, thereby achieving visible-blind NIR photodetection. The double BHJ OPD has a high responsivity of 0.38 A/W at 1050 nm and a specific detectivity of >1013 Jones over the wavelength range from 800 to 1050 nm, making it an ideal candidate for applications in optical communication, food quality detection, wellness monitoring, and NIR image sensors.

Synthesis of double Z-scheme CdS/Bi2O2CO3/BiOCl heterojunction photocatalysts for degradation of rhodamine B under visible light

A novel double heterojunction structure provides rapid electron transfer paths, which can degrade rhodamine B efficiently.

An oxygen-vacancy rich ZnFe2O4/BiOI/AgI heterojunction for enhanced photocatalytic and photo-Fenton performance via double Z-scheme structure

As a bridge, BiOI nanosheets connect ZnFe2O4 nanoparticles and AgI clusters (∼300 nm) via hydrothermal-coprecipitation. A ternary composite with a double Z-type heterojunction structure is obtained through precise electronic regulation. Compared with ZnFe2O4 and ZnFe2O4/BiOI, the photo-Fenton degradation RhB of ZnFe2O4/BiOI-AgI-3 samples is obviously improved under simulated sunlight. Rich oxygen vacancies also provide more active sites for photocatalysis and photo-Fenton process. The main active specie in photo-Fenton degradation is ·OH. The existence of a double Z-heterojunction structure and Ag0 makes the electrons on the interface transfer and separate effectively. Notably, the intensity of PL luminescence ZnFe2O4/BiOI/AgI is lower than that of ZnFe2O4 and ZnFe2O4/BiOI because of reducing the recombination rate of photogenerated carriers. ZnFe2O4/BiOI/AgI composites have excellent stability of photo-Fenton degradation. Furthermore, the formation of a built-in electric field inhibits the recombination of electron-hole pairs, enhances visible light absorption, results in improving the efficiency of ternary composite materials to degrade pollutants.

Surface Electron Localization in Cu-MOF-Bonded Double-Heterojunction Cu2O Induces Highly Efficient Photocatalytic CO2 Reduction.

Truncated octahedron Cu2O (TOC) has attracted more attention for its suitable band gap and high carrier separation efficiency due to introduction of the facet heterojunction, but its practical drawback is still the instability caused by the irreversible disproportionation reaction (Cu2O → Cu + CuO). Here, we design and fabricate the TOC/Cu-MOF (MOF: metal-organic framework) double-heterojunction structures with different Cu-MOF loadings. The introduced heterojunction between TOC and Cu-MOF not only produces a stable interface Cux+ bonding structure with the electronic states localized within the average collisional diameter of electrons 1.72 nm for TOC/2.1 wt %Cu-MOF as the active sites, but also promotes the surface energy level difference between the (100) and (111) facet heterojunctions. Meanwhile, the bonded Cu-MOF with a narrow bandgap effectively consumes holes by recombination with the photoexcited electrons from Cu-MOF itself. In our experiments, the TOC/Cu-MOF double heterostructure with a loading amount of 2.1 wt % Cu-MOF shows an optimal photocatalytic CO2 reduction performance. The CO evolution rate reaches 23.01 μmol g-1 h-1, which is about 2.01 and 4.47 times larger than those of octahedral and hexahedral Cu2O/Cu-MOF, respectively, and an excellent photostability is shown for four cycles with each cycle lasting for 4 h. Such a double heterostructure provides insight into highly efficient electron transfer and photostability in Cu2O-related composite materials.

Double-Layered NiO/SnO2 Sensor for Improved SO2 Gas Sensing with MEMS Microheater Device

Micro Electro mechanical systems (MEMS) sensor is fabricated for testing low concentration gas sensing of sulphur dioxide (SO2) with sensing layers of single layer tin oxide (SnO2) and double layered heterojunction structure of nickel oxide and tin oxide (NiO/SnO2). NiO and SnO2 structures are deposited with RF sputtering and the elemental composition were identified with structural properties such as X-ray diffraction (XRD), Scanning electron microscope (SEM) and Energy Dispersive X-ray Analysis (EDX) analysis. Sensing results proved that NiO/SnO2 double layered sensor had better sensing characteristics than single layered SnO2 sensor due to the formation of p-n junctions. At 400 ppb of SO2 gas concentration, NiO/SnO2 sensor has maximum sensing response of 20% is recorded and at 2000 ppb, 30% sensing response is recoded. The optimal temperature of the sensor is 250 °C (∼63 mW). Selectivity of the sensor is tested with 5 different gases such as VOC, pyruvate, CO, NH3, SO2 and the sensor has high and better response with SO2 gas.

0D/2D/2D ZnFe2O4/Bi2O2CO3/BiOBr double Z-scheme heterojunctions for the removal of tetracycline antibiotics by permonosulfate activation: Photocatalytic and non-photocatalytic mechanisms, radical and non-radical pathways

• Efficient removal of TCs was achieved by the catalyst/sunlight/PMS systems. • The double Z-scheme heterojunction structure was proved by multiple characterizations and DFT calculation. • 1 O 2 was the dominant species for TCs degradation rather than SO 4 ·− or ·OH. • Photocatalytic and non-photocatalytic processes were distinguished. • Radical and non-radical pathways were also elucidated. A series of environmental-friendly and cost-effective 0D/2D/2D ZnFe 2 O 4 /Bi 2 O 2 CO 3 /BiOBr double Z-scheme heterojunctions (abbreviated as xZnCB) were fabricated by a hydrothermal method using an electrostatic self-assembly strategy. The removal efficiencies toward tetracycline (TC), oxytetracycline (OTC), and doxycycline (DOX) were 93%, 90.1%, and 89.4% in 10ZnCB/sunlight/permonosulfate (PMS) systems within 20 min, and the degradation rate constants of 10ZnCB were 5.7–8.2 times higher than those of ZnFe 2 O 4 and 2.6–2.9 times higher than those of Bi 2 O 2 CO 3 /BiOBr (CB) under the same conditions. XPS valence band spectra, quenching experiments, EPR measurements, and density functional theory (DFT) calculations demonstrated the existence of the double Z-scheme heterojunctions. The coexistence of radical and non-radical pathways caused by photocatalytic and non-photocatalytic degradation mechanisms was also elucidated. PMS was effectively activated by a photoinduced e − , · O 2 − , and Fe(II)/Fe(III) pathway, and active species including · O 2 − , 1 O 2 , SO 4 · − , · OH, and h + participated in the catalytic processes. Unexpectedly, 1 O 2 , and · OH were determined to be the primary species during the degradation processes instead of SO 4 · − . Recycling experiments, metal leaching measurements, and TC degradation experiments in the secondary effluent demonstrated the excellent catalytic and structural properties of these xZnCB hybrid materials.

Microbial coupled photocatalytic fuel cell with a double Z-scheme g-C3N4/ZnO/Bi4O5Br2 cathode for the degradation of different organic pollutants

A novel heterostructure of g-C3N4/ZnO/Bi4O5Br2 (ZB-3) was designed, and used in the microbial coupled photocatalytic fuel cell (MPFC). It can effectively improve electron utilization efficiency and pollutant degradation using this double Z-scheme heterojunction structure. The current–time (I–t) curves demonstrated that the current density of ZB-3 was higher than that of ZnO, ZnO/Bi4O5Br2 (ZB-1), g-C3N4/ZnO (ZB-2). Electrochemical impedance spectroscopy (EIS) indicated ZB-3 possessed the minimum charge-transfer resistance. This MPFC for degrading rhodamine B (RhB) and tetracycline (TC) under different conditions were developed using these materials. Even in the dark condition, MPFC with g-C3N4/ZnO/Bi4O5Br2 demonstrated 93% and 82% degradation efficiency for RhB and TC, respectively. Furthermore, the electron transport mechanism of the MPFC and ZB-3 were proposed. It paves the approach for more efficient pollutant degradation via MFC photocatalysis.

Enhancement of the near-band-edge electroluminescence from the active ZnO layer in the ZnO/GaN-based light emitting diodes using AlN-ZnO/ZnO/AlN-ZnO double heterojunction structure

In this work, an AlN-ZnO/ZnO/AlN-ZnO double heterojunction (DH) structure prepared using the cosputtering technology was deposited onto the p-type GaN epitaxial layer. The indiffusion of the oxygen atoms to the p-GaN epilayer was obstructed as the cosputtered AlN-ZnO film inset between n-ZnO/p-GaN interface. The near-ultraviolet (UV) emission from this ZnO/GaN-based light emitting diode (LED) was greatly improved as compared to an n-type ZnO film directly deposited onto the p-GaN epilayer. Meanwhile, the native defects in the n-ZnO layer associated with the green luminescence was less likely to form while it was sandwiched by the cosputtered AlN-ZnO film. As the thickness of the active n-ZnO layer in the DH structure reached 10 nm, the near-band-edge (NBE) emission became the predominated luminescence over the resulting LED spectrum.

Electrically Switchable Color-Selective Organic Photodetectors for Full-Color Imaging.

The present full-color imaging techniques rely on the use of broadband inorganic photodetectors with dedicated color filters, which is one of the practical challenges for large-area, flexible, and high-solution imaging applications. The development of high-performance color-selective photodetectors is one of the key solutions to overcome this challenge. In this work, an electrically switchable color-selective organic photodetector (OPD) comprising a double organic bulk heterojunction structure has been developed for full-color imaging. The color-selective sensing capability over the visible spectrum ranges can be realized by controlling the bias across the OPD, achieving a high responsivity of ∼200 mA/W, a large linear dynamic range of 122 dB, a viewing angle of 120°, and a -3 dB cutoff frequency of >50 kHz. A full-color imaging function has been demonstrated using electrically switchable red-, green-, and blue-color selective OPD sensors with an excellent operational stability. The results of this work provide a practical solution for applications in high-resolution full-color imaging and artificial vision.

High-Performance Organic Solar Cells Featuring Double Bulk Heterojunction Structures with Vertical-Gradient Selenium Heterocyclic Nonfullerene Acceptor Concentrations.

In this study, we prepared organic photovoltaics (OPVs) featuring an active layer comprising double bulk heterojunction (BHJ) structures, featuring binary blends of a polymer donor and concentration gradients of two small-molecule acceptors. After forming the first BHJ structure by spin-coating, the second BHJ layer was transfer-printed onto the first using polydimethylsiloxane stamps. A specially designed selenium heterocyclic small-molecule acceptor (Y6-Se-4Cl) was employed as the second acceptor in the BHJ. X-ray photoelectron spectroscopy revealed that the two acceptors formed a gradient concentration profile across the active layer, thereby facilitating charge transportation. The best power conversion efficiencies (PCEs) for the double-BHJ-structured devices incorporating PM6:Y6-Se-4Cl/PM6:Y6 and PM6:Y6-Se-4Cl/PM6:IT-4Cl were 16.4 and 15.8%, respectively; these values were higher than those of devices having one-BHJ structures based on PM6:Y6-Se-4Cl (15.0%), PM6:Y6 (15.4%), and PM6:IT-4Cl (11.6%), presumably because of the favorable vertical concentration gradient of the selenium-containing small-molecule Y6-Se-4Cl in the active layer as well as some complementary light absorption. Thus, combining two BHJ structures with a concentration gradient of the two small-molecule acceptors can be an effective approach for enhancing the PCEs of OPVs.

Preparation of TiO2/Sb–SnO2 composite by a polymer pyrolysis method for conducting fillers

Study and development of multifunctional titanium dioxide (TiO2)-based composite have been focused on by researchers for years. Herein, TiO2-based antimony doped tin oxide (TiO2/Sb–SnO2) conductive composite was facilely prepared by a polymer pyrolysis method. The obtained composite was characterized by thermogravimetry-differential thermal analysis(TG-DTA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray fluorescence spectrometer (XRF), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and UV–Vis diffuse reflectance (DRS). The Ti–Sn–Sb polyacrylate precursor was pyrolyzed to prepare a highly crystalline TiO2/Sb–SnO2 composite, and the pyrolysis reaction mechanism was discussed. The TiO2/Sb–SnO2 composite whose molar ratio of Ti to Sn was 5:1 with 8.3% Sb being doped into SnO2, had the narrowest equivalent energy gap of 2.79 eV and the lowest charge transfer resistance, which led to the cascading charge transfer fast and enhances the conductivity, with a resistivity of 15.42 Ω cm. The solid-solid phase interfacial synergism effect and the “staggered” type double heterojunction structure of TiO2/Sb–SnO2 composite both promoted the electrons migrating from TiO2(A) to TiO2(R) and further to Sb–SnO2 in a cascade way. The results give a novel strategy for industrially manufacturing conductive composite filler.

Fabrication of flower-globular Bi2WO6/BiOI@Ag3PO4 photocatalyst for the degradation of bisphenol A and cefepime under sunlight: Photoelectric properties, degradation performance, mechanism and biodegradability enhancement

• A new Bi 2 WO 6 /BiOI@Ag 3 PO 4 catalyst was prepared by one-step hydrothermal method. • The introduction of Ag 3 PO 4 enhanced evidently the photoelectric properties. • High-efficient photocatalytic degradation of BPA and cefepime was presented. • The high photocatalytic activity was due to the double-heterojunction structure. • Photocatalytic degradation could enhance the biodegradability of cefepime solution. A flower-globular photocatalyst (Bi 2 WO 6 /BiOI@Ag 3 PO 4 -5) with symmetric double-heterojunction structure and low Ag-content was successfully fabricated for the first time by precipitation and one-step hydrothermal process. The loading of Ag 3 PO 4 evidently enhanced the photoelectric properties of photocatalysts. Bi 2 WO 6 /BiOI@Ag 3 PO 4 -5 provided an ideal photocatalytic activity towards the degradation of bisphenol A (BPA) and cefepime under simulated sunlight, and it also exhibited well photocatalytic stability. For BPA degradation, the recommend photocatalyst provided nearly 100% of degradation efficiency and 65.4% TOC removal by the simulated sunlight irradiation of 120 min, while for cefepime degradation, it offered the degradation efficiency of 98.2% and the TOC removal efficiency of 33.4%. The mechanism of photocatalytic activity enhancement was owing to the loading of Ag 3 PO 4 and the developed double-heterojunction structure, by which the absorbance of visible light, the separation of electron/hole (e – /h + ) pairs and carrier mobility were enhanced evidently. It was found that superoxide anion radical (∙O 2 – ) played the major role in BPA degradation, while hydroxyl radical (∙OH) and hole (h + ) together played the key role in cefepime degradation. The two pathways for BPA and cefepime degradation were respectively proposed by identifying the byproducts formed during the degradation. In addition, it was found that the photocatalytic degradation of cefepime evidently enhanced the biodegradability of the solution.

Ag/Nanodiamond/g-C3N4 heterostructures with enhanced visible-light photocatalytic performance

Efficient separation of photogenerated carriers and light harvesting are the two key factors for high photocatalytic performance. In this work, a novel double-heterojunction silver/nanodiamond/graphite-carbon nitride (Ag/ND/g-C3N4) structured photocatalyst has been synthesized. In comparison to single-heterojunction Ag/g-C3N4, the double-heterojunction Ag/ND/g-C3N4 photocatalyst presents with a 4.9-times enhancement in H2 evolution from water splitting and 2.4-times promotion in degrading organic pollution under visible-light irradiation. Furthermore, the Ag/ND/g-C3N4 exhibits good photostability after four-run test. The enhanced photocatalytic activity can be attributed to the synergetic effect of surface plasmon resonance from Ag, optical confinement from ND and suitable band alignment in the double-heterojunction structure, which boost the light absorption capacity and increase the photogenerated charge separation of g-C3N4. This study provides a reference for the development of efficient carbon based photocatalysts, which can be used not only for the production of H2 from water splitting, but also for the photodegradation of pollutants under visible light irradiation.

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.

Investigation on the Deposition of an AlN-ZnO/ZnO/AlN-ZnO Double Heterojunction Structure Using Radio Frequency Magnetron Cosputtering Technology

A symmetric AlN-ZnO/ZnO/AlN-ZnO double heterojunction structure was consecutively deposited onto silicon substrate using cosputtering technology and then annealed at 700 °C under vacuum ambient for 30 min. The crystalline quality of the ZnO film in the heterojunction structure was significantly improved as verified by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Improvement on the crystalline structure was ascribed to the stress in the ZnO active film, which was effectively buffered by the underlayered AlN-ZnO layer. Native oxygen vacancies in the ZnO film also were effectively suppressed due to a little diffusion of the Al atoms from the cosputtered AlN-ZnO layer, and led to an increase in the carrier concentration. Such ZnO film deposited onto the homogeneous AlN-ZnO buffer layer emitted an intense near-band-edge emission, and the deep level emission was absent. The ultraviolet emission was further enhanced by covering an AlN-ZnO barrier laye, which was a consequence of the improvement on the carrier confinement. Accordingly, single ultraviolet emission with a quality ZnO crystalline structure, which is very promising for application in short-wavelength optoelectronic devices, was realized from the ZnO film sandwiched by the homogeneity of the cosputtered AlN-ZnO layers.

Rectifying behavior in the GaN/graded-AlxGa1−xN/GaN double heterojunction structure

Rectifying characteristics induced by the polarization fields are achieved in the GaN/graded-AlxGa1−xN/GaN double heterojunction structure (DHS). By grading AlxGa1−xN from x = 0.4(0.3) to 0.1, the DHS displays a better conductivity for smaller reverse bias than for forward bias voltages (reverse rectifying behavior) which is opposite to p–n junction rectifying characteristics. The mechanism of reverse rectifying behavior is illustrated via calculating the energy band structures of the samples. The band gap narrowing caused by decreasing Al composition could compensate the for the band tilt due to the polarization effect in AlxGa1−xN barriers, thus lowering the barrier height for electron transport from top to bottom. The reverse rectifying behavior could be enhanced by increasing the Al content and the thickness of the multi-layer graded AlxGa1−xN barriers. This work gives a better understanding of the mechanism of carrier transport in a DHS and makes it possible to realize novel GaN-based heterojunction transistors.