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A high-performance E-mode hybrid ferroelectric charge storage gate (FEG) GaN HEMT with an innovative source-connected field plate (SCFP) structure is reported for the first time. The optimized E-mode FEG-HEMT implements a novel SCFP structure, which creates a cascode-like configuration with a D-mode GaN MISHEMT. This E-mode SCFP GaN FEG-HEMT has a positive ${V}_{\text {th}}$ of 2.81 V, a high ${I}_{\text {D,max}}$ of 757mA/mm, and a BV of 866 V. Dynamic ${R}_{\text {ON}}$ reduced 25% when operated at ${V}_{\text {DSQ}} =400$ V. ON-state stress tests also show improved current collapse phenomena. Additionally, to address the charge storage abilities of FEG-HEMTs, a multi-cycle OFF-state ( ${V}_{\text {DS}} =300$ V, ${V}_{\text {GS}} =0$ V) retention test was conducted. The SCFP FEG-HEMT showed 58.46% less ${V}_{\text {th}}$ shift percentage than the FEG-HEMT without field plates.

The hybrid gate-recessed structure and ferroelectric charge trapping/storage stacked structure were used in AlGaN/GaN enhancement-mode metal–oxide–semiconductor high-electron mobility transistors (E-MOSHEMTs). The stacked structure consisted of a bottom 10-nm-thick Al2O3 tunnel oxide layer, a middle 4-nm-thick HfO2 charge trapping/storage layer, and a top 40-nm-thick LiNbO3 ferroelectric blocking layer. Using the postannealing process, a high polarized C+ domain of the pulsed laser-deposited LiNbO3 ferroelectric blocking layer was formed and the interface quality of the stacked structure was improved simultaneously. Consequently, positive threshold voltage for enhancement-mode operation was obtained. By studying the performances of the resulting devices, it was found that enhancement-mode behaviors were observed for initializing gate more than 5 V, and the threshold voltage of 1.6 V was obtained for initializing gate at 12 V. The high-frequency performances of the extrinsic unit gain cutoff frequency of 5.9 GHz and maximum oscillation frequency of 10.1 GHz were obtained. Furthermore, a low normalized noise power density of about $1.9\times 10^{-14}$ Hz−1 was achieved for the devices operating at a frequency of 10 Hz, a gate–source voltage of 5 V, and a drain–source voltage of 1 V.

In this study, intrinsic Ga2O3 (i-Ga2O3) film was deposited at about 80 K using a vapor cooling condensation system. Its bandgap energy was 5.0 eV. Low oxygen vacancy and defects were verified by using photoluminescence and Hall measurements. When a 40-nm-thick i-Ga2O3 film was used as the gate dielectric layer of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOS-HEMTs), threshold voltage, and gate breakdown voltage were − 3.5 V and − 538.0 V, respectively. The associated gate leakage current of the devices operating at a gate-source voltage of − 100 V was 0.57 μA. Furthermore, a saturation drain-source current of 186.2 mA/mm and a maximum extrinsic transconductance of 85.8 mS/mm were obtained for the devices operating at a gate-source voltage of 0 V and a drain-source voltage of 10 V. The unit gain cutoff frequency and the maximum oscillation frequency were 5.7 GHz and 11.0 GHz, respectively. The normalized noise and Hooge’s coefficient were 3.79 × 10−14 Hz−1 and 5.06 × 10−5, respectively, when the devices operated at a frequency of 100 Hz, with a drain-source voltage of 1 V and a gate-source voltage of 5 V.

In this paper, we demonstrate the AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrate using an AlGaN back-barrier (BB) and super-lattice (SL) buffer to achieve high breakdown and low current collapse (CC) properties for Radio-frequency (RF) applications. The HEMTs also demonstrated low initial vertical leakage current, high thermal stability, and smaller drain-lag with reduced leakage currents as compared to the devices using step-graded (SG) AlGaN buffer. Also, the device showed improved RF performances with higher f T and f max as compared to the devices with SG buffer.

An improvement in off-state leakage current and cutoff frequency for AlGaN/GaN high electron mobility transistors (HEMTs) is investigated. Raman spectroscopy confirms the low stress of GaN heterostructure grown on a silicon-on-insulator (SOI) substrate. The HEMT devices on SOI substrate show lower knee voltage (Vknee) and on-resistance (RON) compared to those on the high-resistive silicon (HR-Si) substrates by 20.8% and 30.4%, respectively. Off-state leakage current is reduced to 10−7 A mm−1, and the cutoff frequency (f t ) is increased by 19.2% as compared to HR-Si substrate. Thus, the GaN/SOI technology is proven to be a potential technology for high-frequency communication applications.

The performances of nitrogen dioxide (NO2) gas sensors using various dimensional tantalum pentoxide-alloyed indium oxide (Ta:In2O3) rod arrays were investigated, in which the rod arrays were patterned by a laser interference photolithography system. The rod number amount and the surface area-to-volume ratio increased with a decrease of the period of rod array. The best responsivity of 76.1 and the lower operation temperature of 110 °C for the NO2 gas sensors using the sensing membranes of 0.6- $\mu \text{m}$ -periodic Ta:In2O3 rod array were obtained. Besides, the limiting sensing concentration of 0.7 ppm could be detected using the associated NO2 gas sensors. Furthermore, the response time and the recovery time were respectively reduced to 48 s and 329 s, respectively, when the NO2 gas sensors operated at 110 °C under NO2 gas concentration of 100 ppm.

In this article, to study the features of the quadruple gate and quadruple T-gate structures in AlGaN/ GaN fin-nanochannel array metal–oxide–semiconductor high-electron-mobility transistors (MOSHEMTs), an ELS-7500 electron-beam lithography system was used to pattern the 80-nm-wide fin-nanochannel array, and the quadruple gate and quadruple T-gate structures. Compared with the conventional single-gate structure, the saturation drain–source current was improved from 1153 to 1958 mA/mm and 2009 mA/mm by using the quadruple gate structure and quadruple T-gate structure, respectively. Furthermore, the maximum extrinsic transconductance was also improved from 265 to 323 mS/mm and 340 mS/mm, respectively. Compared with the unit gain cutoff frequency ( ${f}_{T}{)}$ of 7.7 GHz and the maximum oscillation frequency ( ${f}_{{\text {max}}}{)}$ of 15.1 GHz of the conventional single gate, the ${f}_{T} = {12.0}$ GHz and ${f}_{{\text {max}}} = {18.6}$ GHz were obtained by using the quadruple gate structure. If the quadruple T-gate structure was utilized, the ${f}_{T}$ and ${f}_{{\text {max}}}$ were further improved to 14.8 and 29.5 GHz. By simulating the equivalent circuits, the improved mechanisms were attributed to the enhancement of transconductance and the reduction of parasitic gate resistance and parasitic gate capacitance.

In this study, vanadium (V) was used as a top electrode in a vanadium oxide (VOx)-based selector device. Electroforming was performed to form a threshold region with metal–insulator transition (MIT) at both V electrode and VOx switching layers without annealing during fabrication. The simple V/VOx/TiN structure obtained was scalable and compatible with complementary metal–oxide–semiconductors. All electrical measurement results indicated that V/VOx/TiN selectors exhibited excellent characteristics such as high uniformity, a short switching time of 60 ns, a robust endurance of >109 cycles, and high reliability and stability. Both the simulation analysis and current fitting method were applied to further validate experimental results. Consequently, we developed a nanoscale V/VOx/TiN selector with excellent characteristics, which can be potentially applied to future highdensity crossbar memory devices to overcome sneak-path problems.

In this article, fin-gated nanochannel array gate-recessed AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs) were fabricated, in which the gate oxide layer was directly grown using the photoelectrochemical (PEC) oxidation method, the gate-recessed structure was formed using the PEC etching method, and the nanochannel array was patterned using the electron-beam lithography system. The improved gate controllability was obtained in devices with a narrower channel width due to the lateral field effect in comparison with those of the conventional planar AlGaN/GaN MOSHEMTs. A threshold voltage of −0.30, −0.35, and −2.3 V, and a subthreshold swing of 95, 109, and 372 mV/dec, were respectively obtained for the AlGaN/GaN MOSHEMTs with a channel width of 80 and 100 nm, and with a planar channel. Furthermore, the associated extrinsic transconductance of 269, 253, and 93 mS/mm was obtained to verify the improved performance of AlGaN/GaN MOSHEMTs using a narrower channel array. Besides, the low-noise and high-frequency performances were also enhanced using a narrower channel width in the fin-gated nanochannel array gate-recessed AlGaN/GaN MOSHEMTs.

NiO layer and NiO/Ag/NiO triple layers were respectively deposited on indium-tin-oxide-coated glass substrates as the hole transport layers of MAPbI3 and FAxMA1−xPbI3 perovskite solar cells using a radio frequency magnetron sputtering system. Compared with the 60-nm-thick NiO hole transport layer with an average transmission of 66.8% between the wavelength range from 300 to 800 nm, the average transmission was improved to 70.4% for the NiO/Ag/NiO (30/7/30 nm) hole transport layer. Consequently, the power conversion efficiency of MAPbI3 perovskite solar cells using the NiO/Ag/NiO (30/7/30 nm) hole transport layer was improved from 11.02 to 11.68%. To broaden light absorption, various FAxMA1−xPbI3 active layers were obtained by blending various formamidine iodide (FAI) contents with methylammonium iodide (MAI). The power conversion efficiency of the FA0.1MA0.9PbI3 perovskite solar cells using 60-nm-thick NiO hole transport layer was improved to 12.30%. By integrating the optimal FA0.1MA0.9PbI3 active layer and the optimal NiO/Ag/NiO (30/7/30 nm) hole transport layer, the power conversion efficiency of the resulting perovskite solar cells was further improved to 12.67%.

In this study, to compare the performance of planar, fin-submicron, and fin-nanochannel array-structured AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs), the scaling effect of fin-channels was investigated by decreasing the nanochannel width to 50 nm using an electron-beam lithography system. The photoelectrochemical oxide method was used to directly oxidize the AlGaN layer into a gate oxide layer and to passivate the fin-nanochannel array. Consequently, the low-noise performance and Hooge's coefficient were improved in AlGaN/GaN fin-nanochannel MOSHEMTs with narrower fin-channels. The improvement was attributed to the effective passivation and the screening effect of trapping probability. Moreover, owing to the improvement in gate control capability caused by the fin structure and the improvement in heat dissipation caused by the lateral heat flow, the direct current and high-frequency performances were improved by using a narrower fin-channel in AlGaN/GaN fin-nanochannel array MOSHEMTs.

In this study, the effects of AlN/GaN superlattice (SL) thickness on performances of AlGaN/GaN high electron mobility transistor (HEMT) heterostructure grown by metal-organic chemical vapor deposition on silicon is investigated. Stress in GaN is controlled by varying the total thickness of the AlN/GaN SL. Improved crystal quality and surface roughness accomplished with 2200 nm-thick AlN/GaN SL, leads to an increase in high electron mobility (1760 cm2 (V s)−1) as well as two-dimensional electron gas concentration (1.04 × 1013 cm−2). AlGaN/GaN metal–insulator-semiconductor HEMT (MIS-HEMT) fabricated on the heterostructure with SL buffer layer exhibits a significant improvement in maximum saturation current of 1100 ± 29 mA mm−1 at V GS = 0 V and a low on-resistance of 4.3 ± 0.15 Ω mm for the optimized AlN/GaN SL. The 2200 nm-thick AlN/GaN SL supports the growth of stress-free GaN heterostructure, which can reduce the insertion loss for sub-6 GHz radio frequency (RF) applications. This GaN HEMT structure based on SL buffer layer is suitable for low-frequency RF power applications.

In this paper, we investigated the tri-gate InxGa1−xAs high electron mobility transistors (HEMTs) with different indium compositions for low noise applications. The tri-gate configuration in this work exhibits better device performances of the transconductance (gm), the current gain cutoff frequency (fT), the maximum oscillation frequency (fmax), and the minimum noise figure compared with the conventional planar structure due to the improved gate controllability. The indium contents of 20%, 40%, and 100% tri-gate devices were also fabricated for comparison. The resistances and leakage current were measured to investigate the effect between the indium compositions and the noise figure. Experimental results exhibited that the tri-gate In0.4Ga0.6As mHEMT with neither the highest transconductance nor the lowest gate leakage current exhibited the lowest minimum noise figure (NFmin) of 2 dB when operating at 50 GHz. The study reveals that device architecture and indium composition are important factors in the determination of noise performance.

Multiple-mesa-fin-channel array patterned by a laser interference photolithography system and gallium oxide (Ga2O3) gate oxide layer deposited by a vapor cooling condensation system were employed in double-channel Al0.83In0.17N/GaN/Al0.18Ga0.82N/GaN heterostructured-metal-oxide-semiconductors (MOSHEMTs). The double-channel was constructed by the polarized Al0.18Ga0.82N/GaN channel 1 and band discontinued lattice-matched Al0.83In0.17N/GaN channel 2. Because of the superior gate control capability, the generally induced double-hump transconductance characteristics of double-channel MOSHEMTs were not obtained in the devices. The superior gate control capability was contributed by the side-wall electrical field modulation in the fin-channel. Owing to the high-insulating Ga2O3 gate oxide layer and the high-quality interface between the Ga2O3 and GaN layers, low noise power density of 8.7 × 10−14 Hz−1 and low Hooge’s coefficient of 6.25 × 10−6 of flicker noise were obtained. Furthermore, the devices had a unit gain cutoff frequency of 6.5 GHz and a maximal oscillation frequency of 12.6 GHz.

A high frequency enhancement mode quaternary InAlGaN/GaN MIS-HEMT with recessed gate (Lg = 150 nm) processed using an oxygen-based digital etching technique is presented. The digital etching was performed by cyclic ICP oxygen treatment to oxidize InAlGaN barrier and HCl wet etching to remove the oxidized layer. In this study, we have demonstrated that the threshold voltage can be adjusted in a wide-range from depletion mode to enhancement mode with a nanometer scale gate for high frequency InAlGaN/GaN MIS-HEMT using the digital etching technique. In addition, the etch rate can be controlled from 0.7 nm/cycle to 3.6 nm cycle−1 with RF bias power changing from 0 W to 40 W with high flexibility in etching rate. The post-etching surface roughness was around 0.12 nm regardless of the ICP oxidation voltage. The enhancement-mode InAlGaN quaternary GaN HEMT with maximum drain current of 955 mA mm−1, gm−1 peak of 440 mS mm−1, Vth of 0.2 V, and ft/fmax of 45/59 GHz were achieved using the digital etching for the gate recess structure.

In this work, whole transparent p-NiO/i-Ga2O3/n-Ga2O3:Si deep ultraviolet light-emitting diodes (DUV-LEDs) were fabricated and studied. The p-NiO film was deposited using a radio frequency magnetron sputtering system. The i-Ga2O3 film and the n-Ga2O3:Si film were deposited using a vapor cooling condensation system at about 80 K and then annealed in an oxygen ambience at 900 °C for 60 minutes. Due to the significantly reduced oxygen vacancies and intrinsic defects resided in the i-Ga2O3 active emissive film, UV-C LEDs were obtained. By measuring the electroluminescence spectrum of the p-i-n DUV-LEDs, the peak wavelength of 240.0 nm and the half width at half maximum of 20.2 nm were obtained.

In this paper, various tantalum pentoxide-alloyed indium oxide (Ta:In2O3) films were deposited on quartz substrates as the sensing membranes of nitrogen dioxide (NO2) gas sensors using a radio-frequency (RF) magnetron co-sputte-ring system. Using an energy-dispersive spectrometer (EDS), the Ta content in the deposited Ta:In2O3 films was 0, 1.28, 2.33, and 2.98 at.% corresponded to the RF power of 0, 30.0, 37.5, and 45.0 W applied to the tantalum pentoxide (Ta2O5) target, respectively. The grain size of the Ta:In2O3 films calculated from the X-ray diffraction (XRD) results was reduced from 9.9 to 7.6 nm as the Ta content of the Ta:In2O3 films increased from 0 to 2.98 at.%. The associated electron concentration increased from $1.9\times 10^{16}$ cm $^{-3}$ to $1.0\times 10^{18}$ cm $^{-3}$ by increasing the Ta content from 0 to 2.33 at.%. Compared with the indium oxide (In2O3) gas sensors, since the Ta:In2O3 sensing membrane with the Ta content of 2.33 at.% had the lowest activation energy of 0.39 eV, its responsivity was enhanced from 29.8 to 64.5 and the operation temperature was decreased from 150 °C to 120 °C under a NO2 concentration of 100 ppm. Under a NO2 concentration of 100 ppm at an operation temperature of 120 °C, the associated response time and recovery time of the Ta:In2O3 gas sensor with a Ta content of 2.33 at.% were improved to 59 and 339 s, respectively.

In this work, the influence of V/III ratio on self-catalyzed InSb nanowire (NW) crystal structure is investigated using metal organic chemical vapor deposition. The effective balance of V/III ratio determines the change of InSb NW crystal structure from zincblende to wurtzite. Particularly, as group-V molar flow varies from 6.4 × 10 −5 to 8.0 × 10 −5 mol/min, wurtzite segment length of InSb NW increases from 16 ± 9 nm to 89 ± 9 nm, whereas zincblende crystal phase is observed for other molar flows. We interpret the observed phase transition to a lower surface energy of In-Sb alloy droplet than pure In-droplet, which makes triple phase line nucleation energetically favorable. This study gives insight on single growth parameter (V/III ratio) change to the InSb NW crystal control, such a technique is essential to selectively tune the crystal phase of single NW for various applications.

To improve the conversion efficiency of InGaP/InGaAs/Ge triple-junction solar cells, AuGeNi/Au nanomesh electrode structure and TiO2 nanostructured antireflection structure were designed and fabricated. Laser interference photolithography system was used to pattern 330-nm-wide nanomesh electrode structures with various AuGeNi/Au metal line intervals. Oblique evaporation method using electron beam evaporator was used to deposit TiO2 nanorod arrays with various periods. By using the AuGeNi/Au nanomesh electrode structure with metal line interval of 100 μm, the conversion efficiency of the InGaP/InGaAs/Ge triple-junction solar cells was improved to 35.25% compared with 30.84% of that with conventional bus-bar electrode structure. By using the TiO2 nanorod array with a period of 1.00 μm to replace the TiO2/SiO2 antireflection structure, the conversion efficiency was further improved from 35.25% to 37.00%.

Due to the high absorption at short wavelength and the color conversion effect, CdSe/ZnS core–shell quantum dots were blended into poly (3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as an active layer of invert polymer solar cells (IPSCs). Compared with the IPSCs using P3HT:PCBM active layer, the short-circuit current density was improved from 10.38 to 11.57 mA/cm2 and the power conversion efficiency was improved from 3.37 to 3.73% for the IPSCs using P3HT:PCBM:CdSe/ZnS active layer. Since the carrier mobility of organic materials is very small, the carrier collection ability is limited by the short carrier transport length before they are recombined. Therefore, the power conversion efficiency of the resulting organic solar cells is unavoidably restricted by the low carrier collection ability. In this work, to improve carrier collection ability, indium tin oxide (ITO) nanorod array was embedded. Compared with the short-circuit current density of 11.57 mA/cm2 and the power conversion efficiency of 3.73% of the IPSCs using P3HT:PCBM:CdSe/ZnS active layer, the short-circuit current density of 15.60 mA/cm2 and the power conversion efficiency of 4.86% were obtained for the IPSCs by embedding 1.0 μm periodic ITO nanorod array in the P3HT:PCBM:CdSe/ZnS active layer.

In this study, polyaniline-reduced graphene oxide (PANI-rGO) nanocomposite films were used as sensitive membranes in NH3 gas sensors operated at room temperature (300 K). The responsivity and response speed of the PANI-rGO NH3 gas sensors were improved by homogeneously distributing PANI nanospheres on both surfaces of rGO nanosheets. The optimal weight ratio was 125 which was obtained by changing the weight ratio of PANI/rGO in the NH3 gas sensors. The NH3 gas sensors based on rGO and PANI sensitivity membranes had responsivities of 0.5% and 8.3%, whereas the recovery times were 8.0 and 48.0 min, respectively. The optimal responsivity and recovery time of these PANI-rGO NH3 gas sensors operated at room temperature under a 15-ppm NH3 gas environment were 13.0% and 22.1 min, respectively. However, because the responsivity of a rGO sensing membrane was considerably less than that of a PANI sensing membrane, the responsivity of the PANI-rGO NH3 gas sensors was reduced by incorporating more rGO content. The lowest detectable NH3 gas concentration of the PANI-rGO NH3 gas sensors was 0.3 ppm.

This study applied a vapor cooling condensation system for depositing p-ZnO:LiNO3, i-Ga2O3, and n-Ga2O3:Si layers on sapphire substrates to fabricate solar-blind p-i-n deep-ultraviolet photodetectors (DUV-PDs). The deposited i-Ga2O3 absorption layer was measured to exhibit high performance levels. Therefore, when the resulting solar-blind p-i-n DUV-PDs were operated at a reverse bias voltage of − 5 V, the dark current and the DUV (250 nm)/visible (500 nm) rejection ratio were 1.45 pA and 2.5 × 104, respectively. Moreover, when the solar-blind p-i-n DUV-PDs were operated at a reverse bias voltage of − 5 V, the photoresponsivity, noise power density, noise equivalent power, and specific detectivity were 0.73 A/W, 2.45 × 10−26 A2/Hz, 4.80 × 10−13 W, and 1.97 × 1012 cmHz0.5W−1, respectively. Furthermore, flicker noise was the dominant low-frequency noise source in the photodetectors. These results demonstrated the effectiveness of the proposed solar-blind p-i-n DUV-PDs.

This study investigated the sensitivity dependence of thin-film-structured NO2 gas sensors on the surface morphology of indium oxide-sensing membranes. The indium oxide-sensing membranes were deposited on quartz substrates by using a radio frequency magnetron sputtering system at various oxygen/(argon + oxygen) ratios to modify their In and O atomic percentage, surface morphology, and grain size in the membranes. According to the energy dispersive spectroscopy and x-ray diffraction results, the oxygen atomic percentage and the grain size of the indium oxide-sensing membranes increased and decreased with an increase in the oxygen/(argon + oxygen) ratio, respectively. Through atomic force microscopy, the average roughness of 50-nm-thick indium oxide films deposited at various ratios of oxygen/(argon + oxygen), namely 0% oxygen (pure argon), 20% oxygen, 60% oxygen, and 100% oxygen (pure oxygen), was found to be 0.80 nm, 0.98 nm, 2.68 nm, and 1.25 nm, respectively. The highest sensitivity of the various NO2 gas sensors was observed at an operating temperature of 150°C because of the generated $$ {\hbox{O}}^{ - } $$ ions and the provision of sufficient energy to overcome the required activation energy. At the operating temperature of 150°C, the sensitivity tendency of the various NO2 gas sensors was according to the surface roughness of the indium oxide-sensing membranes. Furthermore, the response and the recovery times of the various NO2 gas sensors corresponded to the surface morphology of the indium oxide-sensing membranes. The response and the recovery times of the NO2 gas sensors operated at 150°C under an NO2 gas concentration of 100 ppm were 70 s and 364 s, respectively.

Because MgO and NiO materials have the same rock-salt cubic structure and similar lattice constants, the MgNiO absorption layers of metal–semiconductor–metal ultraviolet photodetectors (MSM-UVPDs) were deposited by alloying MgO and NiO. To improve the properties of the MgNiO films, the films were annealed at various temperatures. The grain size of 18.5 nm for the as-deposited MgNiO films was improved to 23.4 nm and 24.8 nm for the MgNiO films annealed at 500 °C and 600 °C for 60 min, respectively. For the film-structured MSM-UVPDs with 500 °C-annealed MgNiO films operated at a bias voltage of 5 V, the photoresponsivity, average noise current, noise equivalent power, and specific detectivity were 1.78 × 10−2 A/W, 3.74 × 10−12 A, 2.10 × 10−10 W, and 1.50 × 109 cmHz0.5W−1, respectively. Aluminum (Al) nanospheres were stacked on the film-structured MgNiO MSM-UVPDs. The performance of the resulting Al-nanosphere-stacked MgNiO MSM-UVPDs was improved by increasing the density of the Al nanospheres. With an Al nanosphere density of approximately 2.0 × 107/mm2, when the Al-nanosphere-stacked MgNiO MSM-UVPDs operated at a bias voltage of 5 V, the photoresponsivity, average noise current, noise equivalent power, and specific detectivity were 2.00 × 10−1 A/W, 7.71 × 10−12 A, 3.88 × 10−11 W, and 8.16 × 109 cmHz0.5W−1, respectively. Despite the noise induced by the Al nanospheres, the performance of the Al-nanosphere-stacked MgNiO MSM-UVPDs was better than that of the film-structured MgNiO MSM-UVPDs. The improved detective performance was attributed to the increase in absorbance induced by the surface plasmon resonance and the optical scattering effect of the stacked Al nanospheres.

Using thermal annealing process, graphene oxide (GO) films were synthesized into reduced graphene oxide (rGO) composite films. Compared with the NO2 gas sensors using ZnO and rGO sensing membranes, the performances of the NO2 gas sensors using ZnO-rGO sensing membrane were improved. The improvement mechanisms were attributed to the removal of oxygen-containing functional groups, the supply of electrons from the oxygen vacancies of ZnO material, and the formation of C O Zn bonds. To study the dependence of sensing performances on the ZnO/GO ratio, various ZnO/GO ratios in the ZnO-rGO sensing membrane were used in the NO2 gas sensors. For the best ratio of 0.08, the sensing responsivity, response time, and recovery time of the ZnO-rGO NO2 gas sensors were respectively improved to 47.4%, 6.2 min, and 15.5 min compared with 19.0%, 10.3 min, and 75.9 min of the rGO NO2 gas sensors operated under 100 ppm NO2 environment at room temperature. Furthermore, the minimum detective NO2 concentration of 5 ppm and linear sensing responsivity from 10 ppm to 100 ppm were achieved using the ZnO-rGO gas sensors operated at room temperature.

To extend the sensing wavelength band, triple ultraviolet-band metal–semiconductor–metal ultraviolet photodetectors (MSM-UVPDs) were fabricated and studied in this study. Using a radio frequency magnetron sputtering system, the ZnO films, TiO2 films, and AlN films were sequentially stacked on quartz substrates. The cutoff wavelengths of the ZnO-based, TiO2-based, and AlN-based MSM-UVPDs were 370, 310, and 220 nm, respectively. The associated photoresponsivity of 47, 142, and 83 mA/W and the associated specific detectivity of $2.02 \times 10^{10}$ , $9.15 \times 10^{10}$ , and $2.20 \times 10^{11}\,\,\text {cm}\cdot \text {Hz}^{1/2}\,\,\cdot \,\,\text{W}^{-1}$ were obtained, respectively. By probing the common pad and each UV pad, the performance of the UVA, UVB, and UVC wavelength band could be measured, respectively. The performance of dual-band could be obtained by probing the common pad and the two connected UV pads. Moreover, by probing the common mode and the three connected UV pads in the stacked MSM-UVPDs, the performance of triple ultraviolet bands could be obtained. Because the photoresponsivity in the visible wavelength band was very low, the triple-band MSM-UVPDs can work as solar blind UVPDs. The dominant noise was the flicker noise.