Study and Fabrication of a Au/n-ZnO/p-Si Structure UV-Enhanced Phototransistor

The fabrication and characterization of a Schottky-emitter heterojunction-collector UV-enhanced bipolar phototransistor （SHBT） are presented. The luminescence peak of the ZnO film is observed at 371nm in the PL spectrum. The sensitivity of the ultraviolet response from 200 to 400nm is enhanced noticeably, and the spectrum response at wavelengths longer than 400nm is also retained, The experiments show that the Au/n-ZnO/p-Si SHBT UV enhanced phototransistor enhances the sensitivity of the ultraviolet response noticeably. The UV response sensitivity at 370nm of the phototransistor is 5-10 times that of a ZnO/Si heterojunction UV enhanced photodiode.

Spatially Bandgap-Graded Mo S2（1-x）Se2x Homojunctions for Self-Powered Visible–Near-Infrared Phototransistors

Ternary transition metal dichalcogenide alloys with spatially graded bandgaps are an emerging class of two-dimensional materials with unique features,which opens up new potential for device applications.Here,visible–near-infrared and self-powered phototransistors based on spatially bandgap-graded MoS2(1−x)Se2x alloys,synthesized by a simple and controllable chemical solution deposition method,are reported.The graded bandgaps,arising from the spatial grading of Se composition and thickness within a single domain,are tuned from 1.83 to 1.73 eV,leading to the formation of a homojunction with a builtin electric field.Consequently,a strong and sensitive gate-modulated photovoltaic effect is demonstrated,enabling the homojunction phototransistors at zero bias to deliver a photoresponsivity of 311 mA W−1,a specific detectivity up to^10^11 Jones,and an on/off ratio up to^10^4.Remarkably,when illuminated by the lights ranging from 405 to 808 nm,the biased devices yield a champion photoresponsivity of 191.5 A W−1,a specific detectivity up to^1012 Jones,a photoconductive gain of 10^6–10^7,and a photoresponsive time in the order of^50 ms.These results provide a simple and competitive solution to the bandgap engineering of two-dimensional materials for device applications without the need for p–n junctions.

Light-triggered interfacial charge transfer and enhanced photodetection in CdSe/ZnS quantum dots/MoS_(2)mixed-dimensional phototransistors

Mix-dimensional van der Waals heterostructures(vdWHs)have inspired worldwide interests and efforts in the field of ad-vanced electronics and optoelectronics.The fundamental understanding of interfacial charge transfer is of vital import-ance for guiding the design of functional optoelectronic applications.In this work,type-Ⅱ0D-2D CdSe/ZnS quantum dots/MoS_(2)vdWHs are designed to study the light-triggered interfacial charge behaviors and enhanced optoelectronic performances.From spectral measurements in both steady and transient states,the phenomena of suppressed photolu-minescence(PL)emissions,shifted Raman signals and changed PL lifetimes provide strong evidences of efficient charge transfer at the 0D-2D interface.A series of spectral evolutions of heterostructures with various QDs overlapping concentrations at different laser powers are analyzed in details,which clarifies the dynamic competition between exciton and trion during an efficient doping of 3.9×10^(13)cm^(−2).The enhanced photoresponses(1.57×10^(4)A·W^(-1))and detectivities(2.86×10^(11)Jones)in 0D/2D phototransistors further demonstrate that the light-induced charge transfer is still a feasible way to optimize the performance of optoelectronic devices.These results are expected to inspire the basic understand-ing of interfacial physics at 0D/2D interfaces,and shed the light on promoting the development of mixed-dimensional op-toelectronic devices in the near future.

A gate-free MoS2 phototransistor assisted by ferroelectrics

During the past decades,transition metal dichalcogenides(TMDs) have received special focus for their unique properties in photoelectric detection.As one important member of TMDs,MoS2 has been made into photodetector purely or combined with other materials,such as graphene,ionic liquid,and ferroelectric materials.Here,we report a gate-free MoS2 phototransistor combined with organic ferroelectric material poly(vinylidene fluoride-trifluoroethylene)(P(VDF-TrFE)).In this device,the remnant polarization field in P(VDF-TrFE) is obtained from the piezoelectric force microscope(PFM) probe with a positive or negative bias,which can turn the dipoles from disorder to be the same direction.Then,the MoS2 channel can be maintained at an accumulated state with downward polarization field modulation and a depleted state with upward polarization field modulation.Moreover,the P(VDF-TrFE) segregates MoS2 from oxygen and water molecules around surroundings,which enables a cleaner surface state.As a photodetector,an ultra-low dark current of 10^–11 A,on/off ration of more than 10^4 and a fast photoresponse time of 120 μs are achieved.This work provides a new method to make high-performance phototransistors assisted by the ferroelectric domain which can operate without a gate electrode and demonstrates great potential for ultra-low power consumption applications.

Analysis on high speed response of a uni-traveling-carrier double hetero-junction phototransistor

In this paper, the positive influence of a uni-traveling-carrier （UTC） structure to ease the contract between the respon- sivity and working speed of the InP-based double hetero-junction phototransistor （DHPT） is illustrated in detail. Different results under electrical bias, optical bias or combined electrical and optical bias are analyzed for an excellent UTC-DHPT performance. The results show that when the UTC-DHPT operates at three-terminal （3T） working mode with combined electrical bias and optical bias in base, it keeps a high optical responsivity of 34.72 A/W and the highest optical transition frequency of 120 GHz. The current gain of the 3T UTC-DHPT under 1.55-μm light illuminations reaches 62 dB. This indicates that the combined base electrical bias and optical bias of 3T UTC-DHPT can make sure that the UTC-DHPT provides high optical current gain and high optical transition frequency simultaneously.

High-sensitive phototransistor based on vertical HfSe_(2)/MoS_(2) heterostructure with broad-spectral response

Van der Waals heterostructures based on the two-dimensional(2D)semiconductor materials have attracted increasing attention due to their attractive properties.In this work,we demonstrate a high-sensitive back-gated phototransistor based on the vertical HfSe_(2)/MoS_(2)heterostructure with a broad-spectral response from near-ultraviolet to near-infrared and an efficient gate tunability for photoresponse.Under bias,the phototransistor exhibits high responsivity of up to 1.42×103A/W,and ultrahigh specific detectivity of up to 1.39×1015cm·Hz^(1/2)·W^(-1).Moreover,it can also operate under zero bias with remarkable responsivity of 10.2 A/W,relatively high specific detectivity of 1.43×1014cm·Hz^(1/2)·W^(-1),ultralow dark current of 1.22 f A,and high on/off ratio of above 105.These results should be attributed to the fact that the vertical HfSe_(2)/MoS_(2)heterostructure not only improves the broadband photoresponse of the phototransistor but also greatly enhances its sensitivity.Therefore,the heterostructure provides a promising candidate for next generation high performance phototransistors.

Intrinsic Photoconductivity of Few-layered ZrS2 Phototransistors via Multiterminal Measurements

We report intrinsic photoconductivity studies on one of the least examined layered compounds,ZrS2.Few-atomic layer ZrS2 field-effect transistors were fabricated on the Si/SiO2 substrate and photoconductivity measurements were performed using both two-and four-terminal configurations under the illumination of 532 nm laser source.We measured photocurrent as a function of the incident optical power at several source-drain(bias)voltages.We observe a significantly large photoconductivity when measured in the multiterminal(four-terminal)configuration compared to that in the two-terminal configuration.For an incident optical power of 90 nW,the estimated photosensitivity and the external quantum efficiency(EQE)measured in two-terminal configuration are 0.5 A/W and 120%,respectively,under a bias voltage of 650 mV.Under the same conditions,the four-terminal measurements result in much higher values for both the photoresponsivity(R)and EQE to 6 A/W and 1400%,respectively.This significant improvement in photoresponsivity and EQE in the four-terminal configuration may have been influenced by the reduction of contact resistance at the metal-semiconductor interface,which greatly impacts the carrier mobility of low conducting materials.This suggests that photoconductivity measurements performed through the two-terminal configuration in previous studies on ZrS2 and other 2D materials have severely underestimated the true intrinsic properties of transition metal dichalcogenides and their remarkable potential for optoelectronic applications.

Polymer: Non-fullerene acceptor heterojunction-based phototransistor for short-wave infrared photodetection

It remains full of challenge for extending short-wave infrared(SWIR)spectral response and weak-light detection in the context of broad spectral responses for phototransistor.In this work,a novel poly(2,5-bis(4-hexyldodecyl)-2,5-dihydro-3,6-di-2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thiophene)(PDPPT3-HDO):COTIC-4F organic bulk-heterojunction is prepared as active layer for bulk heterojunction phototransistors.PDPPT3-HDO serves as a hole transport material,while COTIC-4F enhances the absorption of SWIR light to 1020 nm.As a result,smooth and connected PDPPT3-HDO film is fabricated by blade coating method and exhibits high hole mobility up to 2.34 cm^(2)·V^(-1)·s^(-1) with a current on/off ratio of 4.72×10^(5) in organic thin film transistors.PDPPT3-HDO:COTIC-4F heterojunction phototransistors exhibit high responsivity of 2680 A·W^(-1) to 900 nm and 815 A·W^(-1) to 1020 nm,with fast response time(rise time~20 ms and fall time~100 ms).The photosensitivity of the heterojunction phototransistor improves as the mass ratio of non-fullerene acceptors increases,resulting in an approximately two orders of magnitude enhancement compared to the bare polymer phototransistor.Importantly,the phototransistor exhibits decent responsivity even under ultra-weak light power of 43μW·cm^(-2) to 1020 nm.This work represents a highly effective and general strategy for fabricating efficient and sensitive SWIR light photodetectors.

Synaptic a-Si:H/a-Ga_(2)O_(3) phototransistor inspired by the phototaxis behavior of organisms with all-optical and all-electrical stimulation modes

To improve neuromorphic computing performance,neuromorphic system components should mimic the behaviors of organic systems.In this study,a synaptic a-Si:H/a-Ga_(2)O_(3)phototransistor featuring all-optical and-electrical emulation is fabricated in a manner advantageous for complementary metal-oxide-semiconductor process integration.The phototransistor exhibits excitatory and inhibitory synaptic behaviors under stimulation by both optical and electrical signals.It mimics several essential synaptic functions,including excitatory postsynaptic current,inhibitory postsynaptic current,short-term memory,long-term memory,pairedpulse facilitation,and spike-timing-dependent plasticity.The optical and electrical modulation mechanisms are confirmed to arise from the a-Si:H/a-Ga_(2)O_(3) heterojunction structure and interface effects,and the device is shown to operate at low power in both optical and electrical modes.The all-optical weight modulation function is applied to the wavelength-differential behavior response of zebrafish,successfully mimicking the color perception process of the organism.Finally,to verify the translation of the optoelectrical-derived synaptic behaviors of the phototransistor into artificial neuromorphic computation,handwritten digit image recognition of the Modified National Institute of Standards and Technology dataset is performed by a convolutional neural network,with a demonstrated average learning accuracy of 98.46%.These findings verify the applicability of the synaptic a-Si:H/a-Ga_(2)O_(3) phototransistor in neuromorphic computing.

Significantly enhanced of tungsten diselenide functionalization optoelectronic performance phototransistor via surface

Two-dimensional （2D） layered transition metal dichalcogenides （TMDs） have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide （WSe2） exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe2 phototransistor by in situ surface functionalization with cesium carbonate （Cs2CO3）. WSe2 was found to be strongly doped with electrons after Cs2CO3 modification. The electron mobility of WSe2 increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs2CO3 decoration. Furthermore, the photocurrent of the WSe2-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs2CO3. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs2COB/WSe2 interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe2 as a promising candidate for future 2D materials- based optoelectronic devices.

Enhanced photoresponsivity and hole mobility of MoTe_2 phototransistors by using an Al_2O_3 high-κ gate dielectric

Molybdenum ditelluride （MoTe2） has been demonstrated great potential in electronic and optoelectronic applications. However, the reported effective hole mobility remains far below its theoretical value. Herein, taking advantage of high-κ screening effect, we have fabricated back-gated MoTe2 transistors on an Al2O3 high-κ dielectric and systematically investigated the electronic and optoelectronic proper- ties. A high current on/off ratio exceeding 106 is achieved in the Al2O3-based MoTe2 transistors, and the hole mobility is demonstrated to be 150 cm2 V^-1 s^-1, compared to 0.2-20 cm^2 V^-1 s^-1 ever obtained from back-gated MoTe2 transistors in the literatures. Moreover, a considerable hole concentration of 1.2 × 10^13 cm 2 is attained in our Al2O3-based MoTe2 transistors owing to the strong gate control capa- bility, leading to a high on-state hole current of 6.1 μA μm^-1. After optimization, our Al2O3-based MoTe2 phototransistor exhibits outstanding photodetective performance, with a high responsivity of 543 AW^-1 and a high photogain of 1,662 at 405 nm light illumination, which are boosted around 419 times compared to the referential SiO2-based control devices. The mechanisms of photoconductivity in the Al2O3-based MoTe2 phototransistors have been analyzed in detail, and the photogating effect is considered to play an important role. This work may provide useful insight to improve carrier mobility in two-dimensional layered semiconductors and open opportunities to facilitate the development of high-performance photodetectors in the future.

Hybrid dual-channel phototransistor based on ID t-Se and 2D ReS2 mixed-dimensional heterostructures

The com bination of mixed-dimensional semiconducti ng materials can provide additional freedom to construct integrated n anoscale electronic and optoelectronic devices with diverse functionalities. In this work, we report a high-performanee dual-channel phototransistor based on one-dimensional (1D)/two-dimensional (2D) trigonal selenium (t-Se)/ReS2 heterostructures grown by chemical vapor deposition. The injection and separati on efficie ncy of photoge nerated electro n-hole pairs can be greatly improved due to the high-quality in terfacial con tact betwee n t-Se nano belts and ReS2 films. Compared with bare ReS2 film devices, the dual-cha nnel phototra nsistor based on t-Se/ReS2 heterostructure exhibits considerable enhancement with the responsivity (R) and detectivity (D^*) up to 98 A·W^-1 and 6 x 10^10 Jones at 400 nm illumination with an in tensity of 1.7 mW·cm^-2, respectively. Besides, the respo nse time can also be reduced by three times of magnitude to less than 50 ms due to the type-11 band alignment at the in terface. This study opens up a promising ave nue for high-performa nee photodetectors by constructing mixed-dimensional heterostructures.

Gate-tunable high-performance broadband phototransistor array of two-dimensional PtSe_(2) on SOI

Two-dimensional(2D)layered materials have attracted extensive research interest in the field of high-performance photodetection due to their high carrier mobility,tunable bandgap,stability,other excellent properties.Herein,we propose a gate-tunable,high-performance,self-driving,wide detection range phototransistor based on a 2D PtSe_(2)on silicon-oninsulator(SOI).Benefiting from the strong built-in electric field of the PtSe_(2)/Si heterostructure,the phototransistor has a fast response time(rise/fall time)of 36.7/32.6μs.The PtSe_(2)/Si phototransistor exhibits excellent photodetection performance over a broad spectral range from ultraviolet to near-infrared,including a responsivity of 1.07 A/W and a specific detectivity of 6.60×10^(9)Jones under 808 nm illumination at zero gate voltage.The responsivity and specific detectivity of PtSe_(2)/Si phototransistor at 5 V gate voltage are increased to 13.85 A/W and 1.90×10^(10) Jones under 808 nm illumination.Furthermore,the fabricated PtSe_(2)/Si phototransistor array shows excellent uniformity,reproducibility,long-term stability in terms of photoresponse performance with negligible variation between pixel cells.The architecture of present PtSe_(2)/Si on SOI platform paves a new way of a general strategy to realize high-performance photodetectors by combining the advantages of both 2D materials and conventional semiconductors which is compatible with current Si-complementary metal oxide semiconductor(CMOS)process.

Organic single-crystal phototransistor with unique wavelength-detection characteristics

Organic phototransistors based on high-quality 2,8-dichloro-5,11-dihexyl-indolo[3,2-b]carbazo(CHICZ)single crystals show the highest photoresponsivity of 3×10^3 A W^-1, photosensitivity of 2×10^4 and the detectivity can achieve 8.4×10^14 Jones. We also discovered good linear dependence of log(photosensitivity) versus the wavelength when the devices were illuminated with a series of sameintensity but different-wavelength lights. The organic phototransistors based on CHICZ single crystal have potential applications in wavelength-detection.

Performance improvement of organic phototransistors by using polystyrene microspheres

Organic phototransistors （OPTs） have been intensively studied in recent years due to the combined ad- vantages of phototransistors and organic semiconductors （OSCs）. However, the electrical performance of OPTs is lar- gely limited by OSCs themselves, posing a challenge to further improve the performance of the devices. Preparing nano/mi- cro-structures of OSCs is considered as an effective way to improve the performance of OPTs. Polystyrene （PS） micro- sphere, as a kind of insulating and low-cost material, is ex- tensively used in fabricating nano/microporous structures, and the resulting devices exhibit high response to external stimuli. Therefore, we combined PS microspheres with OSCs to fabricate PS/OSC OPTs, and the Ilight/Idark ratio was en- hanced by two orders of magnitude compared with the pris- tine counterparts, which can be modulated from 46 to 1800 by controlling the diameters of PS microsphereso This strategy paves a way for developing high-performance OPTs with nano/microporous structures with potential applications in organic optoelectronics.

Cocrystal engineering:towards high-performance near-infrared organic phototransistors based on donor-acceptor charge transfer cocrystals

Near-infrared organic phototransistors have wide application prospects in many fields.The active materials with the high mobility and near-infrared response are critical to building high-performance near-infrared organic phototransistors,which are scarce at present.Herein,a new charge transfer cocrystal using 5,7-dihydroindolo[2,3-b]carbazole(5,7-ICZ)as the donor and 2,2’-(benzo[1,2-b:4,5-b’]dithiophene-4,8-diylidene)dimalononitrile(DTTCNQ)as the acceptor is properly designed and prepared in a stoichiometric ratio(D:A=1:1),which not only displays a high electron mobility of 0.15 cm^(2)V^(-1)s^(-1) and very low dark current,but also can serve as the active layer materials in the region of near-infrared detection due to the narrowed band gap and good charge transport properties.A high photosensitivity of 1.8×10^(4),the ultrahigh photoresponsivity of 2,923 A W-1and the high detectivity of 4.26×10^(11)Jones of the organic near-infrared phototransistors are obtained.

High gain,broadband p-WSe_(2)/n-Ge van der Waals heterojunction phototransistor with a Schottky barrier collector

Mixed-dimensional van der Waals(vdW)heterostructures based on two-dimensional transition metal dichalcogenides and threedimensional semiconductors have led to a new era in next-generation optoelectronics due to the high-quality interfaces and energy band complementation,especially in broadband photodetectors which can be used for all-weather navigation,object identification,etc.However,the reported photodetectors conventionally operated in photodiode mode with low responsivity and a narrow response spectrum.In this study,we report a p-WSe_(2)/n-Ge vdW heterojunction phototransistor with a Schottky barrier collector on n-Ge for broadband photodetection.Large hole/electron injection ratio from p-WSe_(2)/n-Ge heterojunction under forward bias due to their large bandgap offset renders the high photocurrent gain,while the Ge Schottky barrier limits the dark current.The responsivities of the phototransistor at 1.0 V emitter-collector bias are 55,95,and 120 A·W−1 at 405,1,310,and 1,550 nm,respectively,which is superior to that of the corresponding p-WSe_(2)/n-Ge photodiodes.The phototransistor shows a high photocurrent gain of 80,a specific detectivity of 1011 Jones,as well as a fast response time of 290μs at 1,550 nm.The results suggest that the novel phototransistor being implemented with complementary metal-oxide-semiconductor processing is an ideal strategy for high-performance broadband photodetection.

Highly sensitive and stable β-Ga_(2)O_(3) DUV phototransistor with local back-gate structure and its neuromorphic application

Deep ultraviolet(DUV)phototransistors are key integral of optoelectronics bearing a wide spectrum of applications in flame sensor,military detector,oil spill detection,biological sensor,and artificial intelligence fields.In order to further improve the responsivity of UV photodetectors based onβ-Ga_(2)O_(3),in present work,high-performanceβ-Ga_(2)O_(3) phototransistors with local back-gate structure were experimentally demonstrated.The phototransistor shows excellent DUV photoelectrical performance with a high responsivity of 1.01×107 A/W,a high external quantum efficiency of 5.02×109%,a sensitive detectivity of 2.98×1015 Jones,and a fast rise time of 0.2 s under 250 nm illumination.Besides,first-principles calculations reveal the decent stability ofβGa_(2)O_(3) nanosheet against oxidation and humidity without significant performance degradations.Additionally,the hexagonal boron nitride(h-BN)/β-Ga_(2)O_(3) phototransistor can behave as a photonic synapse with ultralow power consumption of~9.6 fJ per spike,which shows its potential for neuromorphic computing tasks such as facial recognition.Thisβ-Ga_(2)O_(3) phototransistor will provide a perspective for the next generation optoelectrical systems.

Few-layered organic single-crystalline heterojunctions for high-performance phototransistors

Photogating and electrical gating are key physical mechanisms in organic phototransistors(OPTs).However,most OPTs are based on thick and polycrystalline films,which leads to substantially low efficiency of both photogating and electrical gating and thus reduced photoresponse.Herein,high-performance OPTs based on few-layered organic single-crystalline heterojunctions are proposed and the obstacle of thick and polycrystalline films for photodetection is overcome.Because of the molecular scale thickness of the type I organic single-crystalline heterojunctions in OPTs,both photogating and electrical gating are highly efficient.By synergy of efficient photogating and electrical gating,key figures of merit of OPTs reach the highest among those based on planar heterojunctions so far as we know.The production of few-layered organic single-crystalline heterojunctions will provide a new type of advanced materials for various applications.

A low power discrete operation mode for punchthrough phototransistor

This paper proposed a discrete operation mode for a punchthrough（PT） phototransistor,which is suitable for low power application,since the bias current is only necessary during the read-out phase.Moreover,simulation results show that with the new operation mode,the photocurrent is much larger than that of continuous operation mode.An ultra-high responsivity of 2×10~7A/W at 10^（-9） W/cm^2 is obtained with a small detector size of 1μm^2.In CMOS image sensor applications,with an integration time of 10 ms,a normalized pixel responsivity of 220 V·m^2/W·s·μm^2 is obtained without any auxiliary amplifier.