Driven by advancements in artificial intelligence driving, there is need for developing new technologies in mobile communications, new energy vehicles, robotics and drones, as well as miniaturized and multi-functional chips with high performance. Consequently, it is essential to develop optoelectronic devices that integrate multifunctional components. Traditional photodetectors have typically utilized single photoresponses to achieve high efficiency, responsivity, and fast response rate [1–4]. However, there has been limited research on incorporating both positive and negative photoresponses into a single photodetector. Combining positive and negative responses into a single photodetector would increase the versatility of the device for multiple applications, such as artificial vision adaption mimicking [5,6], multimode optoelectronic logic gates [7], and visual signal reception and preprocessing [5]. Among these emerging applications, the optoelectronic logic gate is a new type of logic application to achieve faster and larger capacity data transmission and processing [3,4,7–9].

Compared to the two-terminal devices, junction field-effect transistors (JFETs) provide advantages such as faster response, lower sensitivity to electrostatic discharge, and adjustable photoresponse modulated by gate voltage. These features make JFETs highly suitable for optoelectronic logic gate devices. [10–13]. Two-dimensional transition metal dichalcogenides (2D TMDs) exhibit strong light-matter interactions, free of surface dangling bonds and high carrier mobility, as compared to traditional semiconductors [14–17]. 2D TMD heterojunctions have many advantages like broadband optical absorption, efficient interlayer carrier transport, simple fabrication process, and potential of fabricating flexible devices [18]. Recent research has explored combining 2D materials with gallium oxide (Ga2O3) to fabricate junction field-effect transistors, including WSe2/Ga2O3 [19,20], NbS2/Ga2O3 [21], TaS2/Ga2O3 [21], h-BN/Ga2O3 [22–24], MoTe2/Ga2O3 [20], graphene/Ga2O3 [25,26], α-In2Se3/Ga2O3 [27], and BP/Ga2O3 [28,29]. These devices exhibit excellent performance due to outstanding air stability, electrical characteristics, and transparency of Ga2O3.

One-dimensional (1D) Ga2O3 microwires (MWs) with high crystallinity can serve as the active optical material to enhance light management and charge collection for developing novel optoelectronic devices [30,31]. For instance, Lu et al. constructed a 1×10 photodetector array based on 1D Sn-doped β-Ga2O3 MWs for solar-blind imaging [32]. Lately, the same group simplified the readout circuits of the solar-blind photodetector array by weaving the Sn-doped β-Ga2O3 microbelts into the lattice structures, which exhibited excellent performance with a low dark current and rapid response time [33]. To date, 1D Ga2O3-based transistors have demonstrated excellent performance in terms of low noise current, rapid response, specific detectivity, and high external quantum efficiency [32,33].

In this work, a novel 1D/2D mixed-dimensional Ga2O3/WSe2 heterojunction transistor and a JFET are presented, with a 2D WSe2 nanosheet serving as both the gate dielectric layer and the photosensitive layer. Under optical illumination with a wavelength of 635 nm, the device exhibits both positive and negative photoresponses, which are modulated by the gate voltage. Notably, the negative photoresponse performs better than the positive response. We further elucidate the effects of gate voltage and laser intensity on the JFET’s negative photoresponse. Specifically, the Ga2O3/WSe2 mixed-dimensional heterojunction JFET demonstrates an outstanding transistor performance, including a high IOFF/ION ratio up to 343, response time of 50.1 ms, and decay time of 53.9 ms at Vds=1V and Vg=5V. The transport mechanism of the hetero-structure is also studied. Notably, at the high gate bias (Vg=5V), the carrier transport is dominated by Fowler-Nordheim tunneling (FNT) behavior. Furthermore, using the gate voltage and optical illumination as the two inputs, the function of the NAND logic gate is realized on the Ga2O3/WSe2 JFET. Subsequently, the output signal can be realized as the binary conversion by the computer. Overall, the mixed-dimensional Ga2O3/WSe2 JFET provides a paradigm for Ga2O3-based transistors to implement logical operations, dual-band detection, and optical communication.

A 50 nm Au film was deposited onto the silicon substrate by thermal evaporation. The high-purity (99.999%) Ga2O3 powder and C (99.95%) powder in a ratio of 3:1 were mixed in a mortar. The mixed powder and the Au-film-coated Si substrates were placed in the quartz tube, and the Si substrates were placed 15 cm downstream from the high-temperature zone. The quartz tube was first flushed with ultrapure argon (99.999%) for 6 min, and then the tube furnace was heated at a rate of 10°Cmin−1 up to 1100°C and maintained at this temperature for 60 min under a constant nitrogen flow of 150 sccm. The white flocculent product obtained at the end of the growth process consisted of the Ga2O3 microwires.

A small quantity of Ga2O3 microwires were dispersed in the ethanol solution and then transferred to a SiO2/Si substrate using a rubber-tipped burette. The sample was annealed at 150°C for 30 s to eliminate the residual effects of the ethanol solution. WSe2 nanosheets were exfoliated from single crystals using a mechanical stripping method and then transferred onto a PVA/PDMS film. With the three-dimensional positional adjusting platform, the WSe2 nanosheet was aligned and transferred to the Ga2O3 microwires, thus forming the p-n heterojunctions. The device was spin-coated, prebaked, and fabricated using an ultraviolet maskless lithography machine [TuoTuo Technology (Suzhou) Co., Ltd.]. Subsequently, the source, drain, and gate electrode (10 nm Cr/50 nm Au) were deposited using an electron beam evaporation instrument to form a Schottky contact.

The morphology of the Ga2O3 samples was characterized using scanning electron microscopy (SEM), while elemental analysis of the materials was conducted by the energy dispersive X-ray spectroscopy (EDS). The thickness of the Ga2O3 and WSe2 was determined by the atomic force microscopy (AFM). X-ray diffraction (XRD) was employed to investigate the crystal structure. The micro-Raman spectrometer was used for characterizing the Ga2O3. The electrical and optoelectronic properties of the JFET were measured at room temperature using the Keithley 2636B semiconductor device analyzer. All measurements were performed under ambient conditions at room temperature.

High-crystal Ga2O3 MWs were synthesized by chemical vapor deposition (CVD) to construct the Ga2O3/WSe2 mixed-dimensional heterojunction JFETs [34]. Figure 1(a) shows the SEM image of the as-fabricated Ga2O3 microwires. A single Ga2O3 microwire with a smooth surface features a spherical particle at its tip. To understand the growth mechanism of Ga2O3 microwires, Ga2O3 was subsequently extracted for EDS characterization. In Fig. 1(b), EDS mapping images of β-Ga2O3 MW-tipped nanoparticles fabricated at 1100°C are presented [35]. Here, the β-Ga2O3 MWs have distinct gold particles at their tips [36]. The growth of β-Ga2O3 MWs follows the vapor-liquid-solid mechanism, consistent with existing literature [34–36]. Additionally, the structures and phase purity of the as-grown Ga2O3 nanostructures were characterized by XRD. Comparing with the standard PDF card (PDF# 41-1103) of β-Ga2O3 material, the observed diffraction peaks were consistent with the standard peaks of β-Ga2O3 material [as shown in Fig. 1(c)]. The room temperature Raman spectra of the deposited β-Ga2O3 nanostructures are shown in Fig. 1(d), revealing the 12 Raman active modes at low frequency (below 200cm−1), medium frequency (300–500cm−1) [37], and high frequency (above 500cm−1). The low-frequency modes such as 112.3, 142.6, 167.7, and 198.1cm−1 are attributed to the translational and low-frequency vibrations between the GaO4 tetrahedral chains and the Ga2O6 octahedral chains, while the 318.0, 345.7, 414.2, and 474.9cm−1 signals are attributed to the Ga2O6 octahedral chains. The high-frequency modes such as 628.3, 653.3, 656.9, and 764.4cm−1 are associated with the stretching and bending of the GaO4 tetrahedral chains. The XRD and Raman results confirm the single pure phase of the β-Ga2O3 MWs.

Figures 2(a) and 2(b) show the schematic and the optical microscope image of the Ga2O3/WSe2 mixed-dimensional heterojunction JFET, respectively. To fabricate the JFET, mechanically stripped WSe2 was dry-transferred onto a 1D Ga2O3 MW and deposited with 10 nm Cr/50 nm Au electrodes to form a Schottky contact. The EDS mapping image shows that the Ga2O3/WSe2 JFET consists of Ga, O, W, and Se elements [as shown in Fig. 2(c)], and the elemental distribution is consistent with the spatial distribution of the heterotransistor materials [38]. To determine the thicknesses of each layer in the Ga2O3/WSe2 mixed-dimensional heterojunction JFET, the AFM images were obtained and the thicknesses of the Ga2O3 and WSe2 flake were 688 nm and 44 nm [as shown in Figs. 2(d) and 2(e), respectively]. To further analyze the energy band structure between the WSe2 and Ga2O3, kelvin probe force microscopy (KPFM) is employed to reveal the Fermi level shift of Ga2O3 and WSe2 [as shown in Fig. 2(f)]. Figure 2(g) shows the difference of the surface potential difference (SPD) of the measured Ga2O3/WSe2 interface [as denoted by the red line in Fig. 2(f)]. Before contact, the Fermi level of the Ga2O3 is lower compared to that of the WSe2 as shown in Fig. 2(h). Upon contact, the Fermi energy level difference between Ga2O3 and WSe2 drives electrons flowing from WSe2 to Ga2O3, forming a thin depletion region at the heterojunction interface [19], and creating an accumulation of electrons in Ga2O3 and holes in WSe2 as shown in Fig. 2(i).

To further analyze the electrical performance of the Ga2O3/WSe2 JFET, I–V measurements of the JFET were conducted in the dark, with results as shown in Fig. 3(a). The JFET demonstrated effective rectification behavior in the absence of illumination. To understand the optoelectronic response, the device was further investigated under 635 nm illumination. Figure 3(b) shows the time-dependent photocurrent of the Ga2O3/WSe2 JFET with the gate bias of Vg=−5V and Vg=5V. The device shows a positive photoresponse when Vg=−5V and a negative photoresponse when Vg=5V. Moreover, both the negative and positive photocurrent responses increase with increasing optical power [as shown in Figs. 3(c) and 3(d)].

Comparing to the positive photoresponse, the negative photoresponse shows more excellent performance. Under 635 nm illumination, the Ga2O3/WSe2 JFET demonstrated a remarkably high negative photoresponse achievable with Vg=5V, having the IOFF/ION ratio of 343. To study the stability of the negative photoresponse, a 100 switching cycle test was performed. The current magnitude of the device remained nearly constant, indicating the high reliability and stability of the JFET [as shown in Fig. 3(e)]. Response time and decay time are important parameters for evaluating the phototransistor. The measured 10%–90% response time and decay time are 50.1 ms and 53.9 ms, respectively. The device gives a fast response speed under gate voltage and light illumination [as shown in Fig. 3(f)].

To investigate the influence of gate voltage and light intensity on the negative photoresponse of the Ga2O3/WSe2 JFET, output curves of different gate voltages and laser powers were measured. Under 635 nm illumination with a power density of 194.52mW/cm2, the WSe2 maintains a low resistance. Meanwhile, increasing the gate voltage widens the depletion region of the Ga2O3/WSe2. This depletion reduces the current flow in the Ga2O3 channel, necessitating a larger bias voltage to provide carriers. Consequently, a rightward shift is observed in the output curve [Fig. 4(a)]. Figure 4(d) shows the output curves of the JFET at a constant Vg of 5 V under different laser powers. As the laser power increases, the resistance of WSe2 decreases and the depletion region in the Ga2O3/WSe2 widens at Vg=5V. Consequently, the output curves exhibit a rightward shift.

It is well known that responsivity (R), external quantum efficiency (EQE), and specific detectivity (D*) are important indexes for evaluating phototransistors. To further evaluate the optoelectronic performance of Ga2O3/WSe2 JFET, the responsivity, detectivity, and external quantum efficiency of the JFETs with different gate voltages and laser powers were quantitatively measured. The parameters of R,D*, and EQE are calculated according to the following equations [39–41]: Iphoto=Ilight−Idark,(1)R=|Iphoto|PS,(2)D*=RS2eIdark,(3)EQE=Rhceλ.(4)

In the given formulas, Idark is the current in the dark condition, and Ilight represents the current in the light condition. Iphoto denotes the net photocurrent under light. It is calculated as the difference between the current under light condition (Ilight) and the dark current (Idark) of the device. P and S represent the light power density and the effective area of the device, respectively. The Planck’s constant (h) is 6.63×1034J·s, the light speed (c) is 3×108m/s, and the element charge (e) is 1.6×10−19. λ denotes the excitation wavelength, which is specified to be 635 nm. The R,D*, and EQE are calculated using the four provided formulas under various conditions.

As shown in the Fig. 4(b), net photocurrent significantly increases under 635 nm illumination while the gate voltage increases from 0 V to 5 V, indicating that the gate voltage has a regulatory effect on the Ga2O3 channel. Figure 4(c) demonstrates the relationship between the gate voltage and the EQE and R. With the increase of the gate voltage, the depletion region becomes widened at the heterojunction interface. The R and EQE of the device gradually increase under the combined effect of light illumination and gate voltage, and the R can reach 425 mA/W and the EQE reaches 83.18%. This enhanced modulation by the gate voltage is likely the cause of these results. As shown in Figs. 4(e) and 4(f), the net photocurrent gradually increases with the enhancement of optical power while Vg=5V. However, the R, D*, and EQE decrease, which may be caused by carrier lifetime shortening due to Auger recombination [42,43].

The Ids−Vg curves of the Ga2O3/WSe2 JFET under dark and light conditions at Vds=1V are shown in Fig. 5(a). Under dark conditions, Ids remains constant (33–34 nA) with varying Vg. Under 635 nm illumination, there is a positive net photocurrent for Vg ranging from −5V to −3.2V, and a negative photocurrent for Vg ranging from −3.2V to 5 V. In contrast, under dark conditions, the high resistance of WSe2 hinders the modulation of the Ga2O3 channel current by Vg. However, light illumination generates a number of electron-hole pairs in WSe2, thereby reducing its resistance and creating a conductive path for Vg to modulate the Ga2O3 channel conductance. Varying the gate voltage affects the width of the depletion region, thereby influencing the electrical and optical characteristics of the device. Figure 5(b) shows the Igs−Vg curves of the Ga2O3/WSe2 JFET under dark and light conditions at Vds=1V. When Vg=−0.2V, the magnitude of the current is equal to 0 A. The changes in the depletion region width, as discussed here, explain the variations of the photoresponse characteristics of the Ga2O3/WSe2 JFET at different Vg conditions, as shown in Figs. 5(c)–5(f).

Owing to the difference in Fermi energy levels between Ga2O3 and WSe2, the electrons move from WSe2 to Ga2O3, thus creating the accumulation of electrons in Ga2O3 and holes in WSe2. A thin depletion region forms [19] at the heterojunction interface, and the direction of the built-in electric field points from WSe2 to Ga2O3. Therefore, increasing the gate voltage widens the depletion region, as visually depicted in Figs. 5(c)–5(f). When Vg=−5V [Fig. 5(c)], the photoelectrons in WSe2 flow into the Ga2O3 channel. This narrows the depletion region in Ga2O3, leading to an increased current compared to that in the dark condition [as shown in Fig. 5(g)]. When Vg=−3.2V, the magnitude of the current is the same as in the dark condition [Fig. 5(d)]. This indicates that the Fermi levels of Ga2O3 and WSe2 are aligned, and the width of the depletion region remains the same as in the dark condition. With Vg=−0.2V, the magnitude of the current is equal to 0 A because the electrons are not energetic enough to cross the barrier [Fig. 5(e)]. With the increase of gate voltage, the electrons flow into WSe2; when Vg=5V, the depletion region in Ga2O3 widens significantly, pinching off the Ga2O3 channel, resulting in the low Ids as shown in Fig. 5(h).

In order to further explore the physical mechanism of the device, the Simons approximation is used to model the current transport of the Ga2O3/WSe2 heterojunction, and the model can be described by the following equations [9,44–46]: IFNT∝V2exp(−8πd2m*φ33heV),(5)ln(1V2)∝(−8πd2m*φ33heV),(6)where V is the gate voltage, d is the width of the tunneling barrier, m* is effective mass of the carrier, and φ is the barrier height. Figure 5(i) shows the fitted result, which can be divided into two regions. At the low gate voltage (region II), the electrons flow from Ga2O3 to WSe2; thus the current of the Ga2O3 channel is relatively low. However, as the gate voltage increases, the depletion region becomes larger and the electrons cannot cross the barrier. At large gate voltages, the barrier in the depletion region will become more triangular and thus the electrons will undergo FNT behavior, causing the gate current to increase dramatically, which in turn affects the channel current and causes the Ga2O3 channel current to decrease.

The adjustable positive and negative photoresponses exhibited by the Ga2O3/WSe2 JFET in response to variations in gate voltage and light intensity hold potential applications in logic gate operations and communication systems. Figure 6(a) shows a schematic illustration of the Ga2O3/WSe2 JFET under 635 nm illumination. To demonstrate the potential applications of Ga2O3/WSe2 JFET, its combined optical and electrical characteristics are utilized to design a NAND logic gate, as depicted in Fig. 6(b). More concretely, the 635 nm laser serves as input 1 (represented by “ON” for a logical “1” and “OFF” for a logical “0”). Similarly, the gate voltage is defined as input 2 (represented by “Vg=5V” for a logical “1” and “Vg=−5V” for a logical “0”). The current is defined as “output”, of which the high-level current and low-level current correspond to the values “1” and “0”, respectively. It can be easily found that output 1 is obtained only under the condition of input 1=0 and input 2=0, as shown in Figs. 6(c) and 6(d). Additionally, when the output signals of the Ga2O3/WSe2 JFET exhibit four grounds of “01010011”, “01000011”, “01001110”, “01010101”, the computers take first the input signals to convert into “S”, “C”, “N”, “U” for American Standard Code for Information Interchange (ASCII) as shown in Fig. 6(e). Over all, these results demonstrate that the Ga2O3/WSe2 JFET has great promising prospects in the field of logic device technology and optical communication.

In summary, we have successfully synthesized Ga2O3 microwires using CVD and fabricated a 1D/2D Ga2O3/WSe2 mixed-dimensional JFET. In this structure, WSe2 nanosheets serve as the gate dielectric layer and light photosensitive layer. Notably, they have minimal impact on the Ga2O3 channel currents under the dark condition. Under illumination at 635 nm, the JFET exhibits gate-tunable positive and negative photoresponses. When Vg=5V, a negative response was realized through FNT behavior. Our device exhibits a reliable and consistent negative photoresponse. Notably, the R reached 425 mA/W at Vds=1V and Vg=5V under 635 nm illumination with a power intensity of 194.52mW/cm2. The response and decay times of the Ga2O3/WSe2 JFET have reached 50.1 ms and 53.9 ms, respectively. Additionally, our device achieves logic operation and optical communication by utilizing positive and negative photoresponses. This successful realization of the Ga2O3/WSe2 mixed-dimensional JFET provides a feasible approach for the realization of optical logic gates, optical communication, and photogating transistors in Ga2O3-based devices.