Objective In modern electromagnetic countermeasures (ECM), high power, ultrafast response, higher bandwidth, miniaturization, and multifunctional integration have become the core pursuits of airborne electronic warfare (EW) systems. The bottlenecks of conventional semiconductor switches in high-voltage endurance, switching speed, and power density are being broken by β-Ga₂O₃, a fourth-generation ultrawide-bandgap (UWBG) semiconductor. Owing to its ultrahigh breakdown field strength and sub-nanosecond optoelectronic switching speed, β-Ga₂O₃ serves as an ideal substrate material for next-generation photoconductive semiconductor switches (PCSS), and is expected to become one of the core devices for next-generation optically controlled high-power microwave (HPM) sources and compact pulsed power supplies, holding significant application value in airborne electromagnetic countermeasure technologies. Photoconductive switches achieve on/off switching by modulating the semiconductor conductivity with laser illumination, featuring optical isolation, fast response, and high power capability, with broad application prospects in ultrafast electronics, pulsed power, and high-power microwave sources. To date, research on photoconductive devices based on third-generation semiconductors such as SiC and GaN has become relatively mature; among them, SiC-based photoconductive devices can achieve megawatt-level peak power output and wideband tunability from 0.5 to 10 GHz. However, existing devices still suffer from insufficient high-power output, high-frequency response, and photoelectric conversion efficiency. Aiming at the limited power and frequency performance of existing PCSS, this paper fabricates Fe-doped β-Ga₂O₃ photoconductive devices with metal mesh electrodes and systematically conducts multi-wavelength and ultrafast transient photoelectric response tests.

Methods The β-Ga₂O₃ single crystals used in this work were Fe-doped single crystals grown under gallium-rich conditions, and all Fe-doped β-Ga₂O₃ substrates had a thickness of 0.5 mm. This study adopted a vertical device structure with facing electrodes on the front and back surfaces. The electrodes were required to simultaneously satisfy the requirements of high electrical conductivity and high optical transmittance. Since the β-Ga₂O₃ photoconductive devices need to be excited by 355 nm light, and the commonly used transparent electrodes exhibit very low transmittance at this wavelength, mesh electrodes were adopted instead, which preserve light-transmitting windows while ensuring high conductivity, thereby balancing both electrical conductivity and optical transmittance. The device fabrication process is described as follows. After cleaning, photolithography was performed on the front surface of the β-Ga₂O₃ substrate, and the front-side ring electrode pattern was defined through a series of process steps including spin coating, pre-baking, exposure, post-baking, and development. The ring electrode had an outer diameter of 3.5 mm and an inner diameter of 2 mm. Subsequently, a multilayer Ti/Au metal electrode was deposited sequentially by electron beam evaporation, with layer thicknesses of 30 nm and 100 nm, respectively. After the excess metal was lifted off by ultrasonic treatment, the same procedure was applied to the back surface of the substrate. Finally, the double-sided multilayer electrodes were annealed at approximately 900 °C for 2 min to ensure good ohmic contact between the electrodes and the semiconductor substrate. After device fabrication, insulating packaging was applied, after which experimental verification was carried out using a coaxial switch test circuit, whose link configuration is shown in Figure 3. The laser beam emitted by the light source was collimated and shaped by an optical shaping lens group, propagated in free space in accordance with geometrical optics, and irradiated the device surface, exciting the device to generate photogenerated carriers. The RF tube, comprising six 30 kV/1 nF ceramic energy-storage capacitors, provided charging support for the device, keeping the voltage across the device constant within 10 ns. Under the applied bias electric field, the photogenerated carriers drifted directionally and formed a photocurrent. The photocurrent was transmitted through a coaxial cable, attenuated by a 30 dB attenuator, and then acquired by a high-voltage oscilloscope (Huaxiang/SHX MS2-8000, Shanghai, China) with a bandwidth of 4 GHz and an impedance of 50 Ω.

Results and Discussions The device achieves the minimum on-resistance under 355 nm irradiation, and no carrier saturation occurs within the laser energy range of 1–5 mJ, demonstrating linear absorption characteristics. Band structure analysis indicates that long-wavelength lasers only induce shallow-level ionization of Fe²⁺, while ultraviolet light can activate complex defects such as gallium vacancies. Multi-channel carrier transitions greatly improve the photoelectric conversion efficiency. Ultrafast laser tests with a pulse width of 700 ps verify that the device with Fe doping concentration of 1×10¹⁸ cm⁻³ possesses shorter pulse rise/fall time, negligible pulse tailing and excellent waveform stability, making it more suitable for high-power microwave systems.

Conclusions This study systematically investigates the influence and underlying mechanisms of laser wavelength, laser energy, and doping concentration on the photoelectric response characteristics of Fe-doped β-Ga₂O₃ photoconductive devices. The experimental results reveal that the photoelectric response performance of the devices is highly correlated with the incident laser. The 355 nm ultraviolet laser can excite multiple types of defects and multiple deep-level transitions, significantly increasing the peak current and substantially reducing the on-resistance, with a response capability far superior to that at the 532 nm band. The devices exhibit good linear absorption characteristics in the laser energy range of 1–5 mJ, with no obvious carrier saturation. Different wavelengths correspond to differentiated electron transition channels, and shorter wavelengths can activate more complex defect levels, which is the core mechanism underlying their superior photoelectric performance. Hundred-picosecond ultrafast tests show that, compared with existing β-Ga₂O₃ photoconductive devices, the device developed in this work exhibits a lower on-resistance (a minimum on-resistance of approximately 103 Ω at 355 nm under a 6 kV bias, lower than the reported value of approximately 327.8 Ω for vertical β-Ga₂O₃ photoconductive devices), a pulse full width at half maximum (FWHM) of only 993 ps, no obvious pulse tailing, and excellent waveform stability. These characteristics render the device more suitable for high-power microwave application scenarios featuring high repetition rate, narrow pulse width, and high power density, and provide an experimental basis for the optimized design of Fe-doped β-Ga₂O₃ photoconductive switches in high-power microwave applications.