We present iron-doped beta gallium oxide (Fe- $\beta $ Ga $_{{\text{2}}}$ O $_{{\text{3}}}$ ) as a candidate for photoconductive semiconductor switches (PCSSs) with sub-bandgap light. From a commercially available Fe- $\beta $ Ga $_{{\text{2}}}$ O $_{{\text{3}}}$ wafer, we first did material characterization. This included measurements of absorption coefficient and dopant composition, carrier activation energy up to 200 $^{\circ}$ C, break down field of planar electrodes (limited from material passivation), and free carrier recombination lifetime, and thermal effects up to 203 $^{\circ}$ C on photocurrent with a 447 nm light emitting diode (LED) source. We then demonstrated pulsed operation of a Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ PCSS under different sub-bandgap wavelengths (355, 532, and 1064 nm) and sub-ns pulses. Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ is a candidate for high temperature PCSS with 355 nm responsivity of 7 $\times$ 10 $^{-\text{7}}$ A-cm/W-kV at room temperature and up to 5.5 $\times$ 10 $^{-\text{4}}$ A-cm/W-kV at 200 $^{\circ}$ C. From these investigations, we discuss a simple trap model to describe the illumination process of the PCSS. Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ has a high breakdown field and has moderate responsivity characteristics, but the dark current at high temperature leads to low photo-to-dark current ratio (PDCR). Regardless, we verify its potential as a PCSS material for harsh environment applications.

Due to relatively low responsivity at near infrared (NIR) wavelengths, surface-illuminated silicon (Si) photodiodes (PDs) are not attractive for ultra-fast data communication applications despite their CMOS-compatibility. Metal-semiconductor-metal (MSM) photodiodes are well-known for simplicity in fabrication compared to pin and pn junctions-based counterparts, but they usually work with lower efficiencies due to thin absorption layer that ensures high speed response. In this letter, we demonstrate a high efficiency and high-speed Si MSM PD with innovative photon-trapping surface structures. These wavelength-scale structures decrease the surface reflection and introduce laterally propagating waves parallel to the semiconductor surface. The responsivity of Si MSM PDs with the photon-trapping holes was measured to be ~0.59A/W, while the control devices without holes show ~0.08A/W. Such PDs with $50\mu \text{m}$ diameter and with 1 $\mu \text{m}$ finger spacing provide a pulse response of 60ps and 38ps full-width at half-maximum (FWHM) at 3V and 10V applied bias, respectively. Surface-illuminated PDs with these dimensions can offer easy optical coupling options for fiber optic links while offering complete CMOS compatibility.

Monolithic integration of high-speed, high-efficiency photodiodes with receiver electronics on a single silicon chip is a key to reduce cost and improve the performance of data centers’ short reach optical interconnects. We report a CMOS-compatible surface illuminated silicon PIN photodiode with integrated micro- and nano-scale holes that trap photons for longer interaction with semiconductors resulting in higher absorption efficiencies. The fabricated photodiode demonstrates more than 55% external quantum efficiency (EQE) at 850nm, and a measured Full-Width at Half Maximum (FWHM) impulse response of 29ps, 31ps, and 34ps at 15V, 10V, and 3V reverse biases, respectively. Simulation results indicate the possibility of operation up to 12.5Gb/s (NRZ) with no equalization.

Silicon photodiode-based CMOS sensors with backside-illumination for 300–1100 nm wavelength range were studied. We showed that a single hole in the photodiode increases the optical efficiency of the pixel. In near-infrared wavelengths, the enhancement allows 70% absorption in a $3~\mu \text{m}$ thick Si. It is $4\times $ better than that for the flat pixel. We compared different shapes and sizes of single hole and hole arrays. We have shown that a certain size and shape in single hole-based pixels contribute to stronger enhancement of optical efficiencies. The crosstalk was successfully reduced by employing trenches between pixels. We optimized the dimensions of the trenches to achieve minimal pixel separation for $1.12~\mu \text{m}$ pixels.

Photoconductive semiconductor switching (PCSS) devices have unique characteristics to address the growing need for electrically isolated, optically gated, picosecond-scale jitter devices capable of operating at high voltage, current, and frequency. The state of the art in material selection, doping, triggering, and system integration in PCSSs is presented. The material properties and doping considerations of GaN, GaAs, SiC, diamond, and β-Ga₂O₃ in the fabrication of PCSS devices are discussed. A review of the current understanding of the physics of the high-gain mode known as lock-on is presented.

Highly resistive gallium nitride (GaN) is an essential material for power optoelectronic applications. While carbon doping is widely used to achieve semi-insulating properties in GaN, the persistent photoconductivity (PPC) arising from deep-level defect traps remains a major obstacle for high-speed power switching. This study demonstrates a novel approach: leveraging the ultrahigh photoresponsivity of GaN:C (up to 2.1 A $\cdot $ cm/W $\cdot $ kV, surpassing alternatives such as GaN:Fe) and employing defect-selective optical control to effectively quench the PPC. By synchronizing a short infrared (1064 nm) quenching pulse with UV (385 nm) excitation in an epitaxially grown GaN:C layer on a heavily doped n-type GaN substrate, we achieve a dramatic reduction in photocurrent fall time by approximately $293\times $ (from 470 to 1.6 $\mu $ s), increasing modulation bandwidth from 745 Hz to nearly 218 kHz. This advancement not only establishes a new pathway for controlling PPC in GaN:C but also enables the practical integration of GaN:C in fast power switching devices. Enhanced modulation bandwidth, along with GaN:C excellent photoresponsivity, makes it a promising candidate for optically controlled high-voltage, high-power electronic systems, such as photoconductive semiconductor switches (PCSSs) used in pulsed-power drivers, high-power microwave (HPM) sources, and high-voltage gate drivers for wide bandgap (WBG) power electronics.