DFB laser
At the Integrated Optoelectronic Device Lab (IOED), we focus on developing advanced Distributed Feedback (DFB) laser chips suitable for high-speed optical communications, silicon photonics platforms, and LiDAR systems. Through meticulous design and rigorous manufacturing processes, our research team has achieved significant advancements in these domains.In the realm of high-speed optical communications, our 1.31-μm directly modulated DFB lasers have achieved data transmission rates up to 53.12 Gb/s using PAM4 modulation, while maintaining stable single-frequency operation and low threshold currents. These characteristics make our lasers highly suitable for future 200 Gb and 400 Gb Ethernet communication technologies.
In the field of LiDAR technology, our C-band DFB lasers, characterized by exceptionally low relative intensity noise and sub-100 kHz linewidths, coupled with sufficient optical power, are well-suited for FMCW-LiDAR modules. Additionally, our high-power 1.55-μm DFB lasers have been validated to produce up to 6 W of pulsed optical power, exhibiting superior operational efficiency and suitability for ToF-LiDAR systems in autonomous vehicle applications. Regarding our 1.55-μm high-power DFB lasers, we have achieved more than 100 mW of maximum optical power and low threshold currents, exhibiting excellent linewidth stability and GHz-level modulation rates, making these lasers an ideal choice for optical communications systems in low Earth orbit satellites and for efficient communication.
At IOED, we not only pursue technological innovation but also strive to establish robust partnerships with international research institutions and industrial collaborators. Through these collaborations, we believe in continuously pushing the boundaries of technology to address the technical challenges faced by the contemporary world. Through ongoing research and development, we aim to bring more innovative solutions to the global community.
GaN HEMT & MMIC
In recent years, with the booming development of the high-tech industry and continuous technological advancements, applications in various aspects of human life have significantly expanded. Notably, there is a growing demand for power components in sectors like automotive electronics, power converters, and base stations, where they play a crucial role. The primary goal in developing power electronic components now is to effectively convert power while minimizing conversion losses. Traditionally, silicon semiconductors have been the mainstay for high-voltage power components. However, the inherent limitations of silicon, such as electron mobility and critical breakdown electric fields, have restricted further improvements in conversion efficiency, diminishing its advantages in applications such as automotive electronics. Consequently, the current development direction for power components focuses on wide bandgap materials like Gallium Nitride (GaN) and Silicon Carbide (SiC). Compared to silicon, these wide bandgap semiconductor materials offer superior characteristics, such as higher breakdown voltages and electron mobility, as illustrated in Figure 1. The exceptional properties of wide bandgap materials enable them to replace silicon-based power components for high-voltage and high-frequency operations. Additionally, an excellent switching device must possess characteristics such as high switching frequency, low switching losses, compact size, and high power density, particularly important in DC/DC or DC/AC converters where high switching frequency is crucial.Generally, the quality of switching devices is measured by the Power Device Figure of Merit, defined as the product of the on-resistance (RON) and the gate charge (Qgd), where a lower value indicates lower device losses and better switching speed. Table 1 compares the power device loss properties and high-frequency performance indicators (Johnson’s Figure of Merit) of GaN, SiC, and silicon, showing that GaN excels in these metrics. This demonstrates that using GaN material for switching devices achieves lower switching losses and higher conversion efficiency at the same switching frequency, along with benefits like high-temperature tolerance and low noise, making GaN highly suitable for power electronics components. Figure 2 presents the Yole Group’s 2023 market research results and future trends for GaN power devices, revealing substantial potential for GaN components in the future power device market.
Our research team is currently focused on developing high-frequency GaN HEMTs and GaN MMICs, with FT exceeding 95 GHz and Fmax over 150 GHz.
Light emitting transistor
The Light-Emitting Transistor (LET) represents a pioneering breakthrough in optoelectronics, originating from the seminal work of Professors Holonyak, Jr. and M. Feng at the University of Illinois Urbana-Champaign (UIUC) in 2004. Their innovative approach utilized the radiative recombination observed in the graded base of InGaP/GaAs HBTs, previously considered as heat loss, to generate light. By integrating Quantum Wells (QWs) into the base of HBTs, they invented the Light-Emitting Transistor (LET), a high-speed three-port device capable of both electrical and optical outputs.By incorporating optical cavity and confining layers, QW-HBTs can evolve into Transistor Lasers (TLs), emitting coherent light. TLs offer high-speed modulation through intracavity photon-assisted tunneling (ICPAT), outpacing diode lasers in optical transmission. Unlike diode lasers, TLs exhibit a tilted charge distribution, allowing slow carriers to be swept towards the collector, reducing recombination lifetimes to the picosecond range. This results in a resonance-free frequency response and high optical modulation bandwidth. The light output of TLs depends on the base–collector junction reverse bias, enabling pure voltage modulation of light output—a feature absents in diode lasers. This dependence on collector voltage modulation may extend TL bandwidth further due to its reliance on fast tunneling processes.
Our research group at NTU IOED is dedicated to enhancing LET and TL efficiency through efficient epi-layer development, layout design, fabrication techniques, and comprehensive electrical and optical characterization. Collaborating with esteemed research group, such as Prof. Milton Feng's group at UIUC, we aim to pioneer Electron-Photonic Integrated Circuits (EPICs), leveraging LETs and TLs for future electronic-photonic applications. Our recent achievements include the design and fabrication of a Novel Darlington transistor for Thermal Sensor Technology and the development of optical logic gates using LETs, laying the groundwork for transformative advancements in EPICs.
Micro-LED
Micro-LED, or μLED, signifies a revolutionary flat-panel display technology comprising arrays of microscopic LEDs forming individual pixel elements. Originating in 2000, the pioneering work of Hongxing Jiang and Jingyu Lin at Kansas State University, later Texas Tech University, led to the development of the first high-resolution InGaN micro-LED microdisplay in VGA format by 2009. This milestone showcased the active driving of the micro-LED array via a complementary metal-oxide semiconductor (CMOS) IC.Compared to LCD technology, micro-LED displays excel in contrast, response times, and energy efficiency. Their pixel-level light control and high contrast ratio reduce energy consumption substantially compared to conventional LCDs. Moreover, the inorganic nature of micro-LEDs affords them a longer lifetime advantage over OLEDs and enables brighter images with minimal risk of burn-in.
The sub-nanosecond response time of micro-LEDs, particularly beneficial in 3D/AR/VR displays, positions them favourably for high-speed modulation and chip-to-chip interconnect applications. Additionally, micro-LEDs hold promise in high-frequency applications such as Radio Frequency (RF) and Visible Light Communication (VLC), leveraging their rapid response times and precise control for data transmission and communication.
At NTU IOED, our research group is dedicated to enhancing micro-LED efficiency through epi-layer optimization, layout design, and fabrication techniques. Utilizing methods like solution surface treatment, surface roughness adjustments, and ALD passivation, we optimize micro-LED arrays for display applications. We also conduct comprehensive characterization and analysis of micro-LED devices' optical and electrical properties to drive further optimization. Furthermore, our ongoing research focuses on developing high-speed micro-LED devices for VLC applications.
Silicon Photonics
Silicon photonics (SiPh) has become a mainstream technology, primarily due to advancements in optical communications. The current generation has witnessed an expansion from thousands to millions of integrated photonic devices, predominantly in the form of communication transceivers for data centers. Exciting new applications, including sensing and computing, are poised to introduce innovative products soon.Through the combination and use of various silicon photonic components - including waveguides, edge couplers, grating couplers, directional couplers, Mach-Zhender modulators, multimode modulators, and ring resonators - different light modulation functions can be realized. On the other hand, by means of heterogeneous integration and even co-packaged optics (CPO), Electron-Photonic Integrated Circuits (EPICs) that combines the optical circuits and electronic circuits, could be achieved
Our research group at NTU IOED is looking forward to base our research on the laboratory's expertise in epitaxial design, component simulation, process flow development, and the measurement and analysis of characteristics for optoelectronic components such as VCSELs, DFBs, LEDs, HEMTs, and HBTs. Through the use of a silicon photonics platform and the design and development of heterogeneous integrated interfaces, our goal is to integrate and package these optoelectronic components. This integration will enhance component efficiency, reduce power consumption, and enable usage across a variety of application scenarios. Our recent works include the design of silicon photonics chip combining DFB laser to achieve narrow linewidth laser source for Quantum Key Distribution (QKD), and the development of spot size converter (SSC) integrated high power DFB laser.
VCSEL
Vertical-cavity surface-emitting Lasers (VCSELs) offer a cutting-edge advancement in optoelectronic technology, offering a diverse array of applications and driving progress across numerous industries. VCSELs signify a pivotal shift in laser technology, distinguished by their distinct design and operational benefits. In contrast to conventional edge-emitting lasers, VCSELs emit light perpendicular to the semiconductor substrate, facilitating compact form factors, minimal power consumption, and precise control over wavelengths. These attributes position VCSELs as indispensable components in a wide spectrum of applications, including data communication, optical interconnects, facial recognition systems, automotive LiDAR technology, and medical diagnostics.At IOED LAB, our research endeavors in the realm of VCSELs are characterized by an unwavering commitment to excellence and innovation. Through rigorous investigation, experimentation, and collaboration with esteemed peers in the field, we have contributed significantly to the evolution of VCSEL design, fabrication methodologies, and performance enhancement techniques. From augmenting output power and beam quality to exploring novel materials and integration methodologies, our focus remains on pushing the boundaries of VCSEL technology to overcome emerging challenges and seize new opportunities.
With a firm belief in the promising capabilities of VCSEL technology, IOED LAB remains dedicated to continuous exploration and innovation, striving to make meaningful contributions to the advancement of VCSEL technology in the foreseeable future.
SPAD
Single-photon avalanche diode (SPAD) is an optical detector based on reverse bias operation of the p-n junction, achieved by applying a reverse bias voltage higher than the breakdown voltage (Geiger mode) to reach extremely high electric field values. This allows individual charge carriers injected into the depleted region to trigger self-sustaining avalanche breakdown, the current can rapidly rise to the milliampere level. theoretically resulting in infinite current gain and achieving high sensitivity in single-photon detection.Our research team at NTU IOED is dedicated to studying SPADs in the communication wavelength band, including III-V SPADs and Ge-on-Si SPADs. We optimize the characteristics of these devices through TCAD device structure design, quenching circuit design, and measurement techniques to improve detection efficiency, suppress noise, reduce timing jitter, and minimize afterpulsing effects. Additionally, we collaborate with Taiwan's leading semiconductor company, TSMC, aiming to develop the latest integrated circuit-compatible Ge-on-Si SPAD. This collaboration aims to break through temperature limitations in structural design, obtain a high-gain, commercially valuable optical sensor, and apply it in areas such as LiDAR and optical communications.
Fig. 1 Schematic diagram and band diagram of common HBT.
Fig. 2 Common problems and corresponding solutions in the application of Type I and Type II DHBT.
Fig. 3 (a) Schematic diagram of a dual-port network, (b) equivalent small-signal circuit model, (c) S-parameter measurement and simulation curves.
Our research group at NTU IOED focuses on developing high-frequency InP HBT components and their manufacturing processes. We employ electron beam lithography (e-beam) to fabricate emitters with line widths below 500 nm. Moreover, we are developing alignment technologies for base fabrication, achieving a self-aligned layout structure that facilitates device area reduction. This reduction in overall component area enables us to obtain high-frequency InP HBTs with improved performance.
HBT
In the era of high-speed transmission with 5G/6G technology, compound semiconductors are gaining prominence in applications such as high-frequency communications, high-voltage power conversion, and specialized sensing, owing to their exceptional material and component properties. Among Group III and V semiconductor electronic components, gallium arsenide (GaAs) stands out as the most mature material, utilized in producing RF power amplifiers (Power Amplifier, PA) and low-noise amplifiers (LNA) crucial for devices and base stations. However, as current wireless communications advance towards the millimeter-wave frequency band, GaAs-based components are gradually falling short of meeting the necessary specifications for long-distance communication and power efficiency.Indium phosphide (InP) is emerging as a promising replacement. InP transistors offer superior frequency response and high-frequency power density compared to GaAs, making them suitable for millimeter-wave power transmission and reception in 5G/6G smartphones, and potentially for sub-terahertz (sub-THz) bands in the B5G (Beyond 5G) generation, ranging from 100 to 500 GHz and other high-frequency bands. Moreover, InP materials find applications in radio frequency integrated circuits for satellite communications, showing significant market potential in emerging fields such as long-distance image sensing.
Semiconductor materials suitable for high-frequency transmission and amplification must possess characteristics such as electron mobility, saturation electron drift rate, and power density. InP boasts an electron mobility of up to 5,400 (cm²/V·s), second only to GaAs and surpassing most common semiconductor materials, while offering significantly higher power density in its components compared to GaAs. These material characteristics are summarized in Table 1. Furthermore, the carrier concentration and electron mobility of the InGaAs epitaxial layer, which matches the InP lattice, surpass that of AlGaAs material matching the GaAs lattice. This suggests that InP-based heterojunction transistors can exhibit wider bandwidth responses than their GaAs-based counterparts. In terms of bipolar heterojunction transistors (HBT), the energy gap of InP HBT material is lower than that of GaAs HBT, allowing for operating voltages to be halved in InP components. Consequently, power amplifiers made of InP HBT consume less power than GaAs HBTs currently used in mobile phones and base stations, with higher power added efficiency (PAE), thereby extending the usage time of mobile devices. Hence, InP HBT is considered a critical technology for 5G smartphone millimeter-wave communication. Additionally, InP HBTs find utility in radio frequency integrated circuits for satellite communications and long-distance millimeter-wave sensing in automobiles.