Zhaojun Liu

Monolithic LED Microdisplay on Active Matrix Substrate Using Flip-Chip Technology

A monolithic high-resolution (individual pixel size 300times300 mum2) active matrix (AM) programmed 8times8 micro-LED array was fabricated using flip-chip technology. The display was composed of an AM panel and a LED microarray. The AM panel included driving circuits composed of p-type MOS transistors for each pixel. The n-electrodes of the LED pixels in the microarray were connected together, and the p-electrodes were connected to individual outputs of the driving circuits on the AM panel. Using flip-chip technology, the LED microarray was then flipped onto the AM panel to create a microdisplay.

A Novel BLU-Free Full-Color LED Projector Using LED on Silicon Micro-Displays

In this letter, we have described the design and fabrication of a novel backlight-unit (BLU)-free full-color light emitting diode (LED) based projector. The prototype used three active matrix addressable light emitting diode on silicon (LEDoS) micro-displays with peak emission wavelengths of 630, 535, and 445 nm. The LEDoS micro-displays were realized by integrating monolithic micro-LED arrays and silicon-based integrated circuits using a flip-chip bonding technique. Since the LEDoS micro-displays are self-emitting, conventional BLUs used in liquid crystal displays were not needed. Using a trichroic prism to combine the light from the three LEDoS chips, we have produced the worlds first three-LEDoS projector. This BLU-free three-LEDoS projector consists of much fewer optical components and has significantly higher light utilization efficiency compared with conventional projectors.

Monolithic Integration of AlGaN/GaN HEMT on LED by MOCVD

Monolithic integration of high-performance AlGaN/GaN high-electron mobility transistors (HEMTs) and blue light emitting diodes (LEDs) on sapphire substrates has been demonstrated by metal organic chemical vapor deposition selective growth technique. The integrated HEMT-LED exhibits a peak transconductance (Gm) of 244 mS/mm, a maximum drain current (Id) of 920 mA/mm, and an ON-resistance (Ron) of 2.6 Ω·mm. The forward voltage (VF) of the LED is 3.1 V under an injection current of 10 mA. The integrated LED emits modulated light power efficiently at a wavelength of 470 nm by a serially connected GaN HEMT, showing potential applications such as solid-state lighting, displays, and visible light communications.

360 PPI Flip-Chip Mounted Active Matrix Addressable Light Emitting Diode on Silicon (LEDoS) Micro-Displays

In this paper, we describe the design and fabrication of 360 PPI flip-chip mounted active matrix (AM) addressable light emitting diode on silicon (LEDoS) micro-displays. The LEDoS micro-displays are self-emitting devices which have higher light efficiency than liquid crystal based displays (LCDs) and longer lifetime than organic light emitting diodes (OLEDs) based displays . The LEDoS micro-displays were realized by integrating monolithic LED micro-arrays and silicon-based integrated circuit using a flip-chip bonding technique. The active matrix driving scheme was designed on the silicon to provide sufficient driving current and individual controllability of each LED pixel. Red, green, blue and Ultraviolet (UV) LEDoS micro-displays with a pixel size of 50 μm and pixel pitch of 70 μm were demonstrated. With a peripheral driving board, the LEDoS micro-display panels were programmed to show representative images and animations.

DC and RF Performance of Gate-Last AlN/GaN MOSHEMTs on Si With Regrown Source/Drain

This paper presents the fabrication and characteristics of self-aligned gate-last AlN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) featuring regrown source/drain for low ON-state resistance (RON). Previously, we demonstrated conventional enhancement-mode AlN/GaN MOSHEMTs on Si substrate with excellent DC performance but limited RF characteristics by large parasitic gate-to-source/drain overlap capacitance. In this paper, the self-aligned gate-last process was developed to minimize the parasitic capacitance. SiNx sidewall and supporting layer were inserted to separate the gate head and source/drain. In the gate-last devices, fT has been improved to be ~40 GHz with a channel length (Lg) of 210 nm. Delay time analysis showed that drain delay was relatively small compared with gate transit and parasitic charging time because of the self-aligned structure.

Selective epitaxial growth of monolithically integrated GaN-based light emitting diodes with AlGaN/GaN driving transistors

In this Letter, we report selective epitaxial growth of monolithically integrated GaN-based light emitting diodes (LEDs) with AlGaN/GaN high-electron-mobility transistor (HEMT) drivers. A comparison of two integration schemes, selective epitaxial removal (SER), and selective epitaxial growth (SEG) was made. We found the SER resulted in serious degradation of the underlying LEDs in a HEMT-on-LED structure due to damage of the p-GaN surface. The problem was circumvented using the SEG that avoided plasma etching and minimized device degradation. The integrated HEMT-LEDs by SEG exhibited comparable characteristics as unintegrated devices and emitted modulated blue light by gate biasing.

Full-Color Pixelated-Addressable Light Emitting Diode on Transparent Substrate (LEDoTS) Micro-Displays by CoB

In this paper, we report the design and fabrication of full-color pixelated-addressable light emitting diode on transparent substrate (LEDoTS) micro-displays. The LEDoTS micro-displays were realized by integrating monochromatic LEDs on a transparent quartz substrate using chip on board (CoB) technique. The full-color pixel has a pitch of 1 mm (P1) and includes red, green, and blue monochromatic LED chips. The pixelated-addressable driving circuit was designed on a custom PCB to provide constant current and individual addressing of each LED pixel with a certain frequency of row and column scanning. With a peripheral FPGA board, the LEDoTS micro-displays were programmed to display representative images and animations.

Investigation of Forward Voltage Uniformity in Monolithic Light-Emitting Diode Arrays

This letter reports the design and fabrication of optimized electrode structures for matrix addressable monolithic light-emitting diode arrays. The variation of forward voltages of LED pixels in the same row are greatly reduced from 2.62 V (81.9%) to 0.02 V (0.6%) in an 8×8 LED array. The LED arrays are designed with 8×8 square pixels, 500×500 μm2 with a 50-μm wide gap in between. A 24×24 large-scale blue LED panel is demonstrated with 0.06 V (1.8%) forward voltage variation by integrating nine LED array modules on a 2.2-cm diagonal silicon-based substrate. With a similar concept, a 30×30 green LED micro-display with scaled pixel pitch shows excellent display uniformity.

P-34: Active Matrix Programmable Monolithic Light Emitting Diodes on Silicon (LEDoS) Displays

In this paper, the first full-color active matrix programmable monolithic Light Emitting Diodes on Silicon (LEDoS) displays are fabricated using flip-chip technology. The forward voltage uniformity of the LED pixels was greatly improved by a double-side ground structure. A basic LED micro-array module was fabricated and the AM substrate has scaling-up ability. By integrating a certain number of LED micro-array modules onto a large-scale AM substrate, a large-scale display is obtained. By this scaling-up method, the utilization rate of the LED wafers is increased significantly. The yield of the LED pixels is improved simultaneously. Red, green and blue phosphors were excited by UV light to realize a full-color display.

Metal-interconnection-free integration of InGaN/GaN light emitting diodes with AlGaN/GaN high electron mobility transistors

We report a metal-interconnection-free integration scheme for InGaN/GaN light emitting diodes (LEDs) and AlGaN/GaN high electron mobility transistors (HEMTs) by combining selective epi removal (SER) and selective epitaxial growth (SEG) techniques. SER of HEMT epi was carried out first to expose the bottom unintentionally doped GaN buffer and the sidewall GaN channel. A LED structure was regrown in the SER region with the bottom n-type GaN layer (n-electrode of the LED) connected to the HEMTs laterally, enabling monolithic integration of the HEMTs and LEDs (HEMT-LED) without metal-interconnection. In addition to saving substrate real estate, minimal interface resistance between the regrown n-type GaN and the HEMT channel is a significant improvement over metal-interconnection. Furthermore, excellent off-state leakage characteristics of the driving transistor can also be guaranteed in such an integration scheme.