Seung Kyu Oh

Electron beam irradiated silver nanowires for a highly transparent heater

Transparent heaters have attracted increasing attention for their usefulness in vehicle windows, outdoor displays, and periscopes. We present high performance transparent heaters based on Ag nanowires with electron beam irradiation. We obtained an Ag-nanowire thin film with 48 ohm/sq of sheet resistance and 88.8% (substrate included) transmittance at 550 nm after electron beam irradiation for 120 sec. We demonstrate that the electron beam creates nano-soldering at the junctions of the Ag nanowires, which produces lower sheet resistance and improved adhesion of the Ag nanowires. We fabricated a transparent heater with Ag nanowires after electron beam irradiation, and obtained a temperature of 51 °C within 1 min at an applied voltage of 7 V. The presented technique will be useful in a wide range of applications for transparent heaters.

High Efficiency and ESD of GaN-Based LEDs With Patterned Ion-Damaged Current Blocking Layer

This letter examined the use of a patterned ion-damaged current blocking layer (patterned-IDCBL). A 50-Å-InGaN layer grown as the top epitaxial layer was transformed into an insulator fabricated by oxygen plasma treatment, in which the dot patterns are regularly arranged over the active areas of the light-emitting diode (LED) and inserted beneath the p-electrode. The results showed that the light output power increased by 16.8% at 60 mA compared with the conventional LEDs, and that the electrostatic discharge resistance is effectively improved by the patterned-IDCBL.

Thin-Film-Flip-Chip LEDs Grown on Si Substrate Using Wafer-Level Chip-Scale Package

Demonstrated are visible GaN-based light-emitting diodes (LEDs) on economical large-area Si substrates using an advanced device and packaging architecture to improve optical output power, while reducing manufacturing costs. The process employs thin-film-flip-chip devices and wafer-level chip-scale packages and uses through-Si-via substrate and anisotropic conductive film for bonding. The improved curvature control region is applied in the epitaxial growth of the LED structure on a Si substrate to achieve flat wafers for epitaxial structures at room temperature, which is critical for wafer-level bonding. External quantum efficiency and light-output power at 350 mA increase by ~12% compared with those of conventional flip-chip LEDs grown on a sapphire substrate. The devices also show a reverse-bias leakage current failure rate of <;10%.

Visible Flip-Chip Light-Emitting Diodes on Flexible Ceramic Substrate With Improved Thermal Management

We demonstrate flip-chip light-emitting diodes (FC-LEDs) on a flexible yttria-stabilized zirconia (YSZ) substrate and compare them with FC-LEDs on a polymeric substrate. Degradation of luminescence intensity and red-shift of peak wavelength are not observed for the LED on the flexible YSZ, unlike one on the polyimide substrate, due to improved capability to remove the generated heat from the chip to the substrate. Thermal distribution measurements and finite-element simulations show improved thermal management by the flexible ceramic as compared with previously developed flexible LEDs on polymeric substrates. The results present an improved solution to high power operation of flexible LEDs.

Effect of electron-beam irradiation on leakage current of AlGaN/GaN HEMTs on sapphire

This study examined the effect of electronbeam (E-beam) irradiation on the electrical properties of n-GaN, AlGaN and AlGN/GaN structures on sapphire substrates. E-beam irradiation resulted in a significant decrease in the gate leakage current of the n-GaN, AlGaN and HEMT structure from 4.0×10 -4 A, 6.5×10 -5 A, 2.7×10 -8 A to 7.7×10 -5 A, 7.7×10 -6 A, 4.7×10 -9 A, respectively, at a drain voltage of -10V. Furthermore, we also investigated the effect of E-beam irradiation on the AlGaN surface in AlGaN/GaN heterostructure high electron mobility transistors(HEMTs). The results showed that the maximum drain current density of the AlGaN/GaN HEMTs with E-beam irradiation was greatly improved, when compared to that of the AlGaN/GaN HEMTs without E-beam irradiation. These results strongly suggest that E-beam irradiation is a promising method to reduce leakage current of AlGaN/GaN HEMTs on sapphire through the neutralization the trap.

High-Output Lead-Free Flexible Piezoelectric Generator Using Single-Crystalline GaN Thin Film

Piezoelectric generators (PEGs) are a promising power source for future self-powered electronics by converting ubiquitous ambient mechanical energy into electricity. However, most of the high-output PEGs are made from lead zirconate titanate, in which the hazardous lead could be a potential risk to both humans and environment, limiting their real applications. III-Nitride (III-N) can be a potential candidate to make stable, safe, and efficient PEGs due to its high chemical stability and piezoelectricity. Also, PEGs are preferred to be flexible rather than rigid, to better harvest the low-magnitude mechanical energy. Herein, a high-output, lead-free, and flexible PEG (F-PEG) is made from GaN thin film by transferring a single-crystalline epitaxial layer from silicon substrate to a flexible substrate. The output voltage, current density, and power density can reach 28 V, 1 μA·cm-2, and 6 μW·cm-2, respectively, by bending the F-PEG. The generated electric power by human finger bending is high enough to light commercial visible light-emitting diodes and charge commercial capacitors. The output performance is maintained higher than 95% of its original value after 10 000-cycle test. This highly stable, high-output, and lead-free GaN thin-film F-PEG has the great potential for future self-powered electronic devices and systems.

Output power enhancement in AlGaN/GaN heterostructure field-effect transistors with multilevel metallization

To improve wafer utilization efficiency and heat dissipation performance, this paper proposes multilevel metallization-structured, lateral-type AlGaN/GaN heterostructure field-effect transistors (HFETs) on a 150 mm Si substrate using photosensitive polyimide (PSPI) as the intermetal dielectric layer. The maximum drain current of the HFETs is 46.3 A, which is 240% higher than that of conventional AlGaN/GaN HFETs with the same die size. Furthermore, the drain current drop of the HFETs under high-bias operation is reduced from 14.07 to 8.09%, as compared to that of conventional HFETs.

Development of chip shrink technology for lateral-type GaN based HFETs using SiO2/polyimide dual IMD layers

This study examined chip shrink technology for lateral-type AlGaN/GaN HFETs on 150 mm Si substrates fabricated with a bonding pad above the active area (BPAA) structure. The SiO2/polyimide layers were used as inter metal dielectric (IMD) layers, which yielded a very low leakage current of 0.58 nA/mm2 even at 1 kV and a good adhesion property after O2 plasma treatment. The fabricated AlGaN/GaN HFETs with the BPAA structure exhibited good device characteristics, such as a low leakage current of 7.1 nA at 1 kV and a drain current of 3.6 A at 2 V, which has the same value compared to that of the AlGaN/GaN HFETs without the BPAA structure, even though the BPAA structure reduced the size of chip by 40%. This suggests that the BPAA structure is a promising method for reducing the size and cost of the lateral-type AlGaN/GaN HFETs.

Bonding Pad Over Active Structure for Chip Shrinkage of High-Power AlGaN/GaN HFETs

This paper reports a bonding pad over active (BPOA) structure with photosensitive polyimide (PSPI) as the intermetal dielectric layer to reduce the chip size of high-power enhancement-mode AlGaN/GaN heterojunction FETs (HEFTs) on a 150-mm (6-in) Si substrate. The fabricated AlGaN/GaN HFETs with a BPOA structure exhibited a threshold voltage and a maximum current of 0.6 V and 38.6 A, respectively, at VGS of 6 V. The leakage current was 4.3 × 10-5 A at 700 V, which was the same value as that of the AlGaN/GaN HFETs without a BPOA structure. The reliability of the AlGaN/GaN HFETs with the BPOA structure during the packaging process was improved by the PSPI layer and a modified bonding pad structure, which endured the stress during the wedge wire bonding and epoxy molding processes.

High-power flexible AlGaN/GaN heterostructure field-effect transistors with suppression of negative differential conductance

We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS = 20 V, which is close to that of the HFETs on Si (9.6%), solving the probl...