M. A. Tsai

Efficiency and Droop Improvement in GaN-Based High-Voltage Light-Emitting Diodes

The efficiency and electrical characteristics of GaN-based high-voltage light-emitting diodes (HV-LEDs) are investigated in detail. The spatial distribution of light output and simulation results showed that 100-V HV-LED with smaller microchips had superior current spreading. As a result, under 1-W operation, the luminous efficiency of 100-V HV-LED with smaller microchips was enhanced by 7.8% compared to that of 50-V HV-LED, while the efficiency droop behaviors were reduced from 28% in 50-V HV-LED to 25.8% in 100-V HV-LED. Moreover, smaller microchips exhibited lower series resistance and forward voltage, leading to higher wall-plug efficiency.

Conversion Efficiency Enhancement of GaN/In

In this study, p-i-n double-heterojunction GaN/ In0.11Ga0.89 N solar cells grown by metal-organic chemical vapor deposition on pattern sapphire substrate are presented. The solar cell with standard process has a conversion efficiency of 3.1%, which corresponds to a fill factor of 58%, short circuit current density of 2.86 mA/cm2 , and open circuit voltage of 1.87 V under AM1.5G illumination. To further improve the conversion efficiency of the GaN/ In0.11Ga0.89 N solar cells, two-dimensional polystyrene nanospheres were deposited and self-organized as mask in the anisotropic inductively coupled plasma reactive ion etching process to form a biomimetic surface roughing texture. The surface morphology of the solar cell shows a periodically hexagonal bead pattern and the beads are formed in a diameter of 160 nm with a period of 250 nm. An increase of 15% in short circuit current density is found, thus improving the conversion efficiency to 3.87%. If we optimize the structure for 180 nm of the height and 375 nm of the period, a 10% gain can be expected when compared to the current structure.

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The impacts of soft-breakdown (SBD) on the characteristics of deep sub-micron NMOSFETs were investigated. It is shown that the BD location plays a crucial role in the post-BD switching function of the device. When BD occurs at the channel, the turn-on behavior of the drain current would not be significantly affected, which is in strong contrast to the case of ED at the drain, Nevertheless, significant increase in gate current is observed in the off-state when the gate voltage is more negative than -1 V. Its origin is identified to be due to the action of two parasitic bipolar transistors formed after SBD occurrence at the channel.

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A single-junction InGaP solar cells with polystyrene (PS) nanospheres on the surface has been developed. The self-assembly of PS nanospheres is one of the simplest and the fastest methods with which to build a 2D closely packed periodic structure. Based on the scattering of the PS nanospheres, the light path length can be increased. As compared to the InGaP solar cells without PS nanospheres on the top surface the standard process. An increase short-circuit current form 8.93% to 10.31% is improved when a single-junction InGaP solar cells with PS nanospheres. The conversion efficiency measured can also be improved from 8.79% to 9.71%. The single-junction InGaP solar cell with PS nanospheres was achieved.

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The effects of plasma charging on the negative-bias-temperature instability (NBTI) of p-channel metal-oxide-semiconductor (PMOS) transistors were explored in this work. It is clearly shown that the threshold voltage shift during bias-temperature stressing (BTS) could be enhanced by plasma charging damage. More importantly, we also found that electron trappings are aggravated by plasma charging, even on new devices with large antenna area ratios prior to BTS. Our charge pumping current measurements confirm that the interface-state density is increased for devices with large antennas, both before and after the BTS. This indicates, without ambiguity, that electron trapping is solely responsible for the observed low (in absolute value) threshold voltage in new devices with large antennas. Finally, it is proposed that the NBTI characterization can be used as a sensitive method for characterizing the antenna effects in devices with ultrathin gate oxide, which is particularly attractive in light of the fact that conventional indicators are becoming inadequate as oxide is scaled down.

N Solar Cells With Nano Patterned Sapphire and Biomimetic Surface Antireflection Process

Photoelectric conversion characteristics of a GaAs solar cell with biomimetic antireflective nanostructure are investigated by using an near-field optical beam induced current (NOBIC) technique. The mappings of the nanostructured profile and photocurrent induced by incident light with different wavelengths can be obtained simultaneously. The measured results of NOBIC show the optical conversion efficiency is mostly harvested near the valley of the nanostructures, which serve as light guides for incident light with three different wavelengths. The observation is consistent with the simulation results using a rigorous coupled wave analysis.