Energy efficiency is an important issue for wireless sensor networks (WSNs). A MAC protocol for WSNs may tolerate higher packet latency in order for minimizing energy consumption, but such a MAC protocol may not accommodate real-time applications. In this paper, we present a hybrid MAC protocol that minimizes both energy consumption and packet latency simultaneously. The protocol utilizes a cross-layer approach that dynamically switches the MAC behavior between TDMA and CSMA based on the routing information of AODV. The nodes which are not involved in data transmission are kept in TDMA mode to minimize their energy consumption. When a node receives a routing request message, it switches to CSMA mode for receiving subsequent data packets promptly. We evaluate the performance of our protocol with various network density. The simulation results show that our protocol can improve both energy efficiency and packet latency of WSNs.

Cross-calibration using an autocollimator and a polygon is the traditional method for calibrating a rotary encoder. These angles, which can be calibrated using this method, are limited by the pitch angle of the polygon, which is 15° for a 24-faced polygon. In this work, we propose a new shift-angle method using the same setup as the traditional method. However, the new method can measure smaller than the pitch angle of the polygon, which is the measurement limitation of the traditional method. The proposed method can calibrate every angle of the rotary encoder. In the experiment, we use an autocollimator and a 24-faced polygon to calibrate the SelfA rotary encoder to verify the proposed shift-angle method. The SelfA rotary encoder, which comprises one rotary encoder and 12 read heads, is calibrated using self-calibration. The difference between the calibration results obtained by applying these two methods to the same SelfA rotary encoder is smaller than ±0.1″.

InGaN-based light-emitting solar cell (LESC) structure with an inverted pyramidal structure at GaN/sapphire interface was fabricated through a laser decomposition process and a wet crystallographic etching process. The highest light output power of the laser-treated LESC structure, with a 56% backside roughened-area ratio, had a 75% enhancement compared to the conventional device at a 20 mA operating current. By increasing the backside roughened area, the cutoff wavelength of the transmittance spectra and the wavelength of the peak photovoltaic efficiency had a redshift phenomenon that could be caused by increasing the light absorption at InGaN active layer.

InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.

InGaN-based light-emitting diodes (LEDs) with inverted pyramidal structures at their GaN/patterned-sapphire interfaces were fabricated by laser decomposition and wet crystallographic etching. Partially roughening the LED structure increased the light output power of the by 21% enhancement at an operating current of 20mA over that of a non-treated LED structure. The transmittance of the roughened LED structure (10.5%) was lower than that of a non-treated LED structure (20.3%) at 447nm because the incident light was mostly reflected and scattered by the patterned-sapphire structure and by the roughened structure at the GaN/sapphire interface. The light emission intensity in the laser-treated striped region, with the roughened N-face GaN surface was higher than that in the non-treated patterned sapphire region. The InGaN structure with the roughened inverted pyramidal structure increased the light extraction efficiency of nitride-based LEDs.Gallium nitride materials have attracted much interest in the development of optoelectronic devices such as white light-emitting diodes 1 (LEDs) and laser diodes. Nevertheless, bright blue LEDs require higher internal and external quantum efficiencies. The low external quantum efficiency of InGaN-based LEDs is caused by the large difference between the refractive indexes of the GaN layer and the surrounding air ( n∼1.5). Bottom-patterned Al 2 O 3 substrates, 2 roughened p-type GaN:Mg surface, 3 the formation of photonic crystal structures, 4 amorphous titanium oxide films with porous structures and a graded refractive-index, 5 two-floor air prism arrays as embedded reflectors, 6 overcut side-holes that are formed by wet etching, 7 and anisotropically etched GaN-sapphire interfaces, 8 have all been utilized to increase the light-extraction efficiency of InGaN-based LEDs on Al 2 O 3 substrates. Fujii et al. 9 reported that a laser-lift-off technique that was followed by an anisotropic etching process to roughen the surface of an n-side-up GaN-based LED with a hexagonal ''conelike'' surface, increased extraction efficiency. Structures of InGaN-based LEDs that promote light extraction have also been formed using pulselaser fabrication processes. 10-13 Lee et al. 14 increased the output power of an InGaN-based LED by reducing both the thermal damage to the sapphire substrate, along with the consequently formed debris, using femtosecond laser scribing.In this work, roughened inverted-pyramidal structures of InGaN LEDs were observed at the top and the bottom of the patternedsapphire substrate via a laser decomposition process and a wet etching process. The InGaN LED structure comprised a patterned sapphire substrate and a roughened structure that increased light extraction efficiency. A roughened N-face GaN surface was formed for high light extraction without using a conventional laser lift-off process. The optical properties, light-extraction efficiency, and far-field radiation pattern of the laser-treated LED structures were analyzed in detail. ExperimentalInGaN-based LED structures w...