Tag Archives: letter-leds-tech

Gallium nitride (GaN) based devices are attractive for harsh environment electronics because of their high chemical and the mechanical stability of GaN itself that has a higher atomic displacement energy than other semiconductor materials.

However, degradation mechanisms of GaN device under radiation environments is not clear mainly because devices consist of many different types of semiconductors, such as p-type and n-type layers in light emitting diode (LED), and each layer has different hardness to radiation.

Now, researchers at the Electronics-Inspired Interdisciplinary Research Institute (EIIRIS) and Department of Electrical and Electronic Information Engineering at Toyohashi University of Technology, and the Japan Atomic Energy Agency (JAEA) describe the physical mechanism of an observed increase in the resistance of p-type GaN irradiated with 380 keV protons compared with n-type GaN.

This image depicts two-terminal resistance of p- and n-type GaN as a function of proton fluence. This inset shows schematic of sample, and lines are guide for eyes. Credit: Copyright (c) 2014 Toyohashi University of Technology.

This image depicts two-terminal resistance of p- and n-type GaN as a function of proton fluence. This inset shows schematic of sample, and lines are guide for eyes.
Credit: Copyright (c) 2014 Toyohashi University of Technology.

The GaN-based LED structure shown in Fig.1 was irradiated with protons and the resulting electrical properties measured. Notably, the electrodes to measure the resistance of the p-type and n-type layers were produced independently using the clean room facilities at EIIRIS and the ion implanter in JAEA.

The two terminal resistance of the n-type GaN did not vary from its initial value after 1×1014 cm-2 proton irradiation, and remained of the same order after 1×1015 cm-2 protons. However, a clear increase of the resistance was found in the p-type GaN after 1×1014 cm-2 irradiation. The resistance increased further by six orders of magnitude after 1×1015 cm-2.

The observed increase of the resistance in p-type GaN is explained as being due to the lower initial carrier density than in n-type GaN due to a lack of efficient p-type doping technology for GaN, which is a key for the realization of novel devices operable in harsh environments.

EV Group (EVG), a supplier of wafer bonding and lithography equipment for the MEMS, nanotechnology and semiconductor markets, today announced that it has established the NILPhotonics Competence Center, which is designed to assist customers in leveraging EVG’s suite of nanoimprint lithography (NIL) solutions to enable new and enhanced products and applications in the field of photonics. These include light emitting diodes (LEDs) and photovoltaic (PV) cells, where NIL-enabled photonic structures can improve light extraction and light capturing, respectively, as well as laser diodes, where photonic structures enable the tailoring of device characteristics to improve performance. The NILPhotonics Competence Center includes dedicated, global process teams, pilot-line production facilities and services at its cleanrooms at EVG’s headquarters in Austria as well as its subsidiaries in North America and Japan.

nanoimprint

“Nanoimprint lithography is an enabling technology for the design and manufacture of all kinds of photonic structures, which can significantly shorten time to market and lower cost of production compared to conventional technologies, such as electron-beam writing and stepper systems for optical lithography,” stated Markus Wimplinger, corporate technology development and IP director at EV Group. “For example, compared with conventional lithography, our full-wafer nanoimprinting technology can pattern true three-dimensional structures in the sub-micron to nano-range as well as features as small as 20nm, which opens up a range of new photonic applications. With our NILPhotonics Competence Center, we’re not just providing our customers with the most advanced NIL systems; we’re also working closely with them during product development to help them determine how best to optimize their product designs and processes to take advantage of the resolution and cost-of-ownership benefits that NIL brings.”

The new NILPhotonics Competence Center builds on more than 15 years of NIL experience at EVG with the largest installed base of NIL systems worldwide. EVG’s NIL equipment portfolio includes the recently introduced EVG7200 UV-NIL system, which supports EVG’s next-generation SmartNIL large-area soft NIL process for high-volume manufacturing. The EVG7200 with SmartNIL provides unmatched throughput and cost-of-ownership advantages over competing NIL approaches.

Physicists at the University of Kansas have fabricated an innovative substance from two different atomic sheets that interlock much like Lego toy bricks. The researchers said the new material — made of a layer of graphene and a layer of tungsten disulfide — could be used in solar cells and flexible electronics. Their findings are published today by Nature Communications.

Hsin-Ying Chiu, assistant professor of physics and astronomy, and graduate student Matt Bellus fabricated the new material using “layer-by-layer assembly” as a versatile bottom-up nanofabrication technique. Then, Jiaqi He, a visiting student from China, and Nardeep Kumar, a graduate student who now has moved to Intel Corp., investigated how electrons move between the two layers through ultrafast laser spectroscopy in KU’s Ultrafast Laser Lab, supervised by Hui Zhao, associate professor of physics and astronomy.

 “To build artificial materials with synergistic functionality has been a long journey of discovery,” Chiu said. “A new class of materials, made of the layered materials, has attracted extensive attention ever since the rapid development of graphene technology. One of the most promising aspects of this research is the potential to devise next-generation materials via atomic layer-level control over its electronic structure.”

According to the researchers, the approach is to design synergistic materials by combining two single-atom thick sheets, for example, acting as a photovoltaic cell as well as a light-emitting diode, converting energy between electricity and radiation. However, combining layers of atomically thin material is a thorny task that has flummoxed researchers for years.

“A big challenge of this approach is that, most materials don’t connect together because of their different atomic arrangements at the interface — the arrangement of the atoms cannot follow the two different sets of rules at the same time,” Chiu said. “This is like playing with Legos of different sizes made by different manufacturers. As a consequence, new materials can only be made from materials with very similar atomic arrangements, which often have similar properties, too. Even then, arrangement of atoms at the interface is irregular, which often results in poor qualities.”

Layered materials such as those developed by the KU researchers provide a solution for this problem. Unlike conventional materials formed by atoms that are strongly bound in all directions, the new material features two layers where each atomic sheet is composed of atoms bound strongly with their neighbors — but the two atomic sheets are themselves only weakly linked to each other by the so-called van der Waals force, the same attractive phenomenon between molecules that allows geckos to stick to walls and ceilings.

“There exist about 100 different types of layered crystals — graphite is a well-known example,” Bellus said. “Because of the weak interlayer connection, one can choose any two types of atomic sheets and put one on top of the other without any problem. It’s like playing Legos with a flat bottom. There is no restriction. This approach can potentially product a large number of new materials with combined novel properties and transform the material science.”

Chiu and Bellus created the new carbon and tungsten disulfide material with the aim of developing novel materials for efficient solar cells. The single sheet of carbon atoms, known as graphene, excels at moving electrons around, while a single-layer of tungsten disulfide atoms is good at absorbing sunlight and converting it to electricity. By combining the two, this innovative material can potentially perform both tasks well.

The team used scotch tape to lift a single layer of tungsten disulfide atoms from a crystal and apply it to a silicon substrate. Next, they used the same procedure to remove a single layer of carbon atoms from a graphite crystal. With a microscope, they precisely laid the graphene on top of the tungsten disulfide layer. To remove any glue between the two atomic layers that are unintentionally introduced during the process, the material was heated at about 500 degrees Fahrenheit for a half-hour. This allowed the force between the two layers to squeeze out the glue, resulting in a sample of two atomically thin layers with a clean interface.

Doctoral students He and Kumar tested the new material in KU’s Ultrafast Laser Lab. The researchers used a laser pulse to excite the tungsten disulfide layer.

“We found that nearly 100 percent of the electrons that absorbed the energy from the laser pulse move from tungsten disulfide to graphene within one picosecond, or one-millionth of one-millionth second,” Zhao said. “This proves that the new material indeed combines the good properties of each component layer.”

The research groups led by Chiu and Zhao are trying to apply this Lego approach to other materials. For example, by combining two materials that absorb light of different colors, they can make materials that react to diverse parts of the solar spectrum.

The National Science Foundation funded this work.

Silicon is the second most-abundant element in the earth’s crust. When purified, it takes on a diamond structure, which is essential to modern electronic devices–carbon is to biology as silicon is to technology. A team of Carnegie scientists led by Timothy Strobel has synthesized an entirely new form of silicon, one that promises even greater future applications. Their work is published in Nature Materials.

Although silicon is incredibly common in today’s technology, its so-called indirect band gap semiconducting properties prevent it from being considered for next-generation, high-efficiency applications such as light-emitting diodes, higher-performance transistors and certain photovoltaic devices.

Metallic substances conduct electrical current easily, whereas insulating (non-metallic) materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can move to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the “band gap.” While direct band gap materials can effectively absorb and emit light, indirect band gap materials, like diamond-structured silicon, cannot.

In order for silicon to be more attractive for use in new technology, its indirect band gap needed to be altered. Strobel and his team–Carnegie’s Duck Young Kim, Stevce Stefanoski and Oleksandr Kurakevych (now at Sorbonne) –were able to synthesize a new form of silicon with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved.

The silicon they created is a so-called allotrope, which means a different physical form of the same element, in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new silicon allotrope consists of an interesting open framework, called a zeolite-type structure, which is comprised of channels with five-, six- and eight-membered silicon rings.

They created it using a novel high-pressure precursor process. First, a compound of silicon and sodium, Na4Si24, was formed under high-pressure conditions. Next, this compound was recovered to ambient pressure, and the sodium was completely removed by heating under vacuum. The resulting pure silicon allotrope, Si24, has the ideal band gap for solar energy conversion technology, and can absorb, and potentially emit, light far more effectively than conventional diamond-structured silicon. Si24 is stable at ambient pressure to at least 842 degrees Fahrenheit (450 degrees Celsius).

“High-pressure precursor synthesis represents an entirely new frontier in novel energy materials,” remarked Strobel. “Using the unique tool of high pressure, we can access novel structures with real potential to solve standing materials challenges. Here we demonstrate previously unknown properties for silicon, but our methodology is readily extendible to entirely different classes of materials. These new structures remain stable at atmospheric pressure, so larger-volume scaling strategies may be entirely possible.”

“This is an excellent example of experimental and theoretical collaboration,” said Kim. “Advanced electronic structure theory and experiment have converged to deliver a real material with exciting prospects. We believe that high-pressure research can be used to address current energy challenges, and we are now extending this work to different materials with equally exciting properties.”

This work was supported DARPA and Energy Frontier Research in Extreme Environments (EFree), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Samsung Electronics Co. today introduced new chip-on-board (COB) LED package products, the LC006B and LC008B, with six and eight watts of power respectively. The new packages join five others in Samsung’s popular LC series (LC013B, LC019B, LC026B, LC033B and LC040B), to complete its COB package line-up.

“With the introduction of our new under-10 watt COB packages, we are signaling our intent to aggressively target the indoor LED lighting market,” said Bangwon Oh, Senior Vice President, Strategic Marketing Team, LED Business, Samsung Electronics. “Samsung will continue to advance its LED technology and business objectives by providing lighting manufacturers with the best in LED lighting components, delivering exceptionally high-quality LED package and engine products and services that reliably meet customer needs,” he added. “We remain dedicated to increasing our breadth of market solutions, to further grow our LED lighting component business.”

A chip-on-board LED package provides a single light source that combines multiple LED chips to achieve higher light intensity and uniformity, while simplifying luminaire design.

The LC006B and LC008B offer high-efficacy levels of 140lm/W and 142lm/W at 5000K CCT, respectively. The new packages will support a wide range of CCT (Correlated Color Temperature) specifications from 2700K to 5000K with a CRI (Color Rendering Index) over 80. They also feature a compact package size with an 8mm LES (Light Emitting Surface) and a package structure that can be easily connected with holders or screw mounts for greater installation convenience.

Samsung’s LC series has gained widespread attention for delivering high luminance from a small LES, as well as low heat resistance and outstanding light efficacy. The LC packages also feature high color uniformity with 3-step MacAdam ellipses and consistently superior light quality.

Samsung COB LED lighting solutions now can be used in a significantly wider range of applications, including downlight for home lighting, flood light for industrial lighting, and spotlight and downlight for commercial lighting.

Cree, Inc. announced a breakthrough in lighting-class LED performance with its SC5 Technology Platform. The new platform powers the next generation of lighting with the introduction of Extreme High Power (XHP) LEDs. This new class of LEDs can reduce system costs by up to 40 percent in most lighting applications.

“As a technology company, we’re focused on breaking the performance barriers that really matter to the lighting industry,” said Chuck Swoboda, Cree Chairman and CEO. “The SC5 Technology Platform redefines what is possible in high-power LEDs by doubling the lumens out of a single LED, giving lighting manufacturers the flexibility to innovate significantly lower cost systems. This new platform establishes a new benchmark for LED lumens per wafer, which we believe will define the long-term success of our industry. This also validates our belief that high-power LED technology enables the best lighting system designs and a better lighting experience for end customers.”

The SC5 Technology Platform is built on Cree’s silicon carbide technology and features significant advancements in epitaxial structure, chip architecture and an advanced light conversion system optimized for best thermal and optical performance. With these advancements, the SC5 Technology Platform achieves unparalleled lumen density and longer lifetime at higher operating temperatures than previous LED technology, which can significantly reduce thermal, mechanical and optical costs at the system level.

“LEDs are no longer the most expensive portion of an LED lighting system, but they fundamentally determine the overall system performance and cost,” said Dave Emerson, vice president and general manager for Cree LEDs. “While other LED manufacturers only promise incrementally lower LED cost, our new Extreme High Power (XHP) LEDs leveraging the SC5 Technology™ Platform directly address the increased burden that thermal, mechanical and optical elements now place on total system cost.”

The first available family of XHP LEDs is the XLamp® XHP50 LED, delivering up to 2250 lumens at 19 watts from a 5.0×5.0 mm package. At its maximum current, the XHP50 provides twice the light output of the industry’s brightest single-die LED, the XLamp XM-L2 LED, at a similar lumens per watt and without increasing the package footprint. By leveraging Cree’s latest reliability innovations, the XHP50 is designed to maintain L90 lifetimes above 50,000 hours even at high temperature and current.

The U.S. Patent Office has issued US Patent No. 8,859,310 to Versatilis LLC that shows how fine semiconductor particles, powders or fines, often the waste byproduct of dicing semiconductor wafers into ever smaller chips, can be processed into a sea of low cost solar cells or micro-LEDs.

A principal challenge in making such devices has always been forming the active layer, whether the light absorbing layer in a solar cell or the light-emitting layer in a LED. This has also been the most costly and capital-intensive part of the manufacturing process, since the active layer must be made to high standards of semiconductor crystal quality and uniformity. Leading solar cells, for example, use mono- or poly-crystalline silicon wafers, while LEDs use variants of Gallium Nitride (GaN) on expensive sapphire, Silicon Carbide or even GaN wafers. In many cases, these materials are thicker than needed, the added thickness lending structural support to the end device without adding to efficiency, but contributing to overall cost and weight of the structure.

Versatilis shows instead that the active layer can be made from semiconductor fines or powders of single crystal particles densely packed into a monolayer, in a configuration not unlike sandpaper one particle thick, and then further processed into active diode structures serving as solar cells, for example, or as LEDs. Such particles are readily available, often a byproduct of other processes or made inexpensively off-line, or sometimes chemically synthesized. Silicon fines, for example, are widely available, screened for a desired size distribution, as are CIGS and GaN particles, the latter chemically synthesized. And a small amount of such “dust” can go a long way; for example, a kilogram of one micron single crystal CIGS particles used as micro-solar cells can cover an area over 300 square meters, resulting in very low costs per unit area.

“By levering cheap, ex-situ produced and optimized, single or polycrystalline powders and fines for Si, Ge, CIGS, GaN, ZnO as the starting raw material and wrapping unique processing techniques around that, we can produce highly functional opto-electronic devices with reduced infrastructure, processing, and material utilization cost,” stated Ajay Jain, Versatilis CTO and inventor of the now patented technology.

The potential cost savings have led others to try using semiconductor particles in a variety of ways, however, none have proven commercially practical. A major challenge has been to lay down these particles quickly enough and as a monolayer. Similarly, researchers have shown basic functional devices with nanorods, nanowires and other semiconductor “nanostructures” in the lab, only to be stopped by a general lack of production ready manufacturing technology for nanoscale, including suitable tools for in-line process metrology and characterization.

In addition to processing semiconductor particles into useful devices, Versatilis has unique fluidics technology for rapidly depositing such particles as a monolayer, from nano to microscale, on wafers or in a continuous, high-speed web. It had licensed the technology to VersufleX Technologies (http://www.versuflex.com), who are beginning to sell benchtop process tools to R&D labs based on this technology. The process can tolerate reasonable variation in particle size and shape, and there are a variety of methods possible for orienting particles floating on the surface of a fluid medium.

“This technology will not set performance records for efficiency in PV cells nor in lumens/watt for LEDs, but we believe there is no cheaper, more practical way to realize semiconductor diode based functionality over a large, flexible area,” added George Powch, Company CEO, “We think it can enable low cost Building Integrated Photovoltaics or rival OLEDs with a wholly inorganic large area micro-LED solution.”

Specifications of reed relays, which are used for current switching in ATE and other applications are explained, including carry current, lifetime, minimum switch capacity, hot switching, operating speed and thermoelectric switching.

BY KEVIN MALLETT, Pickering Electronics, Clacton-on-Sea, Essex, U.K.

Reed relays, which use an electromagnet to control one or more reed switches without requiring an armature, are used for instrumentation and automatic test equipment (ATE), high voltage switching, low thermal EMF, direct drive from CMOS, RF switching and other specialized applications.

Reed relays are deceptively simple devices in principle. They contain a reed switch, a coil for creating a magnetic field, an optional diode for handling back EMF from the coil, a package and a method of connecting to the reed switch and the coil to outside of the package. The reed switch is itself a simple device in principle and relatively low cost to manufacture thanks to modern manufacturing technology.

The reed switch has two shaped metal blades made of a ferromagnetic material (roughly 50:50 nickel iron) and glass envelope that serves to both hold the metal blades in place and to provide a hermetic seal that prevents any contaminants entering the critical contact areas inside the glass envelope. Most (but not all) reed switches have open contacts in their normal state.

If a magnetic field is applied along the axis of the reed blades the field is intensified in the reed blades because of their ferromagnetic nature, the open contacts of the reed blades are attracted to each other and the blades deflect to close the gap. With enough applied field the blades make contact and electrical contact is made.

The only movable part in the reed switch is the deflection of the blades, there are no pivot points or materials trying to slide past each other. The reed switch is considered to have no moving parts, and that means there are no parts that mechanically wear. The contact area is enclosed in a hermetically sealed envelope with inert gasses, or in the case of high voltage switches a vacuum, so the switch area is sealed against external contamination. This gives the reed switch an exceptionally long mechanical life

Inevitably in practice the issues are a little more complicated. The ferromagnetic material is not a good conductor and in particular the material does not make a good switch contact. So the reed blades have to have a precious metal cover in the contact area, the precious metal may not stick to the blade material very well so an underlying metal barrier may be required to ensure good adherence. Some types of reed relay use mercury wetted contacts, consequently reed relays that use plated contacts are often referred to as “dry” reed relays. The metals can be added by selective plating or by sputtering processes. Where the reed blade passes through the glass envelope any plating (in many cases there may be none) requires controlling to avoid adversely affecting the glass to metal hermetic seal. Outside the glass seal the reed blades have to be suitably finished to allow them to be soldered or welded into the reed relay package, usually requiring a different plating finish to that used inside the glass envelope.

The materials used for the precious metal contact areas inside the glass envelope have a significant impact on the reed switch (and therefore the relay) characteristics. Some materials have excellent contact resistance stability; others resist the mechanical erosion that occurs during hot switch events. Commonly used materials are ruthenium, rhodium and iridium– all of which are in the relatively rare platinum precious metal group. Tungsten is often used for high power or high voltage reed switches due to its high melting point. The material for the contact is chosen to best suit the target performance – bearing in mind the material chosen can also have a significant impact on manufacturing cost. Sealed in a long, narrow glass tube, the contacts are protected from corrosion, and are usually plated with silver, which has very low resistivity but is prone to corrosion when exposed, rather than corrosion-resistant but more resistive gold as used in the exposed contacts of high quality relays. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. As the moving parts are small and lightweight, reed relays can switch much faster than relays with armatures. They are mechanically simple, making for reliability and long life.

This article reviews and explains common specifications used for reed relays (FIGURE 1).

FIGURE 1. Reed relays are used for current switching in ATE and other applications.

FIGURE 1. Reed relays are used for current switching in ATE and other applications.

Carry current

Carry current is the current that the reed relay can support through its contact without long term damage. The life of the relay should be indefinite under this condition though some reed relays may also have a pulse current rating which can be applied to the relay without damage.

The carry current is determined primarily by the contact resistance of the relay and the heat sinking to the environment. As the current increases the temperature of the reed blades increases until it reaches a temperature where the material is no longer ferromagnetic (Curie Temperature). Once that temperature is reached the relay contacts may open since the blades no longer respond to the magnetic field. The blade temper- ature is clearly dependent upon the current and relay path resistance – the normal assumption is that this is a square law (with current) relationship. In reality, the temperature rise is significantly more than a square law since the metallic resistance also increases with temperature, the magnetic field drops with temperature because of coil resistance rise and the mechanical properties of the blade can change. Consequently like all relays, exceeding the rating can result in a type of thermal runaway.

The packaging of the reed switch has a significant impact on the temperature rise, a lead frame tends to conduct heat to the outside world while the plastic encapsulation materials insulate it. The packaged reed relay will always have a lower current rating than that of the reed switch because manufacturers quote the rating with the reed switch directly exposed (no coil, no plastic packaging). The coil power will also add to the heating effect. Consequently Pickering Electronics always de-rates the reed relay ratings to ensure that the relay switch remains within its design limits.

There is also another subtle effect that occurs as the carry current increases – the signal creates its own magnetic field that twists the blades and therefore can modulate the contact resistance. The blade twisting may start to see a contact resistance rise as the blade contact area reduces or changes.

Care must be taken not to exceed the relays ratings and pulse ratings should take account of the square law relationship between current and temperature.

It becomes difficult to manufacture reed relays with a carry current of greater than 2A because the contact area has to be increased and that tends to make the bladed stiffer and require a higher magnetic field strength to operate them.

Lifetime

The lifetime of reed relays is critically dependent on the load conditions the reed switch encounters. For reed relays which are instrument grade the mechanical lifetime is much greater than 1 billion operations – they are mechanically simple devices that rely purely on the deflection of a blade to operate and there are conse- quently fewer wear out mechanisms.

The blade contact area though stills wears as they are opened and closed. If the signal load when the blade closes or opens is low then the wear out is very slow, as the load increases and hot switching (interruption or closure of a signal live carrying significant current or voltage) occurs higher temperatures are generated at the contact interface and this makes the materials more prone to wear. DC signals can also result in the migration of metal from one contact to another and without regular polarity reversal eventually the underlying contact materials are exposed with their poorer conduction characteristics. Hot switching can also create a temporary plasma in the contact area with high local temperatures, rapid operation of a relay under load can start to raise the contacts temperature to an extent where premature wear out can occur. The life an instrument grade reed relay can vary by three orders of magnitude according to the load conditions, perhaps 5 billion operations under no or light load to 5 million operations at a heavy load.

Minimum switch capacity

Some types of relay have a minimum switch capacity, if the relay is closed on a very low level signal (current or voltage) oxide or debris on the relay contacts can remain at the interface and cause a higher than expected resistance, or even an open circuit. This tends not to be the case with reed relays because the precious metal contacts are sealed in a hermetic glass envelope containing inert gas. Minimum switch capacity tends to be a characteristic of higher power mechanical (EMR) relays.

Hot switching

Hot switching occurs whenever a relay contact is opened or closed with a signal (current and voltage) is present. As the contacts move apart or close an arc can be created which transfers material from one contact to another, or simply redistributing the material. As the contact plating is damaged the resistance will eventually start to rise until the relay is no longer fit for the intended application.

For reed relays hot switching tests are always conducted into resistive loads. The hot switch capacity of a reed relay is typically quoted at a current/voltage that results in the number of operations that the relay will support around 10million operations. The data sheet specifies a hot switch current (the limiting factor at low voltages), a hot switch voltage (limiting factor at low current) and a power (from the product of the open contact voltage and the closed contact current).

Operating speed

The operate time is the time from when the relay coil is energized or de-energized to when the contact reaches a stable position.

For a normally open contact when the coil is energized the current, and therefore the magnetic
field, in the coil rises until the blades start to move closer together until they make contact. The contacts may impact each other sufficiently rapidly that there is bounce where for a short duration the contact is inter- mittently closed then opened. The operate time should be the time from when the relay coil was energized until the contacts are stably closed.

If the coil is driven from a higher than specified coil voltage the closing speed of the relay will be faster, however once the contacts make there may be more contact bounce as they meet with greater force. Overdriving the coil can also increase the release time since the magnetic field takes longer to collapse to the point where the contacts start to open.

For a normally open Form A (SPST) contact the release time is the time from when the coil is de-energised to when the contact is open. This operate time can be dependent on how the reed relay is driven, the presence of a protection diode on the coil will increase the release time. Typically, the release time is around one half the operate time.

Soft and hard weld failures

Operation of reed relays (or EMRs) under high load conditions causes one of the most common failure mechanisms for relays – a failure where the contacts are welded together. By convention these welds are classified as being either soft or hard failures. In the event of hard failure the contacts tend to be welded together and nothing will separate them. This is an easy fault to identify. Soft failures occur where the contacts sick but eventually come apart without any additional assistance. The failure is caused by small areas on the contact welding together, but the weld area is sufficiently small that the reed blades will separate because of their sprung nature. They could spring apart very quickly, or it may take several seconds to spring apart depending on how hard the weld is.

In either case the impact on the user is that the switching function of the relay is impaired and this is likely to have an adverse impact on the user application. So in either case the relay will require replacing since the defect is unlikely to improve with time. The cause of the weld will also need to be investigated and corrected.

Thermoelectric EMF

The cause of thermoelectric voltages is often misunderstood by users, and often misrepresented in articles and on the internet. The effect of thermoelectric EMF’s is to generate a small voltage (measured in microvolts) across the relay terminals when the relay is closed (FIGURE 2).

FIGURE 2. Thermoelectric EMF’s are used to generate a small voltage (measured in microvolts) across the relay terminals when the relay is closed.

FIGURE 2. Thermoelectric EMF’s are used to generate a small voltage (measured in microvolts) across the relay terminals when the relay is closed.

The voltage arises whenever a metal wire has a temperature gradient across it (the Seebeck Effect), if one end of the wire is at a different temperature to the other then a voltage will appear which is dependent on the temperature difference and the materials that make up the wire. Reed relays use a mix of metals, and these can have different temperature drops across them which results in a voltage appearing at the relay connection terminals. The voltage is not created at a connection junction. Nickel iron has quite a strong thermoelectric EMF, so designing reed relays with low thermal EMF’s can be a challenge.

The number and type of materials varies according to FIGURE 2. Thermoelectric EMF’s are used to generate a small voltage (measured in microvolts) across the relay terminals when the relay is closed. to the way the reed switch is designed and how it is packaged. If the relay was perfectly symmetric in construction (so the materials used from each contact to the reed switch were the same and the reed itself was perfectly symmetric in all materials and dimensions) and all heat sources in the relay body (primarily due to the coil) then this would be the case. However in reality the symmetry is not perfect so a residual voltage will arise.

Users can also degrade the performance by how they use the relay. When mounted on a PCB if the PCB has a temperature profile across it then that will generate an additional thermal EMF. Relay manufacturers usually assume that the thermal EMF is zero when the relay is first closed since up to that point no heat source exists inside the relay body. However, a temperature profile across the PCB (caused by the presence of other heat sources or forced air cooling) will create a thermal EMF.

Reed relays that have excellent Thermal EMF performance are typically designed to be as symmetric in design as possible and to use highly efficient coils to avoid heating the reed switch. Typically though, this results in a physically larger relay.

Two pole designs often quote the Differential Thermal EMF, this is the voltage generated between the two switches (usually) in a single package.

Assuming the relay design is reasonably symmetrical to a first order the voltage in one switch is the same as the other, so the differential voltage can be much smaller for the relay. Differential and single ended Thermo Electric EMF numbers should not be directly compared or confused with each other.

Duke University researchers have made fluorescent molecules emit photons of light 1,000 times faster than normal — setting a speed record and making an important step toward realizing superfast light emitting diodes (LEDs) and quantum cryptography.

This year’s Nobel Prize in physics was awarded for the discovery of how to make blue LEDs, allowing everything from more efficient light bulbs to video screens. While the discovery has had an enormous impact on lighting and displays, the slow speed with which LEDs can be turned on and off has limited their use as a light source in light-based telecommunications.

In an LED, atoms can be forced to emit roughly 10 million photons in the blink of an eye. Modern telecommunications systems, however, operate nearly a thousand times faster. To make future light-based communications using LEDs practical, researchers must get photon-emitting materials up to speed.

In a new study, engineers from Duke increased the photon emission rate of fluorescent molecules to record levels by sandwiching them between metal nanocubes and a gold film.

This is an artist's representation of light trapped between a silver nanocube and a thin sheet of gold. When fluorescent molecules -- shown in red -- are trapped between the two, they emit photons up to 1,000 times faster than normal. Credit: Gleb Akselrod, Duke University

This is an artist’s representation of light trapped between a silver nanocube and a thin sheet of gold. When fluorescent molecules — shown in red — are trapped between the two, they emit photons up to 1,000 times faster than normal. Credit: Gleb Akselrod, Duke University

“One of the applications we’re targeting with this research is ultrafast LEDs,” said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. “While future devices might not use this exact approach, the underlying physics will be crucial.”

Mikkelsen specializes in plasmonics, which studies the interaction between electromagnetic fields and free electrons in metal. In the experiment, her group manufactured 75nm silver nanocubes and trapped light between them, greatly increasing the light’s intensity.

When fluorescent molecules are placed near intensified light, the molecules emit photons at a faster rate through an effect called Purcell enhancement. The researchers found they could achieve a significant speed improvement by placing fluorescent molecules in a gap between the nanocubes and a thin film of gold.

To attain the greatest effect, Mikkelsen’s team needed to tune the gap’s resonant frequency to match the color of light that the molecules respond to. With the help of co-author David R. Smith, the James B. Duke Professor and Chair of Electrical and Computer Engineering at Duke, they used computer simulations to determine the exact size of the gap needed between the nanocubes and gold film to optimize the setup.

That gap turned out to be just 20 atoms wide. But that wasn’t a problem for the researchers.

“We can select cubes with just the right size and make the gaps literally with nanometer precision,” said Gleb Akselrod, a postdoc in Mikkelsen’s lab and first author on the study. “When we have the cube size and gap perfectly calibrated to the molecule, that’s when we see the record 1,000-fold increase in fluorescence speed.”

Because the experiment used many randomly aligned molecules, the researchers believe they can do even better. They plan to design a system with individual fluorescent molecule placed precisely underneath a single nanocube. According to Akselrod, they can achieve even higher fluorescence rates by standing the molecules up on edge at the corners of the cube.

“If we can precisely place molecules like this, it could be used in many more applications than just fast LEDs,” said Akselrod. “We could also make fast sources of single photons that could be used for quantum cryptography. This technology would allow secure communication that could not be hacked — at least not without breaking the laws of physics.”

Seoul Semiconductor, a global LED manufacturer, announced the availability of Acrich MJT 3030 a new LED in the Acrich MJT product family which improves on performance and enables lower system costs. Using Seoul Semiconductor’s Acrich MJT technology, the MJT 3030 LED offers improved performance and high lm/$ in a mid-power package.

This new Acrich series has dimensions of 3.0mm x 3.0mm delivering a typical luminous flux of 103 lumens at 40mA at 22V, 25° C, 3000K and can be driven to a maximum current of 60mA delivering upto 155 lumens to address high-lumen applications that require low cost and high reliability solutions. To improve time-to-market, lighting manufacturers seeking ENERGY STAR qualification can take advantage of the completed 6,000 hours LM-80 data of the Acrich MJT 3030 LED.

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Seoul Semiconductor Executive Vice President of Lighting sales, Jay Kim stated that, “The new Acrich MJT 3030 LED combines the improved performance and high lm/$ with the reliability of the MJT technology enabling lighting manufacturers to create new innovative solutions to address a wide range of lighting applications.”