APPLICATIONS OF ULTRABRIGHT LEDs AND DISPLAYS

The LEDs

The Innovation of SOLID STATE LAMP or LIGHT EMITTING DIODE (LED) marks a real breakthrough in the lighting industry. This device is highly versatile in its applications and is distinguished by its high reliability, long life and fast switching speed. Whereas pulsing application is the nemesis of best of filament lamps, a LED actually thrives on pulsing.

There are light generation mechanisms like incandescence, electroluminescence, chemiluminescence, triboluminescence, photoluminescence etc. LEDs employ pn-junction luminescence that occurs as a result of the application of direct current at low voltage to a suitably doped crystal containing a pn junction. It arises in two steps- excitation when electrical energy is absorbed and recombination in which energy is released as light and heat .

The light output, Iv, of an LED is proportional to the forward current, If, which again is a function of the forward voltage applied. In reverse bias beyond Vr, the LED transits from a nonconductive mode to breakdown mode.

A normal LED consists of an LED chip of around 0.3 sq. mm is attached to a 2 leaded lead frame with highly reflective cup forming contact on the p side. A fine .025mm gold /aluminum wire connects top n side pad to the next lead. An epoxy filled lens is molded on to the leads to provide protection as well as to produce the desired beam pattern.

Brief introduction to Electro-Optical Measurements of LEDs

Several areas of "light" measurement are used for defining the performance parameters of an LED. Photometric measurements relating to measurements of visible radiation as the "standard" human eye responds to it are of primary importance. The other system of measurement is radiometric, which is not limited to the eye response. Radiometric measurements are used to cover the full spectrum of radiation/photons emitted from the LED (visible plus ultraviolet and infrared).

Photometric units of measurement are important for LED product designers and those focusing on visible LED product applications (such as Big Screen Video Displays. Radiometric measurements are important, both to the packaging specialists as well as for end-product designers and for the applications requiring an understanding as to a device's emitted radiation profile.

These measurements are conducted with the industry standard drive condition of 20 mA. Photometric Measurements: The "standard" human eye or "standard observer" (photopic) curve(s) has been established by the Commission Internationale de I'Eclairage [.1] (CIE). The most widely used version is the CIE 1931 Chromaticity Diagram. The eye response is not a constant over the entire visible range. It peaks at Green 555 nm and falls off sharply toward the Blue at 400 nm, and in the Red at 700 nm. Detectors required for taking this type of measurement must be spectrally corrected so that their response matches the CIE curve selected. This correction is usually made by adding a specially designed photometric filter to the detector. The basic unit of measurement of light is the lumen. Important photometric quantities are: Luminous Flux (Fv)- total luminous power emitted from a source (lumens), and Luminous Intensity (Iv) or luminous flux emitted from a source per unit solid angle in a specified direction (lumens per steradian or candelas).

Radiometric Measurements: The basic radiometric unit of optical flux (power) is the watt. one watt of radiation in the ultraviolet is equal to onewatt in the visible and one watt in the infrared range as well. Radiometric measurements are determined accurately when both the spectral power distribution of the light source measured is known along with the absolute response [.2] [.3] of the detector are known, The lamp specific power density (SPD) and the detector absolute responsitivity, are then combined and resulting light source specific scaling factor applied to the photocurrent read from the detector. The important radiometric quantities are Radiant Flux or Power (FR): The time rate of flow of radiant energy (watt), and Radiant Intensity (IR) - the radiant flux emitted from a source per unit solid angle in a specified direction (watts per steradian)

High Brightness GaN LEDS

GaN compounds, are understood to be the next generation of higher performance solid-sate capability for LEDs, VCELs, RF, Microwave and Millimeter wave devices, to name a few, that will work faster. It means the advent of High Brightness low wavelength devices that will be applied to markets and product applications heretofore totally unattainable, including digitally compatible general "White" lighting and big screen full color solid-state outdoor video displays.

GaN is the only crystal, the fundamental ingredient of a semiconductor, with band gap characteristics specifically advantageous for many visible short wavelength and ultraviolet LED applications. Bulk GaN cannot be grown by itself as a semiconductor crystal in a cost-effective manner yet, mostly because of physical constraints. Indeed, a major difficulty in growing high-quality GaN crystalline films was in establishing a suitable substrate material.

GaN was first successfully produced for High Brightness LEDs via MOCVD epitaxy on AI2O3 (Sapphire) and this combination continues today in large-scale production from an increasing supplier base. Subsequently, GaN LEDs have also been produced on SiC (Silicon carbide) substrates as well and are currently in large-scale production, albeit with somewhat lower brightness results. The potential for homoepitaxy with GaN wafer substrates would virtually eliminate lattice mismatch characteristics, and is under development at many centers throughout the world. ZnO provides only a 2.2% lattice mismatch with GaN and is matched with InGaN. InGaN/ZnO, as well, is being worked with for use in LEDs.

LED Packaging - Design Issues

Packaging engineering of LED semiconductors is a key contributor to producing better discrete component designs that perform more efficiently in a wide variety of operational and environmental conditions, than current conventional formats especially as demand for LEDs to fulfill new, higher performance, higher brightness applications continues to manifest and gain momentum. Current packaging performance efficiencies, compared to LED die performance attributes, clearly shows that most conventional packages existing to date, are inadequate for the demands of many current and future applications.

This holds true for axial leaded as well as surface mount leaded and leadless packaging technologies (SMT). Most existing LED lamp designs are an outgrowth of traditional formats that support cube type LEDs, with length and width dimensions ranging from 8 to 12 mils and thicknesses of approximately 8 to 10 mils. Most common varieties of packages and the "open" tools that are used to produce them, including the T 1 ¾ (5 mm) lamp, are designed with this size die in mind. Most LED package styles are also designed for conducting substrate LEDs bottom of the LED, as opposed to planar circuit LEDs on insulating substrates (such as InGaN on AI203 LEDs having two contacts for wire bonding on the top side). Much work also goes into ensuring that existing packaging designs are practical to manufacture and can be produced at low cost.

The T 1 ¾ (5 mm) lamp for example, costs pennies to produce via highly automated assembly procedures yet has very poor optical performance characteristics. It is estimated that the LED lamp has an optical efficiency of < 30% with conducting substrate style LEDs and this efficiency is even less with new style InGaN / AI203 LEDs that are generally 13 mil square with heights running < 5 mils in thickness. SMT device packages are generally known to have optical efficiencies even less than this (<20%). Additionally, other factors such as thermal and chemical factors are also associated with traditional style package inadequacies for many new applications. For example, T 1 ¾ packaged Blue LEDs that improperly use encapsulate materials that will prematurely age, will loose an additional 40-50% of their output when subject to ~ 5 - 10 thousand hour aging. It can be clearly seen, that if the goal is higher performance (for example YAG-coated Blue InGaN AI203 "White" T 13/4 LED lamps currently achieve ~10 Im/W and the goal is to quickly achieve 120 Im/Watt (as per Japan's MITI), then to achieve it, this gain will not only have to come from the LED semiconductor, but also the package that it is assembled into.

New package engineering must consider better optical and electrical performance. It must minimize the collective effect of environmental and operational stresses on the resultant package design. Better thermal management needs to be incorporated. Good designs must also consider the variety of new LED die types and wavelengths that they produce and the effects these wavelengths have on the packaging die attach and encapsulating materials and processes selected.

Parameter	Comments
Optical:	Problem: The T 1 ¾ , LED lamp has 30% luminous efficiency Answer Redesign the lens and reflector cup specifically for an InGaN/ AI203 LED sized die using non-imaging optics (NIO) to lower internal absorption and reflection. Attach the AI203-based transparent LED die with a thin layer of non-conductive, optically clear adhesive to a highly reflective surface
Thermal:	Problem: Heat causes lattice to vibrate which eventually alters configuration (feedback loop becomes positive) causing lower emissions and/or failure, as well as embrittlement and cross linking of encapsulating polymers. Tests show that some die formats can be driven up to 130 mA without degradation, yet the T-1 ¾ package is rated at only 20 mA. Answer: Incorporate better thermal paths from the die p-n junction-to-component and the component to-board, for better thermal management.
Electrical:	Problem: Wire-bond failures are generally agreed upon as an early LED lamp failure mechanism. Answer: Use of softer, more compliant polymers that mechanically are more conducive to wire-bond integrity.
Chemical:	Problem: Oxidation causes performance & reliability problems (hydrogen & oxygen break epoxy chemical bonds causing gasses, yellowing and water to occur Answer: Consider additives to the epoxy (antioxidants), and/or different types of materials/approaches.
Radiative	Problem: The "rule-of-thumb" is that 10 Blue photons have approximately the same damaging effect on epoxy as 1 UV photon, which causes chemical bonds to break and brittleness to ensue. "Yellowing" also absorbs a significant proportion of lower wavelength light reducing output. Answer: Consider additives to the epoxy (antioxidants), and/or different types of materials/approaches.

With the successful incorporation of new design features, product performance improvements on the order of > 50% optical, > 100% in power efficiency, better life and reliability, can be realized yielding, even with the T 1 ¾ package, not 10 Im/W but significantly better results with current and future LED die.

Behavior of LED in operation

Light Emitting Diodes (LEDs), from an electrical perspective, can be operated using the same drive considerations as other diodes. Similar limiting properties must be taken into account as with standard electrical diodes including maximum reverse voltage and maximum forward current.

Constant current drive: LEDs are preferably used in a circuit configuration that controls the current through the device rather than the voltage across the device since this will yield the most stable light output, which is critical in most applications. This is because there are small variations in the junction voltage at a specific current due to unavoidable variations in the manufacturing process.

Temperature rise: The temperature dependent behavior of the diode forward voltage (Vf) at a specific drive current (I) implies that at higher drive conditions the electrical power dissipated in the device increases. Basically, power is dissipated in three areas in the device: the p-contact region, the pn junction region, and the n-contact region. For properly designed devices the voltage drop across the resistances in the p and n regions is much smaller than the junction voltage and hence most power is dissipated in a very narrow volume of semiconductor material near the junction. This volume heats up very quickly, typically on the order of microseconds, and then heat flows out of the device by the lowest thermal impedance path(s) to the external thermal environment. Once thermal equilibrium is achieved the Vf, wavelength, and light output of the device stabilizes.

Overheating of the diode junction adversely affects the lamp performance for many reasons. First, the efficiency of an LED drops with increasing temperature. Second, the lifetime of an LED is reduced at higher temperatures, and finally, the packaging material that surrounds the diode can be catastrophically damaged at high temperatures. This last effect can be very significant since it is inherently nonlinear. The encapsulate used to package LED reach a glass transition temperature above which the plastic flows very easily, which can cause several deleterious effects in a lamp. To the end user of the LED lamp this implies that there is a maximum junction temperature, Tj, which should not be exceeded whether using DC or pulsed conditions.

Clustering: In one case one or more LEDs in series are connected to a standard current supply, which is set up to deliver the amount of current specified for a specific application. An advantage of this method is that it accommodates variation in the junction voltage of the LEDs at a specific current level. In the second case LEDs can be run in parallel with a single voltage supply. Normally one or more series resistors are used to control the current through the diode.

The primary advantage of this scheme is that it's simple and is readily used where a DC power source is already available (e.g. from a battery in a car). The primary difficulty is that the drive current through the diode will change with its Vf. A practical example will illustrate this. Assume that standard off the-shelf diodes have a voltage range of 1.90V to 2.2V at a current of 20mA. One design could have six diodes connected in series (i.e. no series resistor is used to limit the current), and they're driven with a 12V battery. If the Vf values for each of the diodes is 2.0V, then exactly 20mA flows in the circuit. However, for diodes with Vf of 1.9V the drop at 20mA will be only 11.4V, which implies that they will draw more than the designed 20mA. For diodes with high values of Vf the situation is reversed and less than the design current flows. The situation is made worse by the fact that in cars the allowed variation in voltage across the battery is very large. In general, it is impractical to drive LEDs with a voltage source without a current-limiting series resistor The disadvantage of this is that excess power is dissipated in the series resistor which lowers the overall efficiency of the circuit. The conclusion is that each design needs to be optimized to a specific application, and either current or voltage drive circuits can be used with LEDs.

Reverse Voltage Considerations

An LED, like any diode, conducts current easily under forward bias, but blocks the current flow when reverse biased. However, since the devices are optimized for high light output their characteristics, as blocking diodes, are not very good. Typically a device will conduct a few microamps (mA) at -5V, but this leakage current becomes significant at more negative voltages. Therefore, using LEDs in an AC circuit (e.g. at 220V/120VAC) where they both emit light and block current in the reverse direction is not normally recommended. Using LEDs under these conditions can lead to unpredictable performance and a significant reduction in the lifetime of the devices.

LED Long Term Life

The long-term reliability of LED (light emitting diodes) semiconductor die provides a very strong value for most applications. If packaged properly, LEDs emit light for a much longer time period than almost every other alternative light source technology. LEDs are semiconductor light sources. Because LEDs operate under the laws of solid state physics, reliability behavior is characterized in a semiconductor context. LEDs are not subject to catastrophic failure when packaged properly and operated within the design parameters.

Light generation in LEDs and incandescent bulbs are distinctly different processes. With incandescent bulbs, light is generated by heating a tungsten filament to high temperatures, and leads to macroscopic materials degradation and loss. Within an LED, light is generated through electron-hole recombination and is a material conserving process. The crystalline structure of the LED is unchanged in the light emission process, although microscopic changes can occur at normal low drive currents and voltages, these microscopic changes are small. The Macroscopic versus microscopic nature of the degradation processes also leads to different failure mechanisms. Incandescent bulbs fail suddenly and catastrophically when the filament ruptures. The performance of LEDs with time typically follows a predictable degradation of light output with time. In some instances, LEDs can fail catastrophically due to environmental (high AI-content LEDs) or packaging related effects (stress).

The most common lighting industry characterization is rated life. Rated life is defined as the length of ' operational time, under standard conditions, that 50% of a large sample of devices will fail catastrophically. For example, one can expect that half of the incandescent lamps rated at 1000 hours will fail after 1000 hours of use. At 1000 hours, there is a 50 / 50 chance that any bulb's filament will break. The bulb is also treated as end of useful life if at 60% of rated life its lumen maintenance falls below 85% of initial value.

The semiconductor industry also refers to catastrophic failure as a definition of device life. In this case, the term used is MTBF (mean time between failures). MTBF is a well-defined and rigorously calculated statistical term. LED physics dictates that this number reflects the time before one can expect a single failure. Once the initial failure occurs, one restarts to time zero, and it will take another MTBF period before another failure can be expected. This definition is appropriate, it is not terribly useful in light output sensitive applications. It also does not integrate many of the device handling, assembly, device drive conditions, and environmental factors that contribute to premature LED failure. No reference is made to light output degradation.

The mean time between failures (MTBF) of high quality LEDs properly packaged is on the order of millions of hours. Therefore, a rated life projection provides little useful information. However, all LEDs experience light output degradation. It is a matter of how long will the light output level remain useful? Because every application is different, there is no simple answer to this question. A determination of the minimum acceptable light level, associated drive requirements, and applicable reliability parameters desired needs to be carefully reviewed and serve as the basis for the LED device packaging design to ensure the minimum light level is not breeched over the useful life. In combination with this review a clear understanding of the package design in terms of mechanical, material and other factors must be reviewed and determined for potential component package related degradation issues ranging from moisture induced corrosion, dark line-defect growth, strain-optic effect, optical transparency, and other chemical and physical effects. In practice, LED die and their life can be extended or diminished based on these packaging choices. LEDs can be packaged to exceed 100,000 hours of life while being cycled from -55 to 125°C, or can be packaged as an indicator lamp (5mm / T 1 ¾) to last 100,000 hours at ambient temperature.

Existing light output reliability data provides characteristic LED packaged component performance under several drive conditions and at various temperatures, over time. Packaged LED operating life is characterized by the degradation of LED intensity over time for that particular package style. The limiting factor in all of these characterizations is almost always package related versus LED die related constraints. When the LED packaged component degrades to half of its original intensity after 100,000 hours, for example, it is at the end of its useful design life although it will continue to operate at diminished levels. LEDs can also be operated in high shock and vibration modes, over wide temperature variations and environments, and can be cycled on/off without excessive degradation. As for the actual LED die, the useful life far exceeds packaged performance component levels.

LEDs & "White" Light

Solid-state "White" light, the many ways to create it and deliver it via LEDs, for a variety of applications and uses, is not well known. Many people, companies and designers are familiar only with traditional analog "White" lighting, with no real appreciation of the beneficial alternatives that LEDs can provide

LEDs not only possess the attributes of energy efficiency, life and ruggedness etc., but are also a new kind of "White" light source that embodies many application specific attributes:

· Ultra-low Voltage and Power

· Fast Response (<100 ns) Read/Write Time

· No DC-AC Inverter/Power Supply

· Full Digital Control (Strobe, Pulse, Color Field Sequential (CFS) and Full Range Dynamic Dimming (15,000:1)

· No Warm-up or Over Voltage Start-up

· No UV Radiation

· No Hazardous Mercury Disposal

· No ELF, RMI

· Addressable Individual or Alternating Component Colors (Red, Green & Blue - RGB = 256 Million Hue Color Pallet)

· Full Spectrum Lighting or Specialized.

"Black" is the absence of all colors. When light from all parts of the color spectrum overlap one another (i.e. The presence of all colors), the additive mixture of colors appears "White': This is referred to as "Polychromatic White Light" - Primary Colors from which all hues are derived are Red, Green and Blue. Secondary Colors also called "additive" - of light are: Magenta (mix Red & Blue); Cyan (mix Green & Blue); and Yellow (mix Red and Green). Any additive Secondary Color and an opposite Primary Color also equals WHITE Light (Yellow & Blue, Cyan & Red, Magenta & Green). >

 "White" lighting, with LEDs, takes on a whole new meaning for many far reaching applications, including: outdoor signs and architectural; night-vision and safety; "entertaining" lighting for the home or mall; general lighting for off the grid applications; "healthy" lighting that eliminates some of the physiological and psychological effects caused by fluorescents; RGB color field sequential lighting for non-emissive LCD displays required for portable electronic appliances; "White" vehicle lighting with built-in safety features, and others.

Polychromatic White with multicolor LEDs enables a one to not only produce "White" light but a wide mixture of hues of light by drive address and control, to create various color effects. White light is also produced by utilizing an LED in conjunction with a phosphor (conversion) layer such as Yttrium Aluminium Garnet, added to the emitting surface(s) during discrete component packaging. The latter approach produces "White" light only.

InGaN	AIInGaP	Phosphor	White Light Plus Other Hues	Applications
Blue and Green	Red	n/a	White + R, G, B, & 256 Million Combinations(Polychromatic)	LCD, Healthy, Architectural, General Lighting, Digital Film Recording, Displays, etc.
Blue	Yellow	n/a	White + B, Y, and various shades (Multicolor)	Automotive, General Lighting, etc, etc
Blue-Green	Red or Red Orange	n/a	White + B, R and various shades (Multicolor)	Automotive, General Lighting, etc, etc
Blue 470 - 450		Add phosphor	White only	"Multicolor White Light'; etc, etc
UV		Add phosphor	White or various Monochromatic colors dependent on phosphors used	"Multicolor White Light'; etc, etc
ZnSe epi	Green-red ZnSe substrate		White +Blue from epilayer, green red from substrate	"Multicolor White Light'; etc, etc

LEDs and Static Electricity - Handle with care

Macroscopically, the effects of static electricity are broadly known: lighting, for example. Microscopically, effects range from build-up of dust on TVs, clinging of clothes, a "static" shock when someone touches a cat. When friction and separation occurs between material, triboelectric generation occurs; the transfer of electrons from the atoms on the object's surface, takes place. The resulting imbalance of electrons is known as an electrostatic charge. This charge is either positive or negative depending on deficiency or abundance of free electrons.

The amount of charge potential is influenced by the extent of the contact, the materials involved, relative humidity, and the texture of the materials. Static charges of up to 30,000 Volts are not uncommon and can be generated quite easily. In comparison, a discharge of only 10 Volts can destroy a Class 1 ESD (Electrostatic Discharge) Sensitive device. ESD damage to electronic devices may lead to premature or intermittent device failure. Studies indicate that ESD damage to electronics and associated equipment is estimated as high as $5 billion annually.

LEDs are static sensitive devices. InGaN die are generally considered "Class 1" devices (Can survive 20 V bias w/ Machine Model Testing, and 130 V from Human Body Model Testing), while AIInGaP LEDs are considered "Class 2" or better

ESD damaged devices can appear dim, dead, shorted, or with low Vf or Vr ESD damaged devices should not be confused with electrical overstress (EOS) induced damage caused by improper circuit design or drive, from damage incurred during die attach, wire bonding or encapsulation, or from normal environmental induced stress. The following devices have been identified by test data and/or analysis applicable to the level shown:

Device Sensitivity To ESD: Human Body Model (HBM)
ESD Class 0-iC:0 to 1999 volts	Microwave & High Freq. Devices (Schottky, Probe Contact, Detector diodes;MOSFETs; SAWs; JFETs; Voltage Regulators; OP Amps; Thin Film Resistors; ICs; VHSICs, SCRs
ESD Class 2: 2000 to 3,999 volts	Some MOSFETs, JFETs, ICs, & VHSICs, Type RZ Resistor Networks; Low Power Bipolar Transistors InGaN LEDs

ESD Safety & Control (ESDS) Programs: include provisions for Charge Protection; Grounding; Shielding; Neutralization; Use of Anti-Static Materials, Work Stations, and other materials and equipment, and all importantly, Education. The primary method of ESD Control is to ground (or bring to the same potential) all conductors that come in contact or near proximity to the electronic device(s). These conductors include humans, tools, mats, other electronic devices, boards, connectors, packaging, etc.

ESD sensitive devices should always be stored in enclosed antistatic shielding bag, tote, and container when being routinely handled.

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