Light Emitting Diode (LED) technology and its impact on DLP television applications

Abstract
This white paper will discuss Light Emitting Diode (LED) technology and its impact on television applications. It will highlight the advantages and challenges for these applications and will explore the specific advantages that LED technology has for DLP product applications.

Introduction
The LED has become a pivotal illumination technology with a wide variety of applications. Since their initial invention, LEDs have been used in many diverse applications such as watches, calculators, remote controls, indicator lights, and backlights for many common gadgets and household devices. The technology is advancing at a rapid pace and new applications continue to emerge as the brightness and efficiency of LEDs increase.

LED History
From the early 1900s, scientists have been discovering ways to generate light from various materials. In 1907, Henry Joseph Round discovered that light could be generated from a sample of Silicon Carbide (SiC). For the next 50 years, scientists continued to discover the light emitting properties that exist with some compounds. In the 1950s, studies around the properties of Gallium Arsenide (GaAs) paved the way for the first official LED discoveries that soon followed.1

LED research began in the early 1960’s, primarily at Bell Labs, Hewlett Packard (HP), IBM, Monsanto, and RCA. Gallium-Aresenide-Phosphide (GaAsP) provided the basis for the first commercially available red LEDs in 1968 by HP and Monsanto. In the early 1970s, the use of LEDs exploded with new applications such as calculators and watches by companies like Texas Instruments (TI), HP, and Sinclair. Other applications such as indicator lights and alphanumeric displays soon became the mainstream use for LEDs and continued to be so for many years.2

LED Technology Background
As the name implies, an LED is a diode that emits light. The diode is the most basic semiconductor whose purpose is to conduct electrical current with some form of controlled variability. The diode in its simplest form is comprised of poor conducting materials that have been modified (or “doped”) to increase the amount of free electrons that are available. High electron materials (referred to as N-type materials) are combined with low electron materials (referred to as P-type materials) to form a junction for these free electrons to flow. This junction is often referred to as the PN junction.

An LED is a PN junction diode semiconductor that emits photons when voltage is applied. This process of photon emission is called injection electroluminescence and occurs when electrons move from the N-type material to fill the lower energy holes that exist in the P-type material. When the high energy electrons fall into these holes, they lose some of their energy which results in the generation of photons. The materials used for the P-type and N-type layers along with the size of the gap between them determine the wavelength and overall energy level of the light that is produced.

Many materials have been developed for manufacturing LEDs. Aluminum-Gallium-Arsenide (AlGaAs), Aluminum-Indium-Gallium-Phosphide (AlInGaP), and Indium-Gallium-Nitride (InGaN) are commonly used for present LED architectures. “AlInGaP” is typically used for Red and Yellow dies while “InGaN” is used for Blue and Green. These materials efficiently produce photons that have wavelengths in the visible spectrum. These materials in combination with new manufacturing architectures have enabled the production of very bright LEDs that are beginning to find their way into general lighting and automotive applications. Some architectures have begun utilizing additional phosphor compounds to generate white light and are now beginning to compete with common incandescent and fluorescent lighting - with much lower power and much longer lifetimes.

The worldwide production of LEDs has risen to about 4 billion units per month. Manufacturing in Taiwan, Japan, and the U.S. comprises the most significant volumes with Taiwan leading with about one half of that volume overall. Much of the manufacturing involves the packaging of the LED die with a limited number of manufacturers creating the actual LED die material. Figure 1 illustrates the market size for low brightness and high brightness LEDs as a function of the total LED market.3

Figure 1 - LED Market Segments

LED Technology Breakthroughs
Recent innovations in the manufacturing of the die material and packaging have resulted in ultra high brightness capabilities. The use of new materials for the substrate have allowed for improved thermal conductivity which allows for higher power consumption and net light output. This increase in light output has enabled new applications for LEDs such as automotive lighting, traffic signals, and more recently, television displays. An example of these new structures is illustrated in Figure 2.

Figure 2 - Basic LED Structure

Significant improvements in the production of Aluminum-Indium-Gallium-Phosphide (AlInGaP) and Indium-Gallium-Nitride structures have allowed for improved brightness in green and blue specifically. Additional colors such as amber and cyan are also being developed at a rapid pace. These improvements enable system designs that can produce better color fidelity at near equivalent brightness to common lamp-based technologies with longer lifetimes. Additional performance enhancements include system level features like instant on, no mercury, no color refresh artifacts, dynamically adjustable brightness, and improved color gamuts. Figure 3 illustrates the gamut area for LED illumination as compared to the common reference standard (Rec. 709).

Figure 3 – LED Color Gamut

LED illumination provides a much larger color gamut (as much as 40% or more than the HDTV color standard [Rec. 709]), providing more accurate color fidelity. These performance attributes can be quite appealing for television applications where long life and excellent color fidelity are required. As LEDs continue to advance, their impact on television applications could be significant. Figure 4 illustrates the evolution of LEDs and their potential brightness efficiency in the coming years.4

Figure 4 – Lighting Technology Evolution