Innovative technology to improve the extraction efficiency of red LED

The red LED at present, have a better extraction efficiency is not impossible, this is due to have a special design of the transmission and reflection layer, the product has been in the online EPISTAR two product development.

At present, many of the AlGaInP based LED are used to provide a red light source for traffic signals and brake lights. However, this type of device can be more successfully applied to commercial products if the cost per lumen is lower. For example, projector, LCD TV backlight and color temperature modulation device, etc..

Methods to reduce the per unit cost of lumens can use new technologies to improve the efficiency of LDE, such as improving the growth conditions or device process, will limit the internal quantum efficiency of this device is to improve the theory, so that it can improve the light extraction efficiency of LED devices.

Many technologies have been developed for this purpose, but none of them is suitable. Join the distributed Prague reflector (distributed Bragg, reflector, DBR) in LED is used to reduce the absorption of light source in GaAs substrate, but the efficiency of light reflection tilt incident angle is relatively low, this is because the optical loss was caused by. Improve the way can replace the substrate to replace the substrate can penetrate, such as sapphire or GaP, but there are still shortcomings, this is because the method is unable to release the effective heat transfer coefficient power, drive current and maximum to use lumen output value. However, the shape of the surface is still able to increase the output of light, but the use of traditional chemical etching technology is not easy to control the boundary and etching range.

The heat conduction problem has recently been used in a novel way to electrify and heat transfer substrates by the conversion of oriented. However, even with this advanced approach, the optical efficiency of LED in many commercialized 620 nm wavelengths is only about 50 lm/W. This means that the production of high brightness LED comes from a combination of a variety of different technologies, so it can not meet customer expectations of efficiency.

In any case, in the Taiwan Jingyuan Au Optronics Co, we have a new series of AlGaInP LED has not yet been published, this product can produce far more than the current technology efficiency. For these products, we named it P and A series (although they originally were named Phoenix and Aquarius LED). The main feature is that the excitation efficiency of light is at least 50%, which is due to the additional multilayer structure of the waveform surface type and different levels of reflection coefficient. More importantly, we use the original equipment can be engaged in manufacturing, and can be loaded.

We call this patented multilayer film structure "Lambertian penetration and reflection film" because they are based on the cosine scattering law of Johann Heinrich Lambert. The reflection or divergence of these structures has great strength in the direction perpendicular to the surface, and the weakest at the most inclined angle (see Figure 1)

Figure 1: (a) Lambertian penetration surface in the vertical penetration rate will have the largest on the surface (light intensity and cos[theta theta] is proportional to the said from the plane of the substrate to the vertical position, the angle between) the length of the arrow represents the light source intensity. (b) the distribution of the same intensity Lambertian reflector is generated, EPISTAR has confirmed that the transmission characteristics of Lambertian (c) and reflector. (d) the angle of the incident beam is 0 degrees, 30 degrees and 60 degrees, and the angle of observation is from 5 to 80 degrees.

Our P- series and the A- series feature the LED for the Lambertian emission and reflection at the top and bottom of the device separately. Most of the light from the emitter is moving forward, and only a very small portion of the light will return to the device, which is likely to be absorbed by the quantum. At the same time, the reflector can return the device to the device through a specific angle, and the majority of the light source is guided to the substrate, so as to avoid the occurrence of multiple reflections inside the chip.

We fabricated our P- series chips created by the Lambertian reflector on the GaP surface, which is the uppermost layer of AlGaInP in the GaAs oriented epitaxial layer (Fig. two a). Before the GaAs is removed, the wafer is bonded to silicon. We then etch the n-type plating layer to form a Lambertian reflector, and clearly define the backside contact between the gold p-type and the silicon substrate. The wafer problem is as follows: before the device is combined, the wafer will be split into separate chips.

A- series of LED have somewhat different design (Fig. two B), in order to combine the use of sapphire wafer and adhesive film, penetration rate high used as binder, in addition, the manufacturing procedure and P- series of similar.

Figure two: (on) the P- series (a) and A- series (b) LED features a Lambertian penetration and reflective surface.

Our A- series chips will be excited at 615~620nm and require only 2V forward voltage (please refer to table 1). At a current of 20 mA, the nm will be released at a rate of 107 lm/W, and the same as the current at the same condition can be produced by the 615 lm/W (see Figure three for the comparison of the current of the other devices in Figure 130) to the nm. This product has a maximum forward current rating of 40 mA, and this device does have the capability to be used as a backlight, building light source, and entertainment and decorative light source.

Figure three. Comparison of current of other devices

P- series chips are the same size as their A- series, but