How to make LED more bright? An article on how to reduce the Droop effect and improve the light extraction efficiency

Reduce the "Droop effect" and improve the light extraction efficiency helps to achieve better and more bright LED.

In the past ten years, LED has changed the solid-state lighting, LED because of its high efficiency and long service life, promote the application in general lighting in. The efficiency of LED at an alarming rate continues to improve, not only reduces the number of a given application LED, but also reduces the hardware cost of the system, so as to improve the rate and reduce the cost. The efficiency of the promotion of high brightness chip becomes small, can be stacked to produce dense array of addressable matrix, it is very suitable for the automobile headlight dynamic beam steering. The inherent InGaN LED high speed switch make it an ideal choice for visible light communication or LiFi.

Figure 1 (a): cool white, warm white phosphor conversion of typical high power LED lighting in J= 350 mA/mm2 and Tj= at 85 C, and the typical high power headlamp LED in J= 1000 mA/mm2 and Tj= 100 C the effect of the composition; (b) high power general lighting LED typical (c); high power automotive lighting LED typical.

LED has penetrated into various markets, largely to this blue LED plug efficiency, conversion efficiency, precise customization and white color control point rise. In this paper, from the San Jose California Lumileds team discussed the technical details of LED, and compared the different structures, thus highlighting the opportunities for future improvement. In the following sections, we will introduce various aspects: white LED efficiency of the typical loss (range); high power LED epitaxial multilayer stack - considering the internal quantum efficiency and attenuation measurement, polar and semi polar and nonpolar GaN; carrier diffusion and light extraction device - patterned substrate chip; structure comparison.

Droop effect

With the LED brightness requirements increase, the working current density increased, extending from the traditional 35 A/cm2 to above 100 A/cm2. This change to extension had a profound impact, because of the increased internal quantum efficiency in 10-20 and A/cm2 increase in the density of 100 A/cm2, the key is significantly different.

At lower current density, the internal quantum efficiency from the material quality, this is because the indirect composite under low current dominant.

By contrast, when the LED drive is more difficult, the focus must be on the Droop effect. Today, the industry generally support Auger recombination is the main reason for the decline of the most advanced industrial equipment efficiency. Because the carrier density in quantum wells increases, so at high driving current, auger loss is very significant, which reinforces the possibility of three particle reorganization.

A selection of Auger recombination reduced is more well into the active region, because it may reduce the carrier density in each of them, but the probability of success is not high. The effective mass of electrons and holes in the asymmetry of the carrier density carrier density active region p side is higher than the N side, and lead to the change of the carrier recombination. So the increase in quantum well benefits may be very small, not even.

A better approach is to use the band structure engineering. This can promote the distribution of carrier, and to ensure that each quantum well low carrier density. To achieve this, the equipment working point to higher efficiency curve (see Figure 2).

One dimensional device active region of LED in figure 2. with different carrier distribution and material quality simulation (a) and the corresponding internal quantum efficiency (IQE) curve (B).

Although the design for activities in the region of low Droop usually can achieve uniform distribution of carriers in the quantum well, but at the expense of their material quality cost, and the increase of non radiative indirect composite. Usually, in the design of the active region of indium content low Droop increase will result in lower quality materials. Obviously, the best LED must be against the efficiency droop and high material quality combine to ensure that the indirect recombinant low (see Figure 2).

Do not want another choice of the Droop effect is the superposition of electron and hole wave function more to increase the radiative recombination rate. Today's LED in the C on the surface and by the internal electric field problems, the electric field opened the electrons and holes and damaged composite radiation. By switching to semi polar and nonpolar substrates to reduce or eliminate the polarization induced electric field in the active area, can be improved. The advantages of not only high radiation, and reduce the Auger recombination rate at higher drive current.

To achieve all this commitment is not easy. Today, half polar and nonpolar devices by the short time non radiative recombination lifetime limits, and the substrate is very expensive, not for commercial purposes. In addition, in spite of these alternative crystal orientation has made progress, but because of the improvement of carrier expansion and material quality, they are chasing a moving target.

To improve the light extraction efficiency

One way to reduce the number of light extraction including Ura Hikaruko jump optimization in modern high power LED, which is usually in the Ura Hikaruko before leaving in the number of trips and cutting chip cavity, cavity absorption pump chip.

The two key characteristics (Ura Hikaruko reflection frequency and pump absorption) are obviously different in the two kinds of structure: flip chip and thin film (see Figure 3 device chart). Provide a small source size film design, and they are preferred in highly oriented applications, and the design of flip chip is connected directly to the board, and do not use the insert. The common point is the high current density and low thermal resistance, can achieve high density array.

In addition to the two kinds of design, there are third kinds, it is a variant of the flip chip: it is through resistance