Dialogue with OSRAM to understand LED technology "UX: 3"

Germany's OSRAM Opto Semiconductors GmbH has developed a new LED structure "UX:3". According to the company, this structure can not only greatly increase the external quantum efficiency of GaN-based LEDs such as blue LEDs, but also significantly suppress the recognized problem that LEDs have reduced luminous efficiency when passing large currents. By reducing the non-radiative recombination that does not accompany the luminescence, "Auger recombination", the decrease in luminous efficiency can be suppressed. A few days ago, the reporter interviewed Osram Opto Semiconductors application engineering engineer Ms. Chuan Qingjie and Glenn-Yves Plaine, the company's marketing deputy manager, on the suppression effect and how to apply UX:3 in the future.

Reporter: UX: 3 An n-type contact electrode is embedded in the LED chip, and is electrically connected to the n-type GaN-based semiconductor layer through the through hole. Why can this Auger recombination be reduced with this electrode configuration or structure?

OSRAM: The higher the current density of the Auger recombination flowing through the LED active layer, the more significant it is. The UX:3 structure has the effect of reducing current density, thereby reducing Auger recombination.

Previously, the "ThinGaN" technology used in the blue LED chip was to provide an n-type contact electrode on the surface of the chip. Specifically, a pad for electrical connection from the outside is provided at one of the four ends of the chip, and an electrode wire (grid) connecting the pad is surrounded on the surface of the chip, thereby causing a current to flow through the entire chip. In fact, the current is not first-class on the surface of the chip. On the surface of the chip, the closer to the pad and the grid, the greater the current flowing, so that a local region with high current density appears. This is because the farther away from the pad, the greater the effect of the grid wiring resistance. As the current input to the chip increases, this tendency will continue to increase, resulting in a significant decrease in luminous efficiency. The reason for this is that there is a significant Auger recombination phenomenon in the region where the current density is high.

UX: 3 is provided inside the LED chip with an n-type contact layer extending into the surface of the chip. From the n-type contact layer, the n-type GaN-based semiconductor layer on the surface of the chip is electrically connected by tens of through holes (when the chip size is 1 mm square). In this way, a voltage can be uniformly applied to the n-type GaN-based semiconductor layer in the surface of the chip. By eliminating the higher portion of the current density, the significant area of ​​the Auger composite can be greatly reduced.

Since the n-type contact layer is provided inside the chip, the light emission from the active layer to the outside of the chip is no longer blocked. This achieves a high light extraction efficiency of 75 to 80%.

Reporter: In addition to changing the configuration and shape of the n-type contact layer, have other measures been taken?

OSRAM: It also thickens the active layer as a light-emitting part. The probability of occurrence of Auger recombination is proportional to the third power of the carrier density. Therefore, if the carrier density is reduced, the occurrence of Auger recombination can be reduced. The carrier density is reduced by thickening the active layer. The active layer is not simply thickened, but the doping amount and the like are optimized.

Reporter: How much improvement can the luminescence characteristics be improved?

OSRAM: The blue LED chip with UX:3 has a light output power that is about 10% higher than that of ThinGaN when the chip is charged at 350 mA with a chip of 1 mm square. The more input current, the greater the difference between UX:3 and ThinGaN. The white LED using this blue LED chip has a luminous efficiency of 136 lm/W.

The forward voltage (VF) is only 100mV. It can be said that this is one of the main reasons for the improvement of luminous efficiency.

Reporter: How much will the manufacturing process change? If the through hole is set, will the yield drop significantly?

OSRAM: Due to the increased number of through-holes, the manufacturing process will increase. However, each manufacturing process is not a special process, and it is only necessary to optimize by the conventional manufacturing process. The through hole depth is only a few hundred nm, and the depth and width are relatively small, so it does not have much impact on the yield.

Reporter: Please talk about the future application plan.

OSRAM: UX: 3 will be used as an evolutionary version of ThinGaN, starting with high-end blue LEDs, white LEDs and green LEDs with high currents. First, the products used in the mobile phone camera flash will be launched, and in 2010, products for general lighting use will be released (Figure 2). It is currently planned to use UX:3 on a large-size chip of 1mm square, but after the technology is mature, it is also possible to use UX:3 on a standard-sized chip.

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