Introduction to the basic structure and key technologies of high-voltage LED

In recent years, due to the advancement of technology and efficiency, the application of LED has become more and more extensive; with the upgrading of LED applications, the market demand for LED has also developed towards higher power and higher brightness, which is commonly known as high-power LED.

For the design of high-power LEDs, major manufacturers currently mainly use large-size single low-voltage DC LEDs. There are two methods: one is the traditional horizontal structure, and the other is the vertical conductive structure. As for the first method, its process is almost the same as that of ordinary small-size grains. In other words, the cross-sectional structure of the two is the same, but unlike small-size grains, high-power LEDs often need to operate under large currents. A slightly unbalanced P and N electrode design will lead to serious current crowding. As a result, in addition to making the LED chip unable to achieve the brightness required by the design, it will also damage the chip's reliability.

Of course, for upstream chip manufacturers/chip factories, this method has high process compatibility and does not require the purchase of new or special machines. On the other hand, for downstream system manufacturers, the peripheral matching, such as power supply design, etc., is not very different. However, as mentioned earlier, it is not easy to evenly spread the current on large-sized LEDs. The larger the size, the more difficult it is. At the same time, due to geometric effects, the light extraction efficiency of large-sized LEDs is often lower than that of small-sized LEDs.

Figure: Differences in driving methods for low-voltage diodes, AC diodes, and high-voltage diodes (click on the image to enlarge).

The second method is much more complicated than the first one. Since almost all commercial blue LEDs are grown on sapphire substrates, to change to a vertical conductive structure, it is necessary to first bond with the conductive substrate, then remove the non-conductive sapphire substrate, and then complete the subsequent process. In terms of current distribution, since there is less need to consider lateral conduction in the vertical structure, the current uniformity is better than the traditional horizontal structure. In addition, in terms of basic physical principles, materials with good conductivity also have the characteristics of high thermal conductivity. By replacing the substrate, we also improve heat dissipation and reduce the junction temperature, which indirectly improves the luminous efficiency. However, the biggest disadvantage of this method is that due to the increased process complexity, the yield is lower than the traditional horizontal structure, and the manufacturing cost is much higher.

Basic structure and key technologies of high voltage light emitting diode (HV LED)

Epistar was the first in the world to propose high-voltage light-emitting diodes (HV LEDs) as a solution for high-power LEDs. Its basic structure is the same as that of AC LEDs, which is to divide the chip area into multiple cells and then connect them in series. Its feature is that the chip can determine the number and size of cells according to the requirements of different input voltages, which is equivalent to customized services. Since each cell can be optimized, it can obtain better current distribution, thereby improving luminous efficiency.

There are three main technical differences between high-voltage light-emitting diodes and general low-voltage diodes. The first is the trench. The purpose of the trench is to separate multiple crystal cells, so the trench needs to reach the insulating substrate. The depth varies depending on the epitaxial structure, generally about 4~8um. There is no definite limit on the width of the trench, but a trench that is too wide means a reduction in the effective light-emitting area, which will affect the luminous efficiency of the HV LED. Therefore, it is necessary to develop a high aspect ratio process technology to reduce the process line width to increase the luminous efficiency.

The second is the isolation layer. If the isolation layer does not have good insulation properties, the entire design will fail. The difficulty lies in the fact that a film with good coverage, tight film quality and good insulation must be coated on the trench with a high aspect ratio. This is also the key to the single-crystal AC LED process.

The third is the interconnection between chips. Generally speaking, to achieve a good connection, the wire needs a relatively flat surface when it is connected. A deep step-like structure will make the wire structure weak and easily damaged under high voltage and high current driving, causing the chip to fail. Therefore, the development of the planarization process becomes important. The ideal state is to flatten the deep grooves at the same time when making the insulation layer, so that the interconnection wire can be smoothly connected.

In addition, the main difference between high-voltage light-emitting diodes and general low-voltage diodes in application is that they can not only be used in constant DC, but can also be used in AC environments as long as an external bridge rectifier is connected, which is very flexible. In high-voltage light-emitting diodes, the external rectifier abandons the AC LED's use of homogeneous gallium nitride and instead uses a silicon rectifier, which not only reduces energy consumption, but also prevents the impact of excessive reverse bias on the chip; finally, because high-voltage light-emitting diodes have less internal bridge rectifier light-emitting areas than AC LEDs, the luminous efficiency is relatively high and the durability is also better.

As a solution for large size, high power LEDs

The efficiency of high-voltage light-emitting diodes is better than that of traditional low-voltage light-emitting diodes. This is mainly due to the small current and multi-cell design that can evenly spread the current, thereby improving the light extraction efficiency. In some applications, in addition to considering the efficiency of the chip itself, the selling price of the final product is also an important indicator; for example, in the current lighting field, LED light sources are still not considered mainstream products, and the key point is that their selling price is still high. The reason for the high price of LED light sources is that in addition to the price of the chip itself, the overall bill of material (BOM) must also be considered. For example, since the light-emitting diode is essentially a polar component, it must be supplied with a forward bias to be lit. Therefore, a general LED lighting source must be equipped with an AC/DC power conversion system, which is a cost that must be paid.

Because the LED itself is small in size, the heat source is easily concentrated, causing the so-called hot spot phenomenon, which shortens the life of the light-emitting element itself. In order to solve the hot spot problem, the heat dissipation design on the LED light source is also indispensable. Currently, metal heat sinks are the most common heat dissipation design, but metal heat sinks not only increase the weight of the light source, but also increase the cost of the light source. Since the high-voltage light-emitting diode itself is highly efficient, it will reduce waste heat and the need for heat dissipation, thereby reducing costs; from the perspective of power conversion, high-voltage and low-wattage power converters such as flyback topology circuits are not only small in size, but also have lower costs because they use fewer components. Therefore, the advantage of high-voltage light-emitting diodes is not only in the chip itself, it can directly or indirectly further improve the efficiency of the overall module.

In summary, single-chip high-voltage light-emitting diodes have the following advantages in terms of application and design:

1. Save transformer energy conversion loss and reduce costs.

2. In addition to high voltage DC applications, the design can also be operated under AC using an external bridge rectifier circuit.

3. It is small in size and does not take up space, and has excellent application flexibility in packaging and optical design.

4. In addition to red phosphors, blue and red HV LEDs can also be used with appropriate yellow and green phosphors to produce more efficient high CRI warm white LEDs.

At present, Epistar will first conduct a basic check of the design criteria based on the various parameter requirements of the customer; then perform simulations based on the relevant optical, electrical and thermal models to determine the size, number and final product presentation form of the unit cell, and then conduct practical verification; and verify the original design based on the data collected in practice, or modify it to achieve the optimal result. Currently, Epistar's R&D center has begun to establish analog optical, electrical and thermal models related to high-voltage light-emitting diodes.