Japan's Sanken Electric has recently developed a new technology that can greatly improve the luminous power of light-emitting diodes (LEDs).
The company's products made with this new technology have nearly a hundred times more luminous power than previous products. Industrial samples began to be available in October 2002 and mass production began in the spring of 2003. In the product operation plan, Sanken Electric firstly produced red (light-emitting wavelength 620nm) and yellow (light-emitting wavelength 590nm) products for signal lights, car brakes and direction indicators. In terms of blue and green LEDs, products of this new technology will also be developed. The company hopes to develop white LEDs made up of red, green and blue LEDs. It is often necessary to solve the following three problems by entering the LCD panel backlights and lighting products. The first three problems must be solved: The luminous efficiency of the LED element is increased by 2.5 times; secondly, the current value per unit area flowing through the LED is increased by a factor of four; and the chip size of the LED is expanded to 10 times. . Among them, the first and second problems are breakthrough factors that greatly increase the luminous power.
In order to improve the luminous power, Sanken Electric has increased its research efforts on LEDs. The solution to the first problem is that the company uses a method of arranging a layer of aluminum metal inside the chip to achieve the light emitted from the luminescent layer of the LED illuminating the outside of the chip without loss, from the luminescent layer to the bottom plate. Light is reflected from the aluminum metal layer to the surface of the chip. Red and yellow LEDs using a GaAs substrate have previously used compound semiconductors in setting the reflective layer, so light reflection is insufficient. In terms of structure, the wiring is spread over the entire chip surface by working on the electrodes of the LEDs. Thus, the electric field applied to the light-emitting layer becomes very uniform throughout the surface of the chip, so that the light emitted from the light-emitting layer becomes uniform on the surface of the chip.
The second solution is to attach the silicon backplane to the LED and set up an aluminum metal layer. Compared to GaAs substrates, silicon substrates have a thermal conductivity of about 2.4 times, so the heat generated by light is easily released outside the LEDs. Previously, since heat was accumulated inside the LED, the life of the component was shortened. Therefore, it is difficult to increase the luminous power by increasing the current.
In the red and yellow LEDs that the company provides for industrial samples, the principle of the silicon substrate bonding process is as follows: First, an LED structure is formed on a GaAs substrate, and then the GaAs substrate is removed in solution by an etching technique. A metal layer is then formed in the order of aluminum and copper on the surface of the LED structure to which the GaAs substrate is connected. Finally, the LED with the metal layer and the copper surface of the n-type silicon substrate on which the copper is formed are pasted together.
This new technology is also suitable for blue and green LEDs. When using these two types of LEDs, a GaN type LED is first formed on a silicon substrate. Then, after the silicon substrate is removed in the solution, a layer of aluminum and copper is formed on the surface of the LED to bond it to the silicon substrate on which the copper is formed. Moreover, in general, when a GaN-type LED is formed on a sapphire substrate used for blue and green LEDs, such a bonding technique is not used.
Da Ke Kang, director of the Semiconductor Research Institute at Sanken Electric Semiconductor, believes that it is very difficult to remove the sapphire substrate in solution. However, the technology of forming GaN type and LED on the silicon substrate has been developed. All preparations are currently in place.
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