The luminous efficiency of the LED is generally called the external quantum efficiency of the module, which is the product of the internal quantum efficiency of the module and the extraction efficiency of the module. The so-called internal quantum efficiency of a module is actually the electro-optical conversion efficiency of the module itself, which is mainly related to the characteristics of the module itself, such as the energy band, defects, impurities, and the barrier crystal composition and structure of the module. The extraction efficiency of a module refers to the number of photons generated inside the module that can actually be measured outside the module after absorption, refraction, and reflection by the module itself. Therefore, factors related to extraction efficiency include the absorption of the module material itself, the geometric structure of the module, the refractive index of the module packaging material, and the scattering characteristics of the module structure. The product of the internal quantum efficiency of the module and the extraction efficiency of the module is the luminous efficiency of the entire module, that is, the external quantum efficiency of the module.
Early module development focused on improving its internal quantum efficiency. The method is mainly to improve the quality of the barrier crystal and change the structure of the barrier crystal, so that the electric energy is not easily converted into heat energy, thereby indirectly improving the luminous efficiency of the LED, and the theoretical internal quantum efficiency of about 70% can be obtained. However, such internal quantum efficiency is almost close to the theoretical limit. Under such conditions, it is impossible to increase the total light emission of the module by simply improving the internal quantum efficiency of the module. Therefore, improving the extraction efficiency of the module becomes an important issue. The current methods used to improve the removal efficiency of modules can be divided into the following directions:
(1) The change of grain shape-TIP structure. Traditional LED dies are made with a standard rectangular appearance, because the refractive index of general semiconductor materials is quite different from that of the encapsulating epoxy resin, and the critical angle of total reflection at the interface is small. The four rectangular cross-sections are parallel to each other, and the probability of photons leaving the semiconductor at the interface becomes smaller, so that the photons can only be totally reflected inside until they are completely absorbed, turning the light into heat, resulting in poor luminous efficiency. Therefore, changing the shape of the LED is an effective way to improve the luminous efficiency. In the TIP (Truncated Inverted Pyramid) type grain structure developed by HP, the four cross-sections will no longer be parallel to each other. This structure can efficiently extract light, and the external quantum efficiency is greatly increased to 55%, and the luminous efficiency is as high as 100lm/W.
However, HP’s TIP-LED is only suitable for easy-to-process quaternary red LEDs, and it is quite difficult for GaN series LEDs using extremely hard sapphire substrates. At the beginning of 2001, Cree used the same structural concept and took advantage of the SiC substrate to successfully make GaN and SiC-LEDs into LEDs with slopes, and greatly increased the external quantum efficiency to 32%; however, SiC substrates are much more expensive than sapphire, so there is no further progress in this technology.
(3) Surface roughening technology. Roughen the internal and external geometric shapes of the module, destroy the total reflection of light inside the module, and improve the light output efficiency of the module. Such a method was first proposed by Nichia Chemical, and its roughening method is basically to form a regular uneven shape on the geometry of the module. And this regularly distributed structure is also divided into two forms according to the location, one is to set a concave-convex shape in the module, the other way is to make a regular concave-convex shape on the top of the module, and set the reflective layer on the back of the module. Concave-convex shapes can be provided on the interface layer of the GaN-based compound semiconductor by using a traditional process. Therefore, the method of providing concavo-convex shapes in the module has high practicability. Using surface roughening technology to develop UV modules with a wavelength of 405nm, an external quantum efficiency of 43% and an extraction efficiency of 60% can be obtained, which is currently the world’s highest external quantum efficiency and extraction efficiency.