# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 17 W/cm\({}^{2}\)-Class Irradiation Modules and Applications  
Figure 19a illustrates the wavelengths of interest for various applications. The wavelengths used with sensors are widely spread between 200 nm and 365 nm, and two representative examples of light sources for urea sensors and high-performance liquid chromatography (HPLC) are shown. The urea sensor in a dialyzer was partially replaced by 285 nm DUV-LEDs and compact HPLC system is expected to be used to measure the water quality of the sewage. Among the sensor applications, these two examples are highly substantial. However, even niche semiconductor industries cannot be free from today's production machines and subcontracting companies. Sensor applications do not need a high output power and considered to be accessible market at the beginning of laboratory production. However, the number of LEDs used in sensor application is not enough, even for the niche semiconductor industries. Accordingly, an irradiation application might be the key to increasing the quantity of production. The sterilization of air and water, UV curing and printing (photochemical applications), deodorizing, the excitation of photocatalyst, phototherapy, medical applications, and so forth, should be indispensable applications for the DUV-LED industry to stand by itself. As described, an output of 100 mW per chip and WPE of over 7% are in sight. This means that W/cm\({}^{2}\) irradiation modules corresponding to the performance of conventional mercury-based bulbs are within reach. Although there are some examples of LED production where LEDs with a specific wavelength are produced on demand, the range of wavelengths tends to be limited. The semiconductor industry is geared to the production of very large quantity. The spectra of conventional mercury lamps are shown in Figure 20. The advantage of LEDs to UV bulbs is thought to be located at wavelength of around 270-300 nm. For UV printing (315 nm) and photocatalyst excitation (325 nm), light sources with a shorter wavelength (280-300 nm) can be used because they are related to excitation. The DUV light required for UV curing has an intensity of about 0.5 W/cm\({}^{2}\). Keeping a UV printing in mind, sub-W/cm\({}^{2}\) modules with a dense layout of dies have been demonstrated. Figure 21 shows photographs of various irradiation modules. For sterilization, DUV-LEDs must be cost-feasible and convenient in comparison with low-pressure mercury lamps. As shown in Figure 19, the absorption of DNA occurs around 265 nm (\(\lambda<290\) nm). DUV-LEDs with wavelengths of 270-280 nm are also expected to be useful because of their high-EQE region. Light flux of about 33 mW/cm\({}^{2}\) at 254 nm is sufficient to kill bacteria, although there is some variation depending on bacteria. The efficiency of sterilization at 280 nm is more than half of that at 254 nm according to Figure 19b. Light flux of 1 mW/cm\({}^{2}\) at 280 nm for 1 min can kill bacteria. DUV-LEDs with a wavelength of 280 nm and 100 mW output can potentiality be used for sterilization. Accordingly, the wavelength that should be focused on mass-production is around 280-300 nm, including sterilization or bacteriostatic applications. However, medical applications (310-315 nm) cannot employ shorter wavelengths owing to the need for weak absorption on the lower-energy side to avoid the destruction of genes. The advantage of using DUV-LEDs is considered to be the tunability of the wavelength and the shorter start-up time before reaching the constant output compared with discharge lamps. Figure 19: (**a**) Map of application wavelengths for DUV-LEDs; and (**b**) absorption of DNA, AGTC nucleic acids, uracil in ribonucle acid, and sterilization efficiency related to their absorption. Sensor applications are expected involve various wavelengths. UV light sources in HPLC (254 nm) and urea sensors (285 nm) in a dialyzer are considered as examples. A larger quantity of DUV-LEDs is expected to be used for the applications of sterilization and photochemistry including UV curing.