Gallium nitride (GaN) has emerged as a cornerstone material for light-emitting diodes (LEDs), enabling advancements in efficiency, brightness, and wavelength versatility. Its wide bandgap of approximately 3.4 eV allows for emission across the visible and ultraviolet spectrum, making it indispensable for solid-state lighting, displays, and specialized UV applications. The development of InGaN/GaN quantum well structures has been pivotal in achieving high-performance LEDs, though challenges such as efficiency droop persist. This article examines the material properties, structural innovations, and key challenges in GaN-based LEDs, along with their diverse applications.

The foundation of GaN LEDs lies in the epitaxial growth of high-quality crystalline layers, typically on sapphire, silicon carbide, or silicon substrates. The lattice mismatch between GaN and these substrates introduces strain, which can lead to threading dislocations. However, advancements in buffer layer engineering have mitigated these defects, improving device performance. The active region of GaN LEDs commonly employs InGaN/GaN quantum wells, where the indium composition determines the emission wavelength. By adjusting the indium content, the bandgap of InGaN can be tuned, enabling emission from violet to green wavelengths. The quantum confinement effect in these structures enhances radiative recombination, boosting internal quantum efficiency.

Despite these advantages, GaN LEDs face a significant challenge known as efficiency droop, where the external quantum efficiency decreases at high current densities. Several mechanisms contribute to this phenomenon, including Auger recombination, carrier leakage, and polarization effects. Auger recombination, a non-radiative process involving three carriers, becomes dominant at high carrier densities, reducing efficiency. Carrier leakage arises from insufficient electron confinement in the quantum wells, exacerbated by the polarization-induced electric fields in wurtzite GaN. Researchers have explored various strategies to mitigate efficiency droop, such as optimizing the quantum well thickness, employing electron-blocking layers, and using nonpolar or semipolar GaN substrates to reduce polarization effects.

Color tuning in GaN LEDs is another critical area of research. Achieving consistent and precise emission wavelengths requires precise control over indium incorporation during growth. However, indium segregation and phase separation can lead to inhomogeneous emission and broad spectral linewidths. Techniques such as growth temperature modulation and strain engineering have been employed to improve uniformity. For white light generation, GaN-based blue LEDs are combined with phosphor coatings, where the blue light excites yellow-emitting phosphors like yttrium aluminum garnet (YAG:Ce). Alternatively, red, green, and blue (RGB) LED arrays can be used for full-color displays, though green LEDs still lag in efficiency compared to their blue and red counterparts.

Ultraviolet (UV) LEDs represent another important application of GaN, with wavelengths ranging from 210 nm to 365 nm. These devices are used in water purification, sterilization, and medical diagnostics. Aluminum gallium nitride (AlGaN) is often employed for shorter wavelengths, as increasing the aluminum content shifts the emission deeper into the UV spectrum. However, achieving high-efficiency UV LEDs remains challenging due to the difficulty in doping AlGaN and the increased defect density at higher aluminum concentrations.

In solid-state lighting, GaN LEDs have revolutionized the industry by offering superior energy efficiency and longevity compared to traditional incandescent and fluorescent sources. The efficacy of commercial white LEDs now exceeds 200 lumens per watt, with lifetimes surpassing 50,000 hours. This has led to widespread adoption in residential, commercial, and industrial lighting. The compact size and directional emission of GaN LEDs also enable innovative designs in architectural and automotive lighting.

Display technologies have similarly benefited from GaN LEDs. Micro-LEDs, which utilize arrays of tiny GaN-based emitters, promise high brightness, contrast, and energy efficiency for next-generation displays. These devices are particularly attractive for augmented reality (AR) and virtual reality (VR) applications, where pixel density and response time are critical. Additionally, GaN-based laser diodes are being explored for projection displays and ultra-high-definition televisions.

The future of GaN LEDs lies in further improving efficiency, reducing costs, and expanding wavelength coverage. Research into novel substrate materials, such as bulk GaN or graphene, aims to reduce dislocation densities and improve crystal quality. Advances in nanostructured LEDs, including nanowires and photonic crystals, could enhance light extraction and mitigate efficiency droop. Meanwhile, the integration of GaN LEDs with emerging technologies like the Internet of Things (IoT) and smart lighting systems will open new avenues for applications.

In summary, GaN has become the material of choice for high-performance LEDs, driven by its versatile bandgap and robust material properties. While challenges like efficiency droop and color tuning persist, ongoing research continues to push the boundaries of what is possible. From energy-efficient lighting to cutting-edge displays and UV applications, GaN LEDs are poised to remain at the forefront of optoelectronic innovation.