Light-emitting diodes for visible light communication

Conventional LEDs always pursue the high brightness for solid-state lighting, however, they always exhibit very low frequency bandwidth of tens MHz. As an application, visible light communication (VLC) is not only for illumination but also for data transmission communication, which is a kind of optical wireless communication that uses the visible white ray as the medium. In this study, we investigate the fabrication and characterization of high-modulation frequency GaN-based light-emitting diodes. The frequency response of LEDs is mainly limited by its diffusion capacitance and resistance, and the injected carriers in the active region of the device. By appropriate device design, gallium-doped ZnO (GZO) film deposited by atomic layer deposition (ALD) is used as the top contact layer with high lateral resistance to self-confine the current injection. In addition, a smaller bonding pad is used to reduce the RC time constant. By the ways, the blue GaN-based LEDs exhibit a 3-dB modulation bandwidth of 225.4 MHz and a light output power of 1.6 mW at the current of 35 mA. Also, the green GaN-based LEDs at 50 mA exhibit the highest 3-dB frequency bandwidth of 463 MHz and a relatively high light output power of 1.6 mW. The 3-dB modulation bandwidth increases with increasing drive current and decreases with increasing temperature. The highest bit rate of 4.4 Gbit/s is achieved at 1 m plastic fiber using nonreturn-to-zero modulation schemes. Such the LEDs can be applied to visible light communication in the future.