Intensity-Stabilized LEDs With Monolithically Integrated Photodetectors

To overcome light output degradations and fluctuations of intensities from light-emitting diodes (LEDs) over time, the monolithic integration of InGaN LEDs and photodetectors (PDs) is demonstrated in this paper. The InGaN/GaN multiquantum wells (MQWs) play the role of light emission and detection from the LED and PD, respectively. Despite the larger bandgap energies of the InGaN layers, the MQWs absorb light emitted by the LED due to the band tail effect, extending the absorption range up to 460 nm, which correspond to the peak wavelength of emission. The tiny-sized PD detects light from the adjacent LED coupled through the sapphire substrate to generate a photocurrent that is proportional to its light output, but remains unresponsive to ambient lighting. Apart from real-time light output monitoring, the photocurrent can be used as a feedback signal for regulation of light output. A microcontroller-based feedback circuit has been implemented to drive the LED and the photocurrent level is maintained to a preset value by adjustment of the driving current. Using this scheme, light output from the LED is stabilized to within ∼0.2% over 1-h periods.