Heterogeneous Integration of Monolithic LED-PD With Circuitry for Intensity Stabilization

The monolithic integration of a photodiode (PD) to a light-emitting diode (LED) enables the on-chip monitoring of light output intensity from the emitter. The light output intensity from the LED, which would fluctuate and degrade over time, can be stabilized with the aid of a driver circuit that reads the photocurrent from the on-chip PD as a feedback signal. Previous demonstration of such systems, implemented with feedback driver circuits built with microcontroller boards or analog circuit assembled on printed circuit boards, showed that the intensity can be stabilized to within 0.03%, albeit with dimensions disproportionate to the LED-PD chip itself. A simplified circuit consisting of a transimpedance amplifier, proportional-integral controller, and a low-dropout regulator is heterogeneously integrated with the LED-PD as a system-in-package, making use of bare dies for the majority of components. Using this approach, the entire system is shrunk to a dimension of 4 mm × 5 mm, which is comparable to the size of the LED-PD chip. The reduced footprint facilitate practical applications of the LED-PD devices for intensity-stabilized lighting or displays. The stability is further improved to 0.01% on average over 1-h periods based on the on-chip PD photocurrent, while the power efficiency is improved to over 70%.