Investigation on Modulation Speed of Photon-Recycling White Light-Emitting Diodes with Vertical-Conduction Structure

The speed performance of a high lumen and high color rendering index (CRI) gallium nitride-based white-light light-emitting diode (LED) has been investigated in detail by measuring its electrical-to-optical frequency responses with different color filters at the receiver end. The ultra-violet/blue/green multiple quantum wells (MQWs) are stacked in the studied LED chips, which have a vertical conducting structure and red phosphor coating for generating white light with high CRI. The typical studied LED exhibits a smaller 3 dB bandwidth (0.6 MHz versus 2 MHz) compared with a reference LED with ultra-violet/blue MQWs and a yellow phosphor coating. This finding suggests that the fluorescent decay time of red phosphor is much longer than that of yellow ones (approximately 270 ns versus approximately 80 ns). In addition, the extracted carrier lifetime inside blue/green In_xGa_1-xN/GaN MQWs under UV light pumping/modulation is as long as approximately 50 ns, which is much slower than that of forward-biased current modulation (approximately 50 ns versus <10 ns). A bandpass UV filter is installed in the receiver side to exclude these slow fluorescent times in MQWs and phosphor. A 17 MHz 3 dB bandwidth and a clear eye opening at 40 Mbit/sec with an on-off keying modulation scheme are demonstrated under a moderate operation bias current (approximately 0.75 A) of studied LEDs.