© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Phys. Status Solidi RRL 3, No. 9, 284 – 286 (2009) / DOI 10.1002/pssr.200903267 www.pss-rapid.com pss Color tunable light-emitting diodes with modified pulse-width modulation Jaehee Cho 1 , Qifeng Shan 1 , Jong Kyu Kim 2 , and E. Fred Schubert *, 1 1 Department of Physics, Applied Physics, and Astronomy; Department of Electrical, Computer, and System Engineering; and Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York 12180, USA 2 Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea Received 18 August 2009, revised 8 September 2009, accepted 9 September 2009 Published online 11 September 2009 PACS 07.60.Dq, 81.05.Ea, 84.37.+q, 85.60.Jb * Corresponding author: e-mail efschubert@rpi.edu, Phone: +1 518 276 8775 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Color tunable and efficient light- emitting diodes (LEDs) are attractive for numerous illu- mination applications [1]. However, it is difficult to simul- taneously achieve both of these qualities, that is, color tunability and high efficiency, with current technologies. Current approaches have relied on vertically stacked GaInN quantum wells (with different quantum well thick- ness or In content) sandwiched in a GaN-based p – n junc- tion diode [2] or laterally distributed GaInN quantum wells (with different quantum well thickness) grown on different facets of GaN [3]. However, because the relative amounts of current that flow through each quantum well determine the final color coordinates of the device, it is difficult to achieve the desired real-time controllability of the chroma- ticity. As an alternative approach, two different colored LEDs with a current-distribution-control unit can be used to achieve color tunability [4]. However, in order to dis- tribute the current between the two LEDs and render the desired coordinates, more than two electrical terminals are necessary with conventional approaches. In addition, add- ing electrical terminals to a lighting system complicates the circuitry and the fabrication process. Pulse width modulation (PWM) is widely used to regu- late the intensity of light sources in applications such as dimmable backlighting units for liquid crystal displays [5]. Whereas conventional PWM [6, 7] generally switches be- tween zero and a positive voltage, the modified PWM [8] is switched between a negative voltage and a positive volt- age, as shown in Fig. 1. By using the modified PWM at high frequency, an efficiency improvement of GaInN LED was reported [8]. The authors hypothesized that the effi- ciency improvement was due to enhanced redistribution of electron – hole pairs by oscillation of the negative bias. Though the reverse bias induced alternately might influ- ence the device reliability, the modified PWM can provide additional functionality to operate LEDs because many pa- rameters such as positive and negative voltages, frequency, pulse width, and the duty ratio can be independently con- trolled, which is not possible with typical continuous wave (cw) LED operation. In this study, a color tunable LED sys- tem is developed by using two LEDs emitting at two dif- ferent wavelengths. Color tunability can be realized by changing the typical cw operation of LEDs to the modified PWM operation and by utilizing only two electrical termi- In this study, a color tunable light source, operated by a modi- fied pulse width modulation method, is investigated. By util- izing this method along with anti-parallel connected discrete light-emitting diodes (LEDs) and two electrical terminals, a wide range of the chromaticity coordinates is attained and va- ried by electrical control. Using the combination of a blue LED and a phosphor-converted yellow LED (blue LED plus yellow phosphor), the chromaticity range is varied by electri- cal control from pure blue to pure yellow. In addition, using the modified pulse-width modulation method and a combina- tion of white and red LEDs, white light with correlated color temperatures ranging from 5000 K to 2000 K is demon- strated.