Efficiency and Droop Improvement in Hybrid Warm White LEDs Using InGaN and AlGaInP High-Voltage LEDs

Kuo-Ju Chen, Hsuan-Ting Kuo, Yen-Chih Chiang, Hsin-Chu Chen, Chao-Hsun Wang,

Min-Hsiung Shih, Member, IEEE, Chien-Chung Lin, Member, IEEE, Ching-Jen Pan, and

Hao-Chung Kuo, Senior Member, IEEE

Abstract—This study investigates the optical and electrical

char-acteristics in hybrid warm white high-voltage light-emitting diodes

(HV-LEDs). The luminous efficiency of the hybrid warm white

LED in this study improved by 11% and 51%, compared to

conven-tional cool and warm LEDs, respectively, solving the warm white

gap in white LEDs. The efficiency droop of the hybrid warm white

LED was reduced to 21.8% from 26.8% for the conventional cool

white LED, and from 26.3% in the conventional warm white LED

at 40 mA (35 A/cm ) the operated current. Furthermore, the color

rendering index (CRI) and angular correlated color temperature

(CCT) were analyzed, indicating a significant improvement in

hy-brid warm white HV-LEDs.

Index Terms—High voltage, light-emitting diodes (LEDs),

phosphor, warm white.

I. I

W

HITE light-emitting diodes (WLEDs) are regarded as

the next generation of environmentally friendly lighting

sources, and are particularly useful in solid-state lighting (SSL)

[1]–[3]. Therefore, because they are an energy-saving lighting

source, it is imperative to increase the quantum or the lumen

ef-ficiency in WLEDs. As mentioned in our previous study,

high-voltage LEDs (HV-LEDs) achieve higher lumen efficiency by

embedding multiple series-connected micro-diodes in one large

chip [4]. The chief advantage of HV-LEDs is that they focus on

the reduction of the operation driving current, which efficiently

eases current crowding and the efficiency droop, compared to

conventional large chip DC-LEDs with the same amount of

power. Furthermore, the high voltage operation can also reduce

the voltage conversion losses from wall plug to SSL devices.

Manuscript received October 01, 2012; revised October 08, 2012; accepted October 08, 2012. Date of publication March 11, 2013; date of current ver-sion March 15, 2013. This work was supported in part by the National Sci-ence Council in Taiwan under Grant NSC 101-3113-E-009-002-CC2 and by NSC-99-2120-M-009-007.

K. J. Chen, H. T. Kuo, H. C. Chen, C. H. Wang, and H. C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: hckuo@faculty.nctu. edu.tw).

M. H. Shih is with the Department of Photonics and Institute of Electro-Op-tical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, and also with the Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan (e-mail: mhshih@gate.sinica.edu.tw).

Y. C. Chiang and C. C. Lin are with the Institute of Photonics System, Na-tional Chiao Tung University, Tainan 711, Taiwan.

C. J. Pan is Helio Photonics Cooperation, Hsinchu 30010, Taiwan. Color versions of one or more of the figures are available online at http:// ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JDT.2012.2227054

To date, phosphor-converted WLEDs have been the critical

technology used in SSL devices. SSLs generally comprise two

types, such as cool and warm WLEDs, which are combined

with blue chips and various phosphor materials [5], [6]. For

cool WLEDs, yellow phosphor, such as Y A O

Ce , is

commonly used. Conversely, regarding warm WLEDs, the

ad-dition of red phosphor, such as nitride-based phosphor, is

re-quired to obtain warm white. However, the differences in

lumi-nous efficiencies of cool WLEDs and warm WLEDs are a

se-rious issue, and are known as the warm white efficiency gap [7].

This could be attributed to several reasons: First, red phosphor

has lower conversion efficiency because of larger stokes shift

losses. Second, the reabsorption phenomenon in yellow light in

the mixture of yellow and red phosphor used in warm WLEDs

causes the cascade excitation process [8]. Furthermore, the

dif-ference in luminous efficiency is approximately 25%–40%

be-tween cool WLEDs and warm WLEDs [7]. Therefore, how to

improve the efficiency gap between two types of WLEDs

be-comes the urgent issue in SSL.

In this study, the hybrid warm white HV-LEDs—including

blue chips, red chips, and yellow phosphor—were shown to

theoretically and experimentally improve the efficiency gaps in

WLEDs. In addition, the efficiency droop was significantly

im-proved by the hybrid warm white HV-LED package.

Further-more, the CRI value achieved 90 in warm white HV-LED LEDs.

II. D

F

In the experiment, the LEDs were grown on c-plane

sap-phire substrate by metal-organic chemical vapor deposition

(MOCVD) system including n-GaN layer, an In Ga

N/GaN

multiple-quantum wells, a p-AlGaN electron blocking layer,

and a p-GaN layer. After that, the inductively coupled plasma

(ICP) etcher method was used to form the isolation trenches

between microchips. To prevent short circuit between each

microchip, the passivation SiO

layer was deposited by

plasma-enhanced chemical vapor deposition (PECVD).

Fi-nally, the bridged Cr/Au were simultaneously evaporated by

e-beam evaporator to serve as cathodes. Each chip process

were referred as [9]. Fig. 1 shows the schematic diagram of

the hybrid warm white LED, conventional cool white LED,

and conventional warm white LED. The hybrid warm white

LED was prepared with four 45 mil 45 mil InGaN HV-LEDs

with domain wavelengths of 452.5 nm, two 50 mil

25 mil

AlGaInP HV-LEDs with domain wavelengths of 617 nm, and

yellow YAG phosphor. The forward voltages of InGaN and

Fig. 1. Schematic diagram of: (a) the hybrid warm white LED, (b) conventional cool white LED, and (c) conventional warm white LED.

Fig. 2. EL spectra of the hybrid warm white LED, conventional cool white LED, and conventional warm white LED.

AlGaInP HV-LEDs under a 40 mA driving current were 53

and 21 V, respectively. For the reference, the samples were

prepared with six 45 mil 45 mil InGaN HV-LEDs, and the

phosphor component of the cool white and warm white LEDs

were yellow YAG phosphor and a mixture of yellow YAG

phosphor and red Nitride phosphor with wavelength of 610 nm,

separately. It is worth to notice the cost of this hybrid white

LED. In the hybrid white LED, we used the two red AlGaInP

HV-LEDs to replace the two blue InGaN HV-LEDs. Since

the price of a red AlGaInP LED is close to the price of a blue

InGaN LED, the cost of the hybrid white LED is comparable

to the conventional cool/warm white LEDs.

The electroluminescence (EL) spectra are shown in Fig. 2,

and were measured at a forward current of 40 mA

(approxi-mately 35 A/cm ) for the hybrid warm LED, the conventional

cool white LED, and the conventional warm white system.

Fig. 2(a) shows the EL spectrum of the hybrid warm LED,

including the blue LED, red LED, and yellow phosphor

Y Al O

Ce

. The full width at half maximum (FWHM)

of the blue LED and red LED were 21 and 25 nm, respectively.

The luminous efficiency and CRI in the hybrid warm white

LED were approximately 130 lm/W and 90 under 3000 K,

respectively. As shown in Fig. 2(b), the emission peak

wave-length of the yellow phosphor Y Al O

Ce

occurred

at 550 nm with a FWHM of 121 nm in the conventional cool

white LED. Fig. 2(c) was composed of blue, yellow, and red

emission bands located at 452.5 nm, 540 nm, and 600 nm,

respectively, which were attributed to the blue LED, yellow

phosphor, and red phosphor white LED achieved a luminous

efficiency 118 lm/W and CRI 70 under 6000 K, whereas the

Fig. 3. Lumen efficiency of hybrid warm white LED, conventional cool white LED and conventional warm white LED.

conventional warm white LED ranks were 86 lm/W and CRI

80 under 3000 K.

III. R

D

Fig. 3 shows the luminous efficiency of the conventional cool

and warm white and the proposed hybrid warm white LED as

a function of a pulsed current at room temperature. There were

significant efficiency differences among the conventional cool

and warm white LEDs, which were calculated as being

approx-imately 27%. The main reason for the large gap was the warm

white gap, as mentioned. At this point, the luminous efficiency

of the warm white LED increased from approximately 11% and

51% compared with the conventional cool and conventional

warm LED.

For an explanation for the improvement in the hybrid warm

LED, the chromaticity coordinates are shown in Fig. 4. Points

A and C represent the cool white light with 6000 K on (0.324,

0.335) and the warm white light with 3000 K on (0.439, 0.409),

respectively. To achieve the warm white light on Point C, red

phosphor is generally added to the cool white light LED on

Point A, which is called the conventional method. However,

red phosphor suffers from the larger Stokes shift losses and the

reabsorption of yellow light by the red phosphor, resulting in

reduced luminous efficiency. Therefore, to solve this problem,

this study demonstrated a different method to achieve

high-effi-ciency warm white light. The proposed method comprises two

steps for creating warm white light: 1) more yellow phosphor

was added into the cool white light on Point A to create the

yel-lowish white light on Point B with 5000 K, resulting in an

ap-proximate 10% enhancement of luminous efficiency, compared

with Point A, and 2) the red chip with a domain wavelength of

617 nm on (0.627, 0.327) was mixed with the former yellowish

white light on Point B to obtain the warm white light on Point

C. Incorporating the red chip to replace the red phosphor solved

the warm white gap problem and increased the luminous

effi-ciency, compared with the reference.

Furthermore, the normalized luminous efficiency droop was

investigated in three types of samples, as shown in Fig. 5. The

efficiency droop of the hybrid warm white LED was improved

from 26.8% in conventional cool white and 26.3% in

conven-tional warm white to 21.8%. These improvements in efficiency

Fig. 4. Comparison of the conventional and proposed method to fabricate the warm white.

Fig. 5. Normalized luminous efficiency of hybrid warm white LED, conven-tional cool white LED, and convenconven-tional warm white LED.

were attributed to the additional AlGaInP HV-LEDs. Some

studies have indicated that the major reason for the efficiency

drop in III-V nitrides has been caused by the carrier overflow

[10], as well as the Auger scattering [11] and charge separation

issues in leading to reduction in efficiency in InGaN QW

LEDs. The LEDs with novel barrier designs had been studied

for efficiency-droop suppression [12], [13], and novel active

region with optimized optical matrix elements had also been

used to suppress charge separation in InGaN QWs [14]–[16].

For the AlGaInP heterostructures, the lower piezoelectric

po-larization electric fields caused the alleviation of the efficiency

droop [17]. Therefore, in this experiment, when the AlGaInP

HV-LED was employed to the InGaN HV-LED, the normalized

efficiency droop improved as the reference. The warm white

LED not only raised the luminous efficiency, but also reduced

the efficiency droop, compared with the conversional warm

and cool LEDs.

In addition to luminosity, the CRI is a quantitative measure of

the ability of a light source to faithfully reproduce the colors of

various objects compared to an ideal or natural light source, and

Fig. 6. CRI of hybrid warm white LED, conventional cool white LED, and conventional warm white LED analyzed at various Munsell codes.

Fig. 7. CCT deviation of the hybrid warm white LED, conventional cool white LED, and conventional warm white LED.

is important for general lighting [18]. As shown in Fig. 6, R1 to

R8 are the color rendering properties of the light source tested

by eight test color samples (TCSs) distributed over the complete

range of hues. Results indicated that the proposed hybrid warm

white achieved the highest value among the eight TCSs.

Espe-cially, the best CRI value was observed from R1, which was

tested by the light grayish red sample. Furthermore, the general

CRI Ra represents the average value among these eight TCSs,

and the highest Ra ranked at 90 was achieved by the proposed

hybrid warm white LED.

To analyze the light quality of the WLEDs, we measured

an-gular correlated color temperature (CCT) deviation for the three

types of LEDs. The angular CCT deviation curves from 70 to

70 deg are shown in Fig. 7.The CCT deviations were 1387 K,

164 K, and 100 K for the conventional cool white, warm white,

and the hybrid warm white LEDs, respectively. Moreover, a

su-perior uniformity of angular-dependent CCT was obtained in

the hybrid warm white LED, compared with the reference. This

could be attributed not only to the excellent light quality in the

warm white region, but also to the proper arrangement of the

chips in the package. The red LED chips were bonded in the

middle of the package, helping lower the CCT deviation caused

by the Lambertian emission of blue LEDs and the isotropic

emission of phosphor.

IV. C

In conclusion, hybrid warm white HV-LEDs were

investi-gated—including InGaN HV-LEDs, AlGaInP HV-LEDs, and

yellow YAG phosphor. The results indicated that the luminous

efficiency of the hybrid warm white HV-LEDs increased 11%

and 51% at 40 mA (35 A/cm ), compared with the conventional

cool white and warm white LEDs, respectively. In addition, the

efficiency droop of the hybrid warm white LEDs improved from

26.8% in conventional cool white and 26.3% in conventional

warm white to 21.8%. Replacing red phosphor with red LEDs

was the chief reason for the improvements. Furthermore, the

CRI was ranked at 90, and the uniform angular CCT within a

100 K deviation was achieved by the hybrid warm white LED.

A

The authors would like to thank Helio Optoelectronics

Cor-poration, Zhubei City, Hsinchu county, Taiwan.

R

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Kuo-Ju Chen was born in Taichung, Taiwan, R.O.C. He received the B.S

degree in industry education from National Kaohsiung Normal University (NKNU), Kaohsiung, Taiwan, in 2008, and the M.S. degree from National Taiwan Normal University (NTNU). He is currently working toward the Ph.D. degree from the National Chiao-Tung University, Hsinchu, Taiwan.

His study is focus on UV excitable phosphate which has high luminous ef-ficiency and high stability. He used the combinatorial chemistry to develop UV-excited phosphor for his master thesis. His Ph.D. research includes fab-rication, simulation, and characterization for high power light-emitted diodes.

Hsuan-Ting Kuo received the B.S. and M.S. degrees in Department of

Pho-tonics from the National Chiao-Tung University, Hsinchu, Taiwan, in 2011 and 2012, respectively. His Master’s research was focused on high voltage white light emitted diodes including fabrication, simulation, measurement, and op-tical characteristic study.

Yen-Chih Chiang was born in Taichung, Taiwan, R.O.C. He received the

M.S. degree from National Chung Hsing University (NCHU), and is currently working toward the Ph.D. degree at the National Chiao-Tung University, Hsinchu, Taiwan. His study is focused on high power & high luminous effi-ciency light-emitting devices (LEDs) for SSL. He used the textured surface & geometry shape to improve the luminous efficiency in AlGaInP red lgiht LED for his master’s thesis. His Ph.D. research includes fabrication, simulation, and characterization for high power light emitted diodes.

Hsin-Chu Chen is currently working toward the Ph.D. degree from Institute

of Electro-Optical Engineering, National Chiao Tung University, Taiwan. My study is focus on quantum dot and nanostructure of solar cells, which can im-prove light harvest and enhance power conversion efficiency. The Ph.D. re-search includes fabrication, simulation, and measurement.

Chao-Hsun Wang received the B.S. and M.S. degrees in Department of

Pho-tonics from the National Chiao-Tung University, Hsinchu, Taiwan, in 2008 and 2009, respectively. He is currently working toward the Ph.D. degree in the De-partment of Photonics, National Chiao-Tung University. His current research interests include the efficiency droop behavior in GaN-based LEDs and high lumen efficiency white LEDs.

integrated photonic circuits, photonic crystals, GaN based lasers, surface plas-monics, and cavity quantum electrodynamics.

Chien-Chung Lin (S’93–M’02) was born in Taipei, Taiwan, R.O.C., in 1970.

He received the B.S. degree in electrical engineering from the National Taiwan University in 1993, and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1997 and 2002, respectively. His thesis work focused on design, modeling, and fabrication of micromachined tunable optoelectronic devices.

Since 2009, he has been with National Chiao-Tung University (NCTU), Tainan, Taiwan, where he holds a position as an assistant professor. The major research efforts in his group are in design and fabrication of semiconductor optoelectronic devices, including LEDs, solar cells, and lasers. Before joining NCTU, he worked for different start-ups in the United States. In 2002, he joined E2O Communications, Inc., Calabasas, CA, as a senior optoelectronic engineer. His main research interests then were in optically and electrically pumped long-wavelength vertical cavity surface emitting lasers. In 2004, he joined Santur Corporation, Fremont, CA, where he initially worked as a member of technical staff then became Manager of Laser Chip Engineering later. He had worked on various projects such as monolithic multi-wavelength DFB Laser arrays for data and telecommunications applications, yield and reliability analysis of DFB Laser arrays, etc. He has more than 30 journal and conference publications.

Dr. Lin is a member of IEEE Photonics Society and IEEE Electron Devices Society.

from National Taiwan University, Taiwan, R.O.C., the M.S. degree in electrical and computer engineering from Rutgers University—The State University of New Jersey, New Brunswick, in 1995, and the Ph.D. degree from Electrical and Computer Engineering Department, University of Illinois at Urbana Champaign, in 1999.

He has an extensive professional career both in research and industrial research institutions that includes: Research Assistant in Lucent Technologies, Bell Laboratories (1993–1995); and a Senior R&D Engineer in Fiber-Optics Division at Agilent Technologies (1999–2001) and LuxNet Corporation (2001–2002). Since October 2002, he has been with the National Chiao Tung University as a Faculty Member of the Institute of Electro-Optical Engineering. He is now the Associate Dean, Office of International Affair, NCTU. His current research interests include semiconductor lasers, VCSELs, blue and UV LED lasers, quantum-confined optoelectronic structures, optoelectronic materials, and Solar cell. He has authored and coauthored 300 internal journal papers, 2 invited book chapter, 6 granted and 12 pending patents.

Prof. Kuo is an Associate Editor of IEEE/OSA JOURNAL OFLIGHTWAVE

TECHNOLOGY and als for the IEEE JOURNAL OF SELECTED TOPICS IN

QUANTUMELECTRONICS(JSTQE) special issue on Solid State Lighting (2009).

He received Ta-You Wu Young Scholar Award from National Science Council Taiwan in 2007 and Young Photonics researcher award from OSA/SPIE Taipei chapter in 2007. He was elected as OSA fellow in 2012.