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High Brightness GaN VerticalLight-Emitting Diodes onMetal Alloy for GeneralLighting ApplicationFlexible chip size, capability for high driving-current and excellent heat dissipation,
high-performance device mounting and packaging technology can meet the
requirements of this application.
By Chen-Fu Chu, Chao-Chen Cheng, Wen-Huan Liu, Jiunn-Yi Chu, Feng-Hsu Fan,
Hao-Chun Cheng, Trung Doan, and Chuong Anh Tran
ABSTRACT | In this paper, we show the many advantages of the
GaN-based vertical light-emitting diodes (VLEDs) on metal alloy
over conventional LEDs in terms of: better current spreading,
vertical current path for low operation voltage, better light
extraction, flexible chip size scaling, higher driving current
density, faster heat dissipation, and good reliability. The GaN
VLED on metal alloy exhibits very good current–voltage
behavior with low operated voltage and low serial dynamic
resistance. The low operation junction temperature of GaN
VLED on metal alloy demonstrates excellent heat dissipation
capabilities. Chip size scaling without efficiency loss shows a
unique property of GaN VLED on metal alloy. The GaN VLED on
metal alloy also enables top surface engineering for efficient
light extraction to further light output. A high-power white LED
having efficiency of 120 lumen/W was achieved through a
combination of reflector, surface engineering, and optimiza-
tion of the n-GaN layer thickness. Coupled with good reliability
and mass production ability, the GaN VLED on metal alloy is
very suitable for general lighting application.
KEYWORDS | Driving current density; GaN-based vertical light-
emitting diodes on metal alloy; general lighting application;
heat dissipation; light extraction; operation voltage; power
efficiency; reliability; size scaling
I . INTRODUCTION
GaN-based wide-bandgap semiconductors are widely used
in various applications, such as the mobile phone handset
keypad, LCD backlighting, camera flash, and full-color
outdoor display. Recently, the most popular application
using high-brightness and high-power LEDs is solid-state
general lighting. However, the high-power LED has its
limitation due to technology problems associated with
conventional LEDs on dissimilar base. In order to fabricatehigh-brightness power LEDs for solid-state lighting appli-
cations, there are several serious factors that should be
considered. First, the heat should be managed efficiently.
In general, the efficacy and lifetime drop rapidly as the
junction temperature (Tj) rises. The light output power
efficiency drops by at least 5% as the junction temperature
increases by 20 �C. Secondly, light output power efficiency
of 100 lumens per watt is needed to replace the con-ventional lighting and save energy. Thirdly, chip size
enlargement is inevitable in order to provide enough total
lumens. Fourthly, reliability of 20 000 hours’ lifetime
under continuous operation for the LED in general lighting
is needed to save the maintenance cost. Lastly, the unit
price per chip must be reduced to achieve a reasonable
price for customers.
Manuscript received April 24, 2009; revised November 4, 2009; accepted
November 5, 2009. Date of publication May 3, 2010; date of current version
June 18, 2010.
C.-F. Chu, C.-C. Cheng, W.-H. Liu, J.-Y. Chu, F.-H. Fan, and H.-C. Cheng are with
SemiLEDs Optoelectronics Co., Ltd., Hsinchu, Taiwan (e-mail: [email protected]).
T. Doan and C. A. Tran are with SemiLEDs Corp., Boise, ID 83702 USA.
Digital Object Identifier: 10.1109/JPROC.2009.2037026
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 11970018-9219/$26.00 �2010 IEEE
II . CONVENTIONAL GaN LED
A. Sapphire SubstrateSemiconductors GaN, AlN, and InN have been grown
primarily on sapphire, most commonly the (0001) ori-
entation but also on the a- and r- planes [1]. In addition, group
III–V nitrides have been grown on SiC, Si, and GaAs. In the
early 1990s, many researchers explored the growth of GaN on
GaAs bases to obtain the metastable zincblende phase of GaN[2]–[4]. Other bases, which were used in order to achieve the
zincblende GaN phase, include Si [5], [6] and SiC [7], [8].
With the exception of SiC, the interest in using these bases
has slowly decreased. The primary reason for the decline in
interest in the zincblende bases is the inherent difficulty in
growing high-quality GaN in the cubic phase. In addition,
the high growth temperature involved precludes the use
of materials with low decomposition temperatures, such asGaAs. Due to the reactivity of nitrogen with Si, amorphous
Si3N4 layers typically form before the GaN deposition,
preventing high-quality GaN films on Si [9], [10] bases.
Therefore, the SiC materials system is challenging the
GaN/sapphire system for dominance in both the opto-
electronic and electronic arena [11]. SiC offers a higher
electrical and thermal conductivity compared to sapphire
and is available in the hexagonal crystal structure. Despitethese advantages, SiC suffers from being substantially more
expensive compared to sapphire and Si. The prohibitive
cost of using SiC has limited its usefulness and availability
to only a small number of groups.
Other suitable base materials have only recently become
commercially available. Almost all the group III–V nitride
semiconductors have been grown on sapphire, despite its
poor structural and thermal match to the nitrides. The pre-ference for sapphire substrates can be ascribed to its wide
availability, hexagonal symmetry, low cost, 8-in-diameter
crystals of good quality,1 transparent nature, and its ease of
handling and pregrowth cleaning. Sapphire is also suitable at
high temperatures ð�1000 �CÞ required for epitaxial growth
using the various chemical vapor deposition (CVD) tech-
niques commonly employed for GaN growth.
However, due to the nonconductivity (electrical resis-tivity ¼ 1011 � 1016 �-cm) and low thermal conductivity
(35 W/m-K) of sapphire substrates, the device process steps
are relatively complicated compared to other compound
semiconductor devices. For devices processed on sapphire
substrates, all contacts must be made from the topside. This
configuration complicates contact and packaging schemes,
resulting in a spreading-resistance penalty and increased
operating voltages [12]. The heat dissipation of sapphiresubstrate was poor; therefore the conventional GaN blue
LEDs on sapphire was typically operated under low current
operation conditions. Therefore, GaN optoelectronics de-
vices fabricated on electrically and high thermally conduct-
ing bases are most desirable. Fig. 1 shows the thermal
conductivity of different bases used today; the vertical LED
on metal alloy has better thermal conductivity than any other
bases such as sapphire, Si, Ge, SiC, and GaN. The betterthermal conductivity suggests heat can be dissipated faster.
B. Configuration of ConventionalGaN LED on Sapphire
1) P-Side Up Configuration: Fig. 2 shows structure
diagrams of the conventional GaN LEDs on sapphire. For
the conventional GaN LEDs on sapphire, the p- and n-
electrode pads are located on the same side as the sapphire,
an insulator. Thus, the total emission area of conventional
GaN LEDs on sapphire is reduced due to the removal ofp-GaN and active layer to expose the n-GaN for n-pad.
2) Current Spreading and Current Path: Fig. 3 shows the
current path of conventional LEDs on sapphire. The current
spreading in conventional LEDs on sapphire from anode to
cathode is laterally along the n-GaN layer. Therefore,
current crowding effects may occur underneath the n-pad,
resulting in higher serial dynamic resistance and higheroperation voltage. Another known issue of the conventional
GaN LEDs on sapphire is the lack of current spreading on
the p-GaN layer due to low conductivity of p-type GaN.
1http://compoundsemiconductor.net/blog/2008/09.
Fig. 1. The thermal conductivity of different base used today.
Fig. 2. The structure diagrams of the conventional GaN LEDs on
sapphire.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
1198 Proceedings of the IEEE | Vol. 98, No. 7, July 2010
Therefore, a semitransparent contact layer (STCL) [13] or
interdigitated designs of electrode [14] on p-GaN for the
conventional GaN LEDs were necessary to spread thecurrent and increase the light output efficiency. Commer-
cially, the specific contact resistance between the STCL and
p-type GaN was typically only about 10�2–10�4 �� cm2.
3) Light Extraction: Due to the absorption of the STCL
[15], the light output power of these conventional LEDs on
sapphire was reduced. To increase the light extraction
efficiency of the conventional LED on sapphire, truncatedhexagonal pyramids or pits were formed on the p-GaN
surface using various techniques such as epitaxy growth [16],
dry etching [17], or wet etching [18]. The drawbacks of the
conventional structure for an effective light extraction lay in
the thickness of the p-GaN layer, which must be thin enough
to avoid reabsorption of photons. In other words, any
engineering of p-GaN surface may affect the active layer
underneath leading to compromised device characteristics.Therefore, the surface engineering on the top of thin p-GaN
(1000–5000 �A) does not leave enough room to engineer.
The second drawback of conventional LEDs on sapphire is
that it lacks a mirror on the backside to reflect photons back
to the rough surface for a better light extraction.
4) Chip Size Scaling: Fig. 4(a) shows the scheme of the
physical geometry emission light direction of the conven-tional LED on sapphire. The GaN/sapphire interface is
transparent. The light generated form the active layer isradiation. Part of the light escapes through the p-GaN to the
air, and the remaining part is discharge from GaN through
the sapphire. The lights were then transmitted from the
four sides and the bottom of the sapphire. Commercially,
the escaped light would then collected by packaging.
However, it is hard to have the surface engineering on the
surface of sapphire due to its chemically stable properties.
Much light could be trapped inside the sapphire because ofthe total internal reflection effect between the sapphire and
air. Once the chip size is enlarged as shown in Fig. 4(b), the
light could be trapped more easily inside the sapphire than
the small chip size. Therefore, the conventional LED on
sapphire has its limitation in chip size scaling. For general
lighting application, the chip size must be scaled up to
obtain enough light for lighting.
5) Heat Analysis: As discussed above, it is tough to form a
good ohmic contact between STCL and p-GaN, and the
current crowding effect that may occur on the bottom of
the n-pad. The majority of heat could be generated and
accumulated from the STCL/p-GaN interface, active layers,
and current crowding area; this generated heat needs to be
dissipated through�5 �m n-GaN layer and�100 �m thick
sapphire. Sapphire’s thickness and its poor thermalconductivity (35 W/mK) make it difficult to dissipate the
generated heat. As a result, the conventional GaN LEDs on
sapphire typically are operated under low current density
conditions in order to prevent thermal problems. The
application of conventional GaN LEDs on sapphire is limi-
ted, especially for the high power LEDs used for solid state
lighting applications.
III . CONFIGURATION OF FLIPCHIP GaN LED
For the flip chip LED, as shown in Fig. 5, the removal of
p-GaN and active layer to expose the n-GaN for n-pad isnecessary, thus the emitting area was reduced. Similarly,
the current transport in flip chip LED from anode to
cathode is lateral along the n-GaN layer. The current
crowding effect generates higher serial dynamic resistance.
Fig. 3. The current path structure diagram of the conventional LEDs
on sapphire.
Fig. 4. (a) The scheme of the physical geometry emission light
direction of the conventional LED on sapphire. (b) The light could be
confined inside the sapphire more than small chip size. Fig. 5. The scheme of flip chip LED structure.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 1199
Furthermore, the heat dissipation is comparatively worsethan the vertical structure on metal due to higher thermal
resistance of the solder to the sub-mount. In addition, the
top surface of LED flip chip is sapphire which is very hard to
be engineered for better light extraction through patterning
or texturing. In addition, the combination of thin-film LED
concept with flip-chip technology is reported [19] to
provide surface brightness and flux output advantages over
conventional flip-chip with sapphire on top as Fig. 5configuration. However, the same side p- and n- electrode
has the similar lateral current transportation; it could
generate the high serial dynamic resistance under high
current density operation.
IV. CONFIGURATION OF VERTICALGaN LED ON METAL
1) N-Side Up Configuration: Fig. 6 shows the structure
scheme of the GaN vertical LED on metal. For the GaN
vertical LED on metal, the single n- electrode pad was directly
made on the top of n-GaN. The active layer remained intact
and could emit more light in comparison to the conventionalGaN LEDs on sapphire with the same chip size [20].
2) Current Spreading and Path: The direction of current
path of the GaN vertical LED is vertically from the bottom
anode to the top cathode. Therefore, the vertical current
path [21] without current crowding effect has much lower
serial dynamic resistance than that of the lateral current
path. In addition, the n-GaN has much higher conductivitythan that of p-GaN. The n-GaN can spread the current well
without using any semitransparent conductive layer. Thus,
no light was absorbed by the semitransparent layer, and
higher light output efficiency could be obtained [19], [21].
The better current spreading of n-GaN can help scale up
the chip size without efficiency loss.
3) Light Extraction: To increase the light extraction, GaNVLED on metal was designed to boost the external quantum
efficiency while relieving the demand for a thin p-GaN layer.
To form the hexagonal pyramids on top of n-GaN, chemical
etching or photoenhance chemical etching were reported
[22]. All the surface engineering occurs on the thick, over4 �m, n-GaN layer. The top surface is roughened,
allowing more photons to escape from the surface.
The reflectivity of metal/GaN interface also plays an
important role to enhance the light extrication. The ref-
lectivity from the interface of metal/GaN interface can be
estimated by using the well-known formula 1-1 [23], [24]
for reflection of a wave perpendicularly incident from
media 1 onto the plane boundary of a solid with refractiveindex n. The ratio R of reflected-to-incident irradiance is
given by the Fresnel expression
R ¼ ðns � nmÞ2 þ k2m
ðns þ nmÞ2 þ k2m
(1-1)
where ns is the refractive index of semiconductor, nm is
the refractive index of metal, and km is extinction coef-
ficient of metal. Table 1 [25]–[28] gives the parameters
for different materials and GaN material at the wave-
length of 400, 460 nm. The calculated reflectivity (R) ofFig. 6. The structure diagrams of the GaN vertical LED on metal.
Table 1 The Parameters for Different Metals and GaN Material at the
Wavelength of 400, 460 nm. [25]–[28]
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
1200 Proceedings of the IEEE | Vol. 98, No. 7, July 2010
GaN/material interface was also listed in Table 1. For
the blue GaN LEDs, the emission wavelength is at about
460 nm. Fig. 7 shows the reflectivity of different metalsversus wavelength measured by depositing the metals on
sapphire. Both silver and aluminum show the higher
reflectivity for the standard blue emission range. To form
the high-reflectivity metal or its alloy as the reflector, more
light output power can be obtained.
4) Chip Size Scaling: Fig. 8(a) shows the scheme of the
physical geometry emission light direction of the GaN VLEDon metal. The GaN VLED on metal has a high reflectivity
metal as a mirror to reflect the light. Because the thickness of
GaN LED epitaxy layer structure is only about 4–6 �m, the
light from the active layer is directly reflected by the bottom
reflector. Almost all of the light is vertically emitted through
the top surface. Once the chip size is enlarged, as shown in
Fig. 8(b), the light emission behaves similarly as the small
chip size. Almost all of the light is emitted vertically throughthe top surface. Therefore, there is no limitation of GaN
VLED on metal in chip size scaling. For the general lighting
application, the emission area should be large enough to
provide enough lumens. The GaN VLED structure is a
suitable solution to achieve the requirement.
5) Heat Analysis: Fig. 9 shows the current path scheme of
the GaN VLED on metal. For GaN VLED on metal, the heat
could be generated from the active layers and the metal
contact to p-type GaN interface. Nevertheless, the thin p-type
GaN ð�0.2 �mÞ directly laid on the layers of high thermal
conductivity metal alloy material (400 W/mK) can dissipate
the heat more quickly. The current path in the GaN VLEDsstructure flows vertically from the bottom anode to the top
cathode. Therefore, the vertical current path without current
crowding effect has much lower serial dynamic resistance
than that of the lateral current path. Consequentially, faster
heat dissipation and higher current operation can be achieved.
The structure with GaN VLED on metal is most suitable for
high-power solid-state lighting application.
V. HIGH-BRIGHTNESS GaN VERTICALLIGHT-EMITTING DIODE ONMETAL ALLOY
A. StructureA schematic cross-section image of VLED on metal
alloy is shown in Fig. 10. The LED structure consists of a
mirror, directly deposited on metal alloy and acting as
anode and reflector, the 0.2-�m-thick p-GaN/p-AlGaN
layer, an InGaN/GaN multiple quantum wells active layer,and a 4-�m-thick n-GaN layer. Then to enhance the light
extraction, the n-GaN surface is patterned. In this
configuration, the current can pass from anode to cathode
vertically, avoiding the current crowding effect observed
with conventional GaN LED on sapphire. The photons
generated in the active layer can escape without passing
through any semitransparent conductive contact layer.
Meanwhile, the photons can be reflected by the mirror atvisible wavelength range to avoid the geometry limited
effect observed from the conventional configuration of
large GaN LEDs on sapphire.
B. Results and DiscussionFig. 11 shows the current–voltage (I–V) characteristics
for the GaN VLED on metal alloy and the conventional LEDs
Fig. 7. The reflectivity of different metals versus wavelength
measured by depositing the metals on sapphire.
Fig. 8. The scheme of the physical geometry emission light direction
of the GaN VLED on metal for (a) small chip size and
(b) large chip size is enlarged.
Fig. 9. The current path scheme of the GaN VLED on metal.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 1201
on sapphire. The operation voltage at 350 mA is about 3.4 V
for the conventional LEDs on sapphire and 3.0 V for the GaN
VLED on metal alloy. From the slope of I–V curve, theaverage dynamic series resistances of these two different
device configurations are 1.1 � for the conventional LEDs on
sapphire and 0.7 � for the GaN VLED on metal alloy. The
higher average dynamic serial resistance of the conventional
LEDs on sapphire could be due to the lateral current path
and the current crowded effect on the bottom of n-pad. The
lower operation voltage and average dynamic series
resistance of the GaN VLED on metal alloy has better lightoutput efficiency and operation performance compared with
the conventional GaN LEDs on sapphire.
Fig. 12 shows the comparison of the light output power–
current (L–I) characteristics for the GaN VLED on metal
alloy and the conventional LEDs on sapphire. The light
output power of the conventional LEDs on sapphire peaked
around 1000 mA and then declined quickly after 1000 mA.
Poor heat dissipation capability was reasoned for thedegradation. On the contrary, the light output power of
the GaN VLED on metal alloy can sustain higher current of
3000 mA or higher without light output power saturation.
The high current operation behavior showed the superior
heat dissipation of the metal alloyed and allowed higher
current injection to obtain higher light output power, which
is especially needed for the general lighting application.
A conventional LED on sapphire and the GaN VLED onmetal alloy of the same size of 40 mil were mounted onto
lead-frames using silver epoxy, and the junction temper-
ature was measured as a function of current. Fig. 13 shows
the junction temperature as a function of current for the
two types of LED structure. The GaN VLED on metal alloy
Fig. 10. A schematic cross-section image of VLEDs on metal alloy.
Fig. 11. The I–V characteristics of a 40 mil chip for the GaN VLED
on metal alloy and the conventional LEDs on sapphire.
Fig. 12. The comparison of the light output L–I characteristics of a
40 mil chip for the GaN VLED on-metal alloy and the conventional
LEDs on sapphire.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
1202 Proceedings of the IEEE | Vol. 98, No. 7, July 2010
has an estimated lower thermal resistance of 74.2 K/W
than the 89.1 K/W of conventional LED on sapphire.
A striking difference between the GaN VLED onmetal alloy and the conventional LEDs on sapphire is the
scaling effect shown in Fig. 14. The normalized
extraction efficiency of the GaN VLED on metal alloy
is not sensitive to the chip size, while the conventional
LEDs on sapphire exhibit a significant drop in efficiency
for larger chips. For lighting application, the chip size
must be large in order to generate enough brightness
required for this application.We use the same technology platform with different
n-electrode patterns design to scale up and down all sizes.
The design rule is taking the n-GaN current spreading into
account. Each one n-electrode designed line can spread the
current to be about 150� 200 �m. Therefore, bigger chip
size needs additional n-electrode line to help for better
current spreading. Fig. 15 shows the blue emission pictures
and the n-electrode design patterns for different chip size
ranging from 15 to 80 mil. The GaN VLED on metal alloy
fabrication technology can apply to all chip sizes withoutefficiency drop.
Fig. 16 shows structural diagrams of the conventional
GaN LED on sapphire. For the conventional GaN LED on
sapphire, the p-GaN layer thickness is usually kept below
2000 �A, as p-GaN absorbs photons in the wavelength of the
interested spectrum. To increase the light extraction effi-
ciency, either truncated hexagonal pyramids or pits were
formed on the p-GaN surface using various techniques. Thedrawbacks of the conventional structure for an effective light
extraction lay in the thickness of p-GaN layer, which must be
thin enough to avoid reabsorption of photons. In other
words, any engineering of p-GaN surface may affect the
active layer underneath, leading to compromised device
characteristics. The second drawback of conventional LEDs
on sapphire is that it lacks a mirror on the backside to reflect
photons back to the rough surface for better light extraction.To overcome these shortcomings, the GaN VLED on
metal alloy was designed to boost the external quantum
efficiency while relieving the demand for a thin p-GaN
Fig. 13. The junction temperature as a function of current for the
two types of LED structure.
Fig. 14. The scaling effect for the GaN VLED on metal alloy and the
conventional LEDs on sapphire. The efficiency was normalized to a
chip size of 350 �m.
Fig. 15. The emission pictures for different chip size ranging from
15 to 80 mil.
Fig. 16. The structural diagrams of the conventional GaN LED
on sapphire.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 1203
layer. Fig. 17(a) and (b) shows the first and second ver-
sions of the GaN VLED on metal alloy. All the surface
engineering occurs on the thick n-layer (> 4 �m). In the
first version of the GaN VLED on metal alloy, the GaN LED
structure is placed on a high-reflectivity surface, as shown
in Fig. 17(a), with the top surface roughened by etching to
allow more photons to escape the surface. To further
enhance the number of photons extracted from the surface,a new technique to form a corrugated pyramid shaped
(CPS) surface, as shown in Fig. 17(b), was developed. The
new CPS surface increases the surface areas having angular
randomization, allowing more scattering of photons.
Fig. 18 shows the scanning electron microscope (SEM)
surface of the GaN VLED on metal alloy of version 1. Fig. 19
Fig. 17. The (a) first and (b) second versions of the GaN VLED
on metal alloy.
Fig. 18. The (a) top view and (b) side view of the SEM surface for
the GaN VLED on metal alloy of first version.
Fig. 19. The (a) top view and (b) side view of the SEM surface for the
GaN VLED on metal alloy of second version with CPS structure.
Fig. 20. Comparison of the light output L–I characteristics for the GaN
VLED on metal alloy of first version, and second version with CPS
structure.
Fig. 21. The far-field emission pattern for the first version
and second version.
Fig. 22. The brightness distribution of the GaN VLED on metal alloy
with different N-GaN thicknesses.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
1204 Proceedings of the IEEE | Vol. 98, No. 7, July 2010
shows the top view and side view of the SEM surface for the
GaN VLED on metal alloy of version 2 with CPS structure.
At 350 mA, an improvement of 20% is observed for the GaN
VLED on metal alloy with CPS surface, as shown in Fig. 20.
Fig. 21 shows the far-field emission pattern for the first
version and the second version. The uniform and good
Lambertian light patterns can be obtained by the vertical
LED structure. The GaN VLED on metal alloy with CPS topsurface had higher total light out power intensity. The light
pattern of the GaN VLED on metal alloy with CPS surface
also showed a higher ratio of light output power in the
power angle from 30� to 150� compared to the first version
GaN VLEDs.
Another aspect of the GaN VLED on metal alloy is that
the n-GaN layer averaged thickness can be tailored to be
thinner. Any reabsorption of photons in the n-layer by freeelectrons or by the mid-gap states will lead to a loss of
efficiency. Besides providing higher extraction (more
scattering sites), the GaN VLED on metal alloy with CPS
technology also lowers the effective n-GaN thickness,
minimizing the reabsorption effects without sacrificing the
quality of the active layers. Fig. 22 shows the brightness
distribution of the GaN VLED on metal alloy with different
n-GaN thicknesses. A reduction of n-GaN layer thickness
from 6 to 4.5 �m improves 12% of brightness.
C. ReliabilityFor a reliability study, we selected 1 mm2 GaN VLED
on metal alloy chips with a light output power equivalent
to converted white light of over 120 lumens/W (Table 2).
The chips then were packaged using silicone as filling.
The packaged chips were mounted on a heat sink, and
measurements were carried out in a close space with stable
ambient temperature. A life test was performed on the
GaN VLED on metal alloy at a forward current of 350 mA
at ambient room temperature. As shown in Fig. 23, thedrop of light output power is quite little and can be kept
below 10% even after the burn-in test of 5000 h. In case of
daily life application at the ambient room temperature, no
Table 2 The 40 mil Chip Packaged With Phosphor to Produce White Light.
(IS System, ISP-150, Keithley 2430 at 350 mA)
Fig. 23. The drop of light output power after the burn-in test of 5000 h.
Fig. 24. The stability of light output power after 1000 times
thermal shock.
Fig. 25. The stability of operation voltage at 350 mA after 1000
cycles thermal shock.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 1205
degradation of light output has been observed so far as
referred to the BRT[ points.The thermal shock aging for the packed VLED chips
is measured ranging from the temperature of �40 �C(15 min) to 125 �C (15 min). After 1000 times high and low
recycle temperature aging, the packaged VLED chips show
no degradation of the light output power (Fig. 24) and no
fail of the operation voltage at 350 mA (Fig. 25).
In some special solid-state lighting application, such as
lighting equipment in the desert or outside of an airplane,the packaged VLED chips can be sustained in harsher
environments. Fig. 26 shows the reflow testing tempera-
ture profile. Fig. 27 shows the packaged VLED chips have
no light output degradation and no operation voltage
failures after ten times reflow cycle testing.
VI. CONCLUSION
In conclusion, the GaN VLED on metal alloy was presented
and characterized. Very low dynamic resistance, low
operation voltage, excellent heat dissipation, and good
reliability of the GaN VLED on metal alloy were proven.An efficiency of 120 lumens/W or better was achieved,
rendering the GaN VLED on metal alloy very suitable for
general lighting application. h
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Fig. 26. The reflow testing temperature profile.Fig. 27. The reliability of packaged VLED chips after ten times reflow.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
1206 Proceedings of the IEEE | Vol. 98, No. 7, July 2010
ABOUT T HE AUTHO RS
Chen-Fu Chu received the B.S. degree in electrical
engineering from Tamkang University, Taiwan,
in 1995, the M.S. degree in optical sciences from
National Central University, Taiwan, in 1998, and
the Ph.D. degree in Electro-Optical Engineering
from National Chiao-Tung University, Taiwan,
in 2003.
He is a Research Director with SemiLEDs
Optoelectronics Co., Ltd. In 2003, he joined High-
link Technology Corp. as a Scientist. In 2005, he
joined SemiLEDs Cooperation as a Chief Scientist. He has published more
than 25 scientific and technical journal papers and 50 conference papers.
Chao-Chen Cheng, photograph and biography not available at the time
of publication.
Wen-Huan Liu, photograph and biography not available at the time of
publication.
Jiunn-Yi Chu, photograph and biography not available at the time of
publication.
Feng-Hsu Fan, photograph and biography not available at the time of
publication.
Hao-Chun Cheng, photograph and biography not available at the time of
publication.
Trung Doan, photograph and biography not available at the time of
publication.
Chuong Anh Tran received the B.S. degree in
physics from Czech Technical University, Czech
Republic, in 1987 and the Ph.D. degree from the
University of Montreal, Montreal, PQ, Canada, in
1993.
He is President and COO of SemiLEDs Corp. He
has extensive educational and professional expe-
rience, including nearly 15 years of working
experience, in the field of optoelectronics. He
joined Emcore in 1993 as a Senior Technical Staff
Member and became one of the key members of the team that developed
the first commercial reactor for InGaN LED. In 1999, he joined Gelcore,
then a joint venture between Emcore and GE Lighting, focusing on solid-
state lighting. In 2001, he joined Highlink Technology Corp. as Vice
President. He has published more than 70 technical papers in numerous
scientific journals and has participated as a Guest Speaker at numerous
conferences, including SPIE, Photonics West, MRS meetings, European
MRS, and International MOVPE conferences.
Dr. Tran received the Quebec Government FCAR Excellence
Scholarship.
Chu et al. : High Brightness GaN Vertical Light-Emitting Diodes
Vol. 98, No. 7, July 2010 | Proceedings of the IEEE 1207