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RESULTS AND DISCUSSION
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FIG. 3. The I-V characteristic of the cascaded p-n junctions with N=, 3, and 4 (a) in linear scale (b) semi-log
scale. The inset to (b) shows the change in series resistance with the number of p-n junctions (N).
 The turn-on voltage of the cascaded device increased
with the number of p-n junctions stacked epitaxially
 All the structures showed rectifying behavior
 the device with 4 junctions does have higher leakage
current
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FIG. 3. The I-V characteristic of the cascaded p-n junctions with N=, 3, and 4 (a) in linear scale (b) semi-log
scale. The inset to (b) shows the change in series resistance with the number of p-n junctions (N).
 Estimate 5.7 × 10−4 Ω𝑐𝑚2 resistivity for each tunnel junction
 resistance would result in relatively low voltage drop of 57 mV
for a current density of 100 𝐴/𝑐𝑚2
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Use the measured results to model cascaded multiple active region LEDs.
The modeled reference LED emits at 470 nm
1.02𝑚𝑚2 mesa area
ZnO layer as contact to p-GaN
The differential series resistance (Rs) of the reported LED was~0.02 Ω𝑐𝑚2
modified for the case of “N” identical LEDs cascaded to
• “n” is the ideality factor of a single junction device
• “C=Is” is saturation current
• Rs(TJ)=6.4 × 10−4 𝑐𝑚2
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 The input voltage at turn-on increases as the number of junctions is increased.
 At the same input power level, relatively lower currents and higher voltages
are needed for the cascaded LED structures
 The total power dissipated due to series resistance of an LED 𝑃𝑅 = 𝐼 2 × 𝑅𝑠
Rs ∝N and I ∝1/N
 higher efficiency while reducing overall heat generation.
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FIG. 5. The change (b) WPE in logarithmic scale
of the modeled reference single junction LED and
cascaded LEDs with N=5, 20, and 50. The
experimental data from reference 18 and the
modeled fit is also shown in Fig.5(b)
 The external quantum efficiency was modeled by curve fitting the
experimental data using ABC model.
 Note here that this model was used as an empirical fit, rather than to
convey any physical information.
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3.5W
FIG. 5. The change in (a) output power of single
junction LED and cascaded LEDs with N=5, 20,
and 50.
 assume that a structure with N cascaded stages has EQEN=N× EQE(single LED).
 absorption loss via free-carriers and InGaN QW layers are expected to increase as
“N” increases, these are ignored here.
 Conventional LED output power increases and saturates below 1 W under 10 W
input power.
 As the number of cascade regions is increased,the saturation is pushed out further,
with negligible saturation in output power for the LEDs with N=20 and N=50 cases
up to 3.5 W
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14% droop
enhancement
of 420%
83% droop
FIG. 5. The change in (b) WPE in logarithmic scale (c) WPE in linear
scale of the modeled reference single junction LED and cascaded
LEDs with N=5, 20, and 50. The experimental data from reference 18
and the modeled fit is also shown in Fig.5(b)
 the maximum efficiency point occurs at approximately Ntimes higher power than
a single LED.
 This implies that the maximum device efficiency can be obtained at much higher
drive powers
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