The document discusses LED power output and efficiency, detailing the processes of carrier generation, recombination, and the resulting optical power produced. It covers key concepts like internal and external quantum efficiency, as well as the impact of minority carrier densities and external conditions on LED performance. Additionally, it provides examples to illustrate calculations of internal power generation and external power efficiency measurements.

1. LED Power Output and
Efficiency
MEC

2. Contents
• Excess Minority Carrier Density
• Carrier Generation.
• Carrier Recombination
• Power Generated.
• Quantum Efficiency.
• Power Efficiency.
• Problems and Solutions.

3. Internal Power Generated
• Power generated internally determined by
considering excess electrons and holes in
p- and n-type material (minority carriers)
when forward biased.
• Charge Neutrality - excess density of
electrons & holes equal, Δn = Δp - injected
carriers created and recombine in pairs.
• Extrinsic materials - one carrier type have
much higher concentration than other.

4. Excess Minority Carrier Density
• In p-type region hole concentration >
electron concentration.
• Excess minority carrier density decays
exponentially with time
Δn(0) - initial injected excess electron
density, τ - total carrier recombination
lifetime.
• Δn is only a small fraction of the majority
carriers, all of minority carriers.

5. Carrier Generation
• Carrier recombination lifetime becomes
minority/injected carrier lifetime τi.
• Constant current flow into the junction
diode – equilibrium.
• Total rate of carrier generation - sum of
externally supplied and thermal generation
rates.

6. Carrier Recombination
• Current density J (A/m2) = J/ed electrons/
m3/s , d - thickness of recombination
region.
• Rate equation for carrier recombination in
LED
• Condition for equilibrium - derivative to
zero.

7. Carrier Recombination
• Steady-state electron density when
constant current flows into the junction
region.
• Recombination rate - total number of
carrier recombinations per second.

8. Carrier Recombination
• Total number of recombinations/sec
i - forward biased current into the device
• Excess carriers can recombine radiatively
(photon generated) or nonradiatively
(lattice vibration, energy release as heat).
• DH device with thin active region (few
microns) - nonradiative recombination
dominate by surface recombination at the
heterojunction interfaces.

9. Carrier Recombination
• Rr - total no. of radiative recombinations per
second .
• Rr is equivalent to total number of photons
generated per second
Internal quantum efficiency

10. Recombination Coefficient
• Obtained from measured absorption coefficient,
for low injected minority carriers relative to
majority carriers.
• Related to radiative minority carrier lifetime
N, P - majority carrier concentrations in n and p
regions.
• In p type region, hole concentration determine
radiative carrier life time such that 1
r
rB N
 
1
( )
r
rB N P
 


11. Optical Power Generated
• Each photon has energy = hf joules.
• Optical power generated internally by LED
• Internally generated power in terms of
wavelength (i – drive current, linear)

12. Power Emitted
• External quantum efficiency - ratio of the
photons emitted from the device to the
photons internally generated
• Also ratio of number of photons emitted to
total number of carrier recombinations
(radiative and nonradiative).
• Optical power emitted from LED - constant
of proportionality ηint multiplied by external
quantum efficiency ηext

13. Internal
Quantum Efficiency
• Internal quantum efficiency - ratio of photons
generated to injected electrons, ratio of radiative
recombination rate to total recombination rate.
• Absence of optical amplification through
stimulated emission in LED limits internal
quantum efficiency.
• Double-heterojunction (DH) structures give
internal quantum efficiencies of 60 to 80%.

14. Internal Quantum Efficiency
• Radiative minority carrier lifetime τr = Δn/rr
• Nonradiative minority carrier lifetime τnr =
Δn/rnr.
• Internal quantum efficiency
• Total recombination lifetime τ = Δn/rt

15. Example
• Radiative & nonradiative recombination
lifetimes of minority carriers in the active
region of a DH LED are 60 ns and 100 ns,
peak emission wavelength is 0.87 μm at a
drive current of 40 mA. To find total carrier
recombination lifetime, power internally
generated within the device…..

16. Example
• Total carrier recombination lifetime
• Internal quantum efficiency
• Power generated internally

17. Example
• LED has an internal quantum efficiency of
62.5% generates 35.6 mW of optical
power, internally. However, this power
level will not be readily emitted from the
device.

18. Radiation Geometry
• Radiation geometry is
Lambertian.
• Surface radiance is
constant in all
directions.
• Maximum intensity
perpendicular to the
planar surface,
reduced on the sides
as per cosine of
viewing angle.
•Coupling Efficiency

19. External Power Efficiency
• Ratio of optical power emitted externally
Pe to electric power provided to device P.
• Optical power emitted Pe into a medium of
low refractive index n from face of planar
LED fabricated from material of refractive
index nx (F - Transmission Factor, Pint -
Power Generated Internally)

20. Example
• Planar LED fabricated from gallium
arsenide has a refractive index of 3.6. To
calculate the optical power emitted into air
as a percentage of internal optical power
for the device when transmission factor at
crystal–air interface is 0.68. When optical
power generated internally is 50% of
electric power supplied, to determine the
external power efficiency…..

21. Example
Refractive index n for air = 1.
Power emitted is only 1.3% of optical
power generated internally.
External power efficiency

22. Example
• Optical power generated internally Pint =
0.5P.
• External power efficiency

23. Thank You