LED Power and EfficiencyGenerally the excess minority carrier density decays exponentially with time time t

Δn = Δn(0) exp (-t/ζ)

d/dt[Δn] = (J/ed)- (Δn/ ζ)(m-3s-1)

LED Power and Efficiency

The condition for the equilibriumis obtained by setting thederivative of the above to zero.

Therefore[Δn] = (J ζ /ed) (m-3)

LED Power and EfficiencyThe total No. of carrier recombination rate

per second or recombination rate

LED Power and EfficiencyWhen the forward biased current into the device is I, then total no of recombination per second

Rt = i/eηint = rr/(rt) = rr /(rr +rnr)

= Rr/Rt

LED Power and EfficiencyRr = = (i/e )ηint

P = hf (i/e )ηint

P int = (hc/λ) (i/e )ηint

LED Power and Efficiency

ηint = [1/{1+(rnr/rr)}= [1/{1+(ζr / ζ nr)}

LED Power and Efficiency

1/ζ =1/ ζr+ 1/ ζnr

ExampleThe radiative and non-radiativerecombination life times of the minority carrier in active region of DH LED are 60 ns and 100 ns respectively. Determine the total carrier recombination lifetime and the power internally generated within the device when the peak emmision wavelength is 0.87 micrometer at a drive current 40 mA.

Ans: ζ = 37.5 ns ηint = 62.5%Pint= 35.6mW.

Edge Emitter LED This has a similar geometry to a

conventional contact stripe ILDIt takes advantage of transparent

guiding layers with very thin activelayer(50-100 μm)in order that the lightproduced in the active layer spreadsinto transparent layer, reducing selfabsorption in the active layer.

Edge Emitter LED

The Effective radiance at theemitting end face can be veryhigh giving an increasedcoupling efficiency into smallNA fiber compared withsurface emitter.

Edge Emitter LED

However Surface emittergenerally radiate more powerinto air (2.5-3 times) than ELEDSince the emitted light is lesseffected by re-absorption andinterfacial recombination.

Edge Emitter LED

But it has been seen thatELED couples more opticalpower into low NA (lessthan0.3) whereas SLEDjust does it’s opposite

Edge Emitter LED

The enhance wave guidingof the edge emitterenables it to couple 7.5times more power intolow NA than SLED

Edge Emitter LED

The Stripe geometryof ELED allows veryhigh carrier injectiondensities for a givendriving current.

Edge Emitter LED

Last but not the least ELEDhave better modulationBW (order of hundredsMHz) than comparablewith SLED

Coupling : ILD To Fiber