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Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
Semi conductor led
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Semi conductor led

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  • 1. Vertical Cavity Surface Emitting Lasers (VCSELs)Advantages over the edge-emitting lasers: Its design allows the chips to be manufactured andtested on a single wafer. Large arrays of devices can be created exploitingmethods such as flip chip optical interconnectsoptical neural network applications to becomepossible. In the telecommunications industry, the VCSELsuniform, single mode beam profile is desirable forcoupling into optical fibres. The cavity length of VCSELs is very short typically1-3 wavelengths of the emitted light. As a result, ina single pass of the cavity, a photon has a smallchance of a triggering a stimulated emission event atlow carrier densities. Therefore, VCSELs requirehighly reflective mirrors to be efficient. For VCSELs,the reflectivity required for low threshold currents isgreater than 99.9%. Such a high reflectivtiy can notbe acheived by the use of metalic mirrors. VCSELsmake use Distributed Bragg Reflectors. (DBRs). Theseare formed by laying down alternating layers ofsemiconductor or dielectric materials with adifference in refractive index.
  • 2. Light Emitting Diode For optical communication systems requiring bit rates less than approximately 100-200 Mb/s together with multimode fiber-coupled optical power in the tens ofmicrowatts, semiconductor light emitting diodes (LEDs) are usually the best lightsource choice. These LEDs require less complex drive circuitry than laser diodessince no thermal or optical stabilization circuits are needed, and they can befabricated less expensively with higher yields.To be useful in fiber transmission applications an LED must have 1. A high radiance (or brightness)- a measure in watts, of the power radiated into aunit solid angle per unit area of the emitting surface. 2. A fast response time - the time delay between the application of a current pulseand the onset of optical emission. 3. A high quantum efficiency - the fraction of injected electron-hole pairs thatrecombine radiatively. In a heterostructure LED, carrier confinement is used to achieve a high level ofradiative recombination in the active region of the device, which yields a highquantum efficiency. Optical confinement is of important for preventing absorptionof the emitted radiation by the material surrounding the pn junction. This dualconfinement leads to both high efficiency and high radiance.LED structures The two basic LED configuration being used for fiber optics are surface emitters(also called Burrus or front emitters) and edge emitters.
  • 3. Surface Emitter LED (SLED) In the surface emitter, the plane ofthe active light-emitting region isoriented perpendicularly to the axisof the fiber. Normally, a well isetched through the substrate of thedevice, into which a fiber is thencemented in order to accept theemitted light. The circular active areain practical surface emitters isnominally 50μm in diameter and upto 2.5mm thick. The emission patternis essentially isotropic with a 1200half-power beam width and is calleda lambertian pattern. In this pattern, the source is equally bright when viewed from any direction, but the power diminishesas cosθ, θ is the angle between the viewing direction and the normal to the surface. The power coupledPc into a multimode step index fiber may be estimated from the relationship ,where r is the Fresnel reflection coefficient at the fiber surface, A is the smaller of the fiber core crosssection or the emission area of the source and RD is the radiance of the source. The power coupled into
  • 4. Edge emitter LED (ELED) The high radiance ELED usedin optical communications issimilar to a conventionalcontact stripe injection laser. This structure forms awaveguide channel thatdirects the optical radiationtoward the fiber core. Most of the propagation lightis emitted at one end faceonly due to a reflector on theother end face and anantireflection coating on theemitting end face. Theemission pattern of this ELEDis more directional than thatof the SLED. In the planeparallel to the junction, wherethere is no waveguide effect,the emitted beam is lambertian with half-power width of . In the plane perpendicular to thejunction, the half-power beam width has been made as small as 25-350 by a proper choice of thewaveguide thickness.
  • 5. LED Optical output power Intrinsically the LED is a very linear device in comparison with the majority ofinjection lasers and hence it tends to be more suitable for analog transmission.The surface emitter radiates significantlymore optical power into air than the edgeemitter, and that both devices arereasonably linear at moderate drivecurrents.
  • 6. Output spectrum The spectral linewidth of an LED operating atroom temperature in the 0.8 to 0.9 mm wavelengthband is usually between 25 and 40nm at the halfmaximum intensity points (FWHP). For materialswith smaller bandgap energies operating in the 1.1to 1.7mm wavelength region the linewidth tends toincrease to around 50 to 160nm. Typical spectral output characteristics forInGaAsP devices. The output spectralwidths of SLEDs tend to be broader thanthose of edge-emitting LEDs because ofdifferent internal-absorption effects. The output spectra tends to broaden at arate of between 0.1 and 0.3 nm0C-1withincrease in temperature due to thegreater energy spread in carrierdistributions at higher temperatures. Thepeak emission wavelength is also shiftedby +0.3 to 0.4nm0C-1for AlGaAsdevices and by +0.6nm0C-1 for InGaAsPdevices.
  • 7. Carrier Recombination in LED Reliance on spontaneous emission allows nonradiative recombination to take place within the LEDstructure due to crystalline imperfections and impurities. Nonradiative recombination includeoptical absorption in the active region (self-absorption), carrier recombination at theheterostructure interfaces, and the Auger process in which the energy released during an electron-hole recombination is transferred to another carrier in the form of kinetic energy. When LED is forward biased, minority carrier of electrons and holes in p- and n-type material,respectively is created by carrier injection at the device contacts. The excess density of electronsDn and holes Dp is equal since the injected carriers are created and recombined in pairs such thatcharge neutrality is maintained within the structure. In general, the excess minority carrier densitydecays exponentially with time t according to the relation: where n0 is the initial injected excess electron density and t is the carrier recombination lifetime. The total rate at which carriers are generated will be where d is the thickness of the recombination region.externally supplied thermal generation rates. When there is a constant current flow into the junction diode, an equilibrium condition isestablished and gives the steady state electron density For the exponential decay of excess carriers, the radiative recombination lifetime is the nonradiative recombination lifetime is Generally,τr and τnr are comparable for direct-band-gap semiconductors, such as GaAlAs andInGaAsP. Thus, the internal quantum efficiency is about 50% for simple homojunction LEDs. LEDshaving double-heterojunction structures can have quantum efficiencies of 60-80%.
  • 8. Internal Quantum Efficiency is defined as the ratio of the radiative recombination rate to the total recombination rate:where τ is the bulk recombination lifetime, If the current injected into the LED is I, then the total number of recombination per second is Rr +Rnr = I / q and rearrange to give . Since Rr is the total number of photons generated persecond and that each photon has an energy of hv, the power generated internally by the LED is Note that a linear relationship between theoptical power generated in LED and the drive current into the device. External Quantum Efficiency , is defined as the ratio of the photons emitted from the LEDto the number of internally generated photons. Only light falling within a cone defined by the critical angle where will be emitted from an optical source. Optical power emitted Pe into a medium of lowreflective index n2 from the face of planar LEDfabricated from a material of reflactive index n1is given approximately by where F is the transmission factor of thesemiconductor-external interface.
  • 9. Modulation of an LED The frequency response of an LED is largely determined by the following threefactors: 1. The doping level in the active region. 2. The injected carrier lifetime τi in the recombination region. 3. The parasitic capacitance of the LED. The parasitic capacitance can cause a delay of the carrier injection into the activeregion and thus delay the optical output. However, this delay is negligible if asmall, constant forward bias is applied to the diode. Under this condition, themodulation response is limited only by the carrier recombination time. If the drivecurrent is modulated at a frequency ω, the power of the device will vary aswhere P0 is the power emitted at zero modulation frequency. The modulation bandwidth of an LED is defined as the point where the electricalsignal power, p(ω) has dropped to half its constant value resulting from themodulated portion of the optical signal. This is the electrical 3-dB point. In optical terms, the detected currentis directly proportional to the opticalpower, thus
  • 10. Modulation of an LED The frequency response of an LED is largely determined by the following threefactors: 1. The doping level in the active region. 2. The injected carrier lifetime τi in the recombination region. 3. The parasitic capacitance of the LED. The parasitic capacitance can cause a delay of the carrier injection into the activeregion and thus delay the optical output. However, this delay is negligible if asmall, constant forward bias is applied to the diode. Under this condition, themodulation response is limited only by the carrier recombination time. If the drivecurrent is modulated at a frequency ω, the power of the device will vary aswhere P0 is the power emitted at zero modulation frequency. The modulation bandwidth of an LED is defined as the point where the electricalsignal power, p(ω) has dropped to half its constant value resulting from themodulated portion of the optical signal. This is the electrical 3-dB point. In optical terms, the detected currentis directly proportional to the opticalpower, thus

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