Sources

Sources

1. LED

   • Structure
     • Basically a P-N Junction under forawrd Bias. 
     • Hetero Junction
       • They are better than homojunctions and the best of the two is the double hetero junction. It places the photons in a specifc place nad increases the light iintensity.  Also contributes to more efficiency. 
     • Substrate
       • Has to be cheap, smooth and cover a large area. It is also prefered that the substrate is either reflecteve or transparant. By transparant, it means that the wavelength of absorbtion/emission of the substrate is lower than the intended wavelength of emission  (Band gap higher then ..). This allowes the implimentation of SiO2 mirrors at the bottom , to reflect the light that goes through the substrate up. 
     • Shape
       • Panic
         • Hemispherical, which is half of a sphere shape, spreads the light everywhere 
       • Communication
         • Paraboloc. Which is more half of an egg shape, focuses light in a nawrrow space. So its the most important
     • Material Composition
       • It affects the Color and the effecincy of the device.  For white light, Blue LED, with yellow chemiclal lights are used.
       • Direct Band Gap
         • All good, Radiative, only needs a photon for emission. 
       • Indirect Band Gap
         • You have to have both the Photon and the Phonon at the exact same time and the exact place to make that momentum shift. 
       • Transition in Bandgap type
         • in GaAsP for example, which is meant to produce green light, the band structure contains direct and indurect bands. Eg in the indirect bandgap is is lower, at lower concentrations (0.1 <y<0.45  IR red)  of Phosphorus So all recombination happens there and its good. As the concentration of phosphorus increases. Both energy gaps start to raise, except that, the indirect gap raises on a slower rate, so at a concentration of P that is higher than 0.45 (Red Yellow Orange) , the Indirect energy gap is bellow the direct gap. To see why this is a problem, check the indirect gap bubble. The thing is that, we still want to use this high P concentration because it is what gives us (yellow) to make green.
         • Isoelectronic Trap
           • So what we do, is to trap the electron on another energy level called the isoelectronic trap. This level needs to be small to not change the wavelength of emitted light. What happens is that, by the hesinberg uncertainity principle. The position of the electron is known, therfore its momentum has high uncertainity. Therefore it could be anywhere withim the momentum plane. Causing a high probability of it existing over the other band, and hence direct recombination.  Nitrogen is used. By using nitrogen the effecincy gain of the device raises by x100 ...  
   • Spectral Width
     • Directly related to the different levels of energies at which recombination occures.  For a single recombination, the wavelengths is inversly propotional to Eg by  (Wavelength = 1.24/Eg)  The spectral width is Affected By : 
     • Tempreture
       • Tempreture affects the spectral width by the equation. Also this equation denotes that at higher wavelengths of emmission. We get even larger uncertainites, which deem LEDs to be unprecice and unsuitable for Optical communications. 
     • Wavelength squaring
       • For blue light for example. Because of the high wavelenght, the spectral uncertainity increases to be too high covering almost all visible spectrum, this is because Delta lambda is propotional to lambda square 
   • Frequency Response
     • Directly affected by QE
     • Quantum Efficiency
       • The amount of radiation, or light emmited over the amount of recombination that occured. Not all recombination results in photons, some recombination energy is dissipated as phonons ( Vibrations).  QE = Rr/ (Rr+ Rnr)Tnr/(Tr+Tnr)  R= recombinations T= lifetime nr=nonradiatve r = radiatve The higher QE is, the better Frequency response 
       • Dopant level
         • Controlling dopant level means controlling Radiative recombination Tr = Delta n / R = Rec/ NA  Delta n = Excess carriers  R = recombination rate NA = Doping level.   By increasing dopant level, we  decrease the radiatve lifetime. By Tr, We increase QE.  QE directly propotional to 1/Tr
       • Material Parameters
         • Tnr = 1 / segma Vth Nt  Nt = trap density, this cant be put down to 0. (I think it can be changed by tempreture) Vth = Thermal Velocity,  can be c changed by changin material parameters By increasing Tnr, we increase QE.  Tnr has to be more than Tr for more QE. 
   • Recombination
     • Has three types 
     • Radiative
       • Consisting of two recombination mechanisims 
       • Thermal
         • Normal transitions with a spectral uncertainity created by tempreture. e-h recombine from different positions within the band. 
       • Deep Levels
         • Donor and acceptor create deep energy levels. The depth of their level depends on the amount of Doping
     • Impurity controlled
       • Sometimes, impurities within the junction create energy levels that trap electrons. Leading to generation of different spectrum of wavelength. or could be just phonons 
     • Auger
       • Its non radiative. 

2. LASER

   • Recombination
     • Population inversion
     • Stimulated emission
   • Structure
     • Degenerate Doping
     • Double Hetero Junction
     • Confinment
       • Active layer thickness
       • Refrective Index difference
         • Optical Gain
     • Geometry
       • Double Hetero
       • Stripe Gemoetry Double Hetero
         • Tunneling
       • Vertical Cavity Surface
         • Confinment Reduction
           • Optical gain
         • Junction Tempreture
           • Heat sink estimation
           • Wavelength Change
     • Quantum Well
       • Quantum Confinment
         • Well Width
       • Multiple structures
         • Stairs and smooth ?
         • Multi-Dementional
           • Gain
           • Frequency
           • Threshold Current
   • Electrical Charchteristics
     • Threshold current
   • Spectral Width
     • Wavelength Squaring
     • Length but not Refractive index
     • More controlable than LED
   • Frequency response
     • Higher power Vs Higher speed
       • J threshold and J off
     • Better than LED
   • Less reliable than LED
     • Gradual Degredation and other problems