LIGHT EMITTING DIODE (LED) & LASER
LED IS A P-N JUNCTION DIODE - DIRECT BANDGAP SEMICONDUCTOR
MINIMUM CONDUCTION AND MAXIMUM VALENCE WITH THE SAME K VALUE
TRANSITION IS DIRECT
RECOMBINATION OF ELECTRON WITH HOLE EMITS LIGHT
E OF THE EMITTED LIGHT IS ABOUT BANDGAP ENERGY (Eg)
UNBIASED CONDITION
FERMI ENERGY LEVEL IS CONTINUOUS ACROSS THE JUNCTION
THE DEPLETION LAYER FORMS AT THE JUNCTIOM CAUSES AN ELECTRIC FIELD AT THE PN JUNCTION
INDUCED ELECTRIC FIELD
DIFFUSION POTENTIAL (= PREVENTS DIFFUSION AND ELECTRON AND HOLES TO THE JUNCTION) Vo ESTABLISHED PN SIDE
FORWARD BIASED LED
WHEN POSITVE TERMINAL OF VOLTAGE SOURCE V IS CONNECTED TO P SIDE, EFFECTIVE VOLTAGE ACROSS JUNCTION DECREASES BY V THAT IS ( Vo - V)
THE DECREASE POTENTIAL BARRIER ALLOW ELECTRON AND HOLE MOVE TO THE JUNCTION
THE JUNCTION IS AN ACTIVE REGION WHERE ELECTRON IN CONDUCTION BAND TRANSIT DOWN TO RECOMBINE WITH HOLES IN VALENCE BAND
EACH RECOMBINATION OF ELECTRON AND HOLE WILL PRODUCE ONE PHOTON OF LIGHT WITH ENERGY WITH ABOUT BANDGAP ENERGY (Eg)
PHOTONS ARE EMITTED RANDOM IN ALL DIRECTIONS, THIS CAUSE SPONTANEOUS EMITTION
TO PRODUCE LIGHT, LED MUST BE CONNECTED TO AN EXTERNAL VOLTAGE SOURCE IN A FORWARD BIASED CONDITION
THE BRIGHTNESS OF LIGHT DEPENDS ON THE NUMBER OF PHOTON EMIT FROM LED
LED WITH VERY HIGH BRIGHTNESS MEANS THE NUMBER OF PHOTON EMITTED VERY HIGH
LED STRUCTURE
LED FABRICATED BY GROWING DOPED SEMICONDUCTOR LAYERS THAT IS PN JUNCTION ON A SUBSTRATE
TO PROVIDE SUPPORT TO PN JUNCTIONS
SUBTRATE CAN BE OF MANY DIFF MATERIALS , eg GaAs
P SIDE IS REGION WHERE LIGHT IS EMITTED
TO AVOID THE EFFICIENCY REDUCTION OF LED, P SIDE IS GROWN VERY THIN LAYER ( A FEW MICRONS)
SIMPLEST LED STRUCTURE
PLANAR LED
SURFACE EMITTING LED (SLED)
EDGE-EMITTING LED ( ELED)
LIGHT IS EMITTED FROM THE SURFACE OF THE STRUCTURE
HAS BROADER BANDWAVE THAN EDGE
LIGHT IS EMITTED FROM THE EDGES OF THE STRUCTURE
HAS BROADER BANDWAVE THAN EDGE
SIMPLE LED STRUCTURE ( NOT ALL CAN EMIT INTO THE AIR)
= ENCAPSULATION OF PN JUNCTION WITH DOME SHAPE ( TO REDUCE TIR) TRANSPARENT PLASTIC MEDIUM OF HIGHER REFRACTIVE INDEX THAN AIR
GREATER CRITICAL DUE TO TIR
LIGHT WITH TETA INCIDENT > TETA CRITICAL = NO LIGHT TRANSMIT INTO AIR BUT LIGHT REFRACTED INTO LED
TIR OCCURS WHEN LIGHT INCIDENT AT INTERFACE AT HIGH TO LOWER INDEX
LED MATERIALS
DIFFERENT MATERIALS GIVE OUT DIFFERENT COLOUR OF LIGHT
EACH MATERIAL HAS ITS OWN ENERGY BANDGAP
DIFFERENCES BETW. ENERGY IN CONDUCTION BAND AND IN VALENCE BAND
LED
HOMOJUNCTION
HETEROJUNCTION
JUNCTION FORMED BY DOPING THE SAME SEMICONDUCTOR
TO OVERCOME LIMITATION OF HOMOJUNCTION
LOW POWER INPUTS
LOW EFFICIENCY
PHOTON REABSORPTION
JUNCTION FORMED BETW. 2 SEMICONDUCTOR WITH DIFFERENT BANDGAPS
REFRACTIVE INDEX OF A SEMICONDUCTOR MATERIAL DEPENDS ON ITS BANDGAP
THE WIDER THE BANDGAP THE LOWER THE RFRACTIVE INDEX
LED CAN BE DESIGNED SUCH THAT IT GUIDES THE PHOTON OUT FROM LED AT A PARTICULAR DIRECTION
CHARACTERISTIC OF LED
ENERGY EMITTED PHOTON IS NOT EXACTLY EQUALS Eg
LED -> P-I RELAY
BECAUSE ENERGY OF ELECTRON AND HOLES ARE DISTRIBUTED IN THEIR RESPECTIVE BANDS
ELECTRON CONCENTRATION PER UNIT ENERGY = g(E) {DENSITY OF STATES , DISTRIBUTION OF ENERGY} . f(E){FERMI-DIRAC FUNCTION , PROBABILITY OF ELECTRON OCCUPIED IN EACH STATE}
WAVELENGTH AT PEAK INTENSITY AND SPECTRUM LINEWIDTH RELATED TO
ENERGY DISTRIBUTION OF ELECTRON AND HOLES IN CONDUCTION BAND AND VALENCE BAND
DENSITY OF STATES IN CB AND VB
INDIVIDUAL SEMICONDUCTOR PROPERTIES
PHOTON ENERGY AT PEAK EMISSION = Eg + KbT
LINEWIDTH IS TYPICALLY , 2.5KbT < WAVELENGTH SEPERATION < 3KbT
OUTPUT SPECTRUM AND INTENSITY VS LAMBDA CHARACTERISTICS NOT ONLY DEPEND ON SEMICONDUCTOR MATERIAL BUT ALSO ON STRUCTURE OF PN JUNCTION
SPONTANEOUS AND STIMULATED EMISSION
ELECTRON UNDERGOUS TRANSITION BETW TWO ENERGY LEVELS. IT EMITS OR ABSORBS A PHOTON WITH ENERGY
EMISSION PROCESS MAY OCCUR IN TWO WAYS
SPONTANEOUS EMISSION
STIMULATED EMISSION
ELECTRON DROPS TO LOWER ENERGY LEVEL IN A RANDOM WAY
ELECTRON IS "TRIGGERED" TO UNDERGO THE TRANSITION BY THE PRESENCE OF PHOTON OF ENERGY
PROCESS RESULTS IN COHERENT RADIATION WITH THE WAVES HAVING
IDENTICAL FREQUENCIES ( SAME WAVELENGTH)
IN PHASE WITH ONE ANOTHER
SAME STATE OF POLARIZATION
TRAVEL IN SAME DIRACTION
THE NUMBER OF THESE WAVES (PHOTON) INCREASES AS IT PASS THROUGH THE GAIN MEDIUM - CAUSES AMPLIFICATION PROCESS
STIMULATED ABSORPTION IS RELATED TO THE PHOTON ABSORPTION THAT WOULD CAUSE STIMULATED EMISSION
THE TRANSITION IS INITIATED BY THE PRESENCE OF STIMULATING PHOTON TO PRODUCE STIMULATED EMISSION
DIFFICULT TO OBSERVE STIMULATED EMISSION BECAUSE THE PROBABILITY OF SPONTANEOUS PROCESS IS HIGHER THAN STIMULATED PROCESS
SPONTANEOUS RADIATION FROM ANY ATOM IS RANDOM - THE EMITTED PHOTONS ARE INCOHORENT
BEAM PROPERTIES OF LASER DIODE
DRIECTIONALITY
LINEWIDTH
COHERENCE
BRIGHTNESS
LASER BEAM vs LED
LASER BEAM
LED
HIGHLY DIRECTIONAL
HIGHLY COLLIMATED
HIGH MONOCHROMATICITY
COHERENT
HIGH BRIGHTNESS
NON-DIRECTIONAL
NON-COLLIMATED
BROADER SPECTRAL WIDTH
INCOHERENT
WEAK BRIGHTNESS