IN-214 Semiconductor Devices and Circuits Panel Discussion #2 Group - IV
Light Emitting Diodes (LEDs)
Light Emitting Diodes (LEDs)
LEDs are semiconductor p-n junctions, that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The quanta of light energy released is approximately proportional to the band gap of the semiconductor.
Light Emitting Diodes (LEDs)
Direct Band Gap Semiconductors
Light Emitting Diodes (LEDs)
Luminescence: the process behind light emission
Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy. In electro-luminescence excitation results from the application of an electric field. In a p-n junction diode, injection electro-luminescence occurs resulting in light emission when the junction is forward biased.
Light Emitting Diodes (LEDs)
Injection Luminescence in LED
p Eg eVo n+ p Eg n+ h =Eg
Light Emitting Diodes (LEDs)
Electrons in CB Holes in VB
Under forward bias majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side. Minority carriers will diffuse and recombine with the majority carrier. The recombination causes light to be emitted. Such process is termed radiative recombination.
Recombination and Efficiency
Ideal LED will have all injection electrons to take part in the recombination process.
Light Emitting Diodes (LEDs)
In real device not all electron will recombine with holes to radiate light.
Sometimes recombination occurs but no light is being emitted (non-radiative).
Efficiency of the device therefore can be described. Efficiency is the rate of photon emission over the rate of supply electrons
Emission wavelength,
The number of radiative recombination is proportional to the carrier injection rate. Carrier injection rate is related to the current flowing in the junction. If the transition take place between states (conduction and valance bands),
Light Emitting Diodes (LEDs)
the emission wavelength, =
we know, EC-EV = Eg
hc (ECEV)
thus, = hc/Eg
LED Construction
Efficient light emitter is also an efficient absorbers of radiation therefore, a shallow p-n junction required. Active materials (n and p) will be grown on a lattice matched substrate. The p-n junction will be forward biased with contacts made by metallization to the upper and lower surfaces. Ought to leave the upper part clear so photon can escape.
Light Emitting Diodes (LEDs)
Efficient LED
Need a p-n junction - preferably the same semiconductor material only different dopants. Recombination must occur: Radiative transmission to give out the right colored LED. Right colored LED : hc/ = Ec-Ev = Eg so choose material with the right Eg Direct band gap semiconductors to allow efficient recombination. All photons created must be able to leave the semiconductor. Little or no reabsorption of photons.
Light Emitting Diodes (LEDs)
Direct Bandgap
Correct Bandgap
Efficient radiative pathways must exist
Material can be easily doped
Light Emitting Diodes (LEDs)
Materials Requirements
Direct band gap materials e.g. GaAs not Si
UV-LED ~0.5-400nm Eg > 3.25eV V-LED ~450-650nm Eg = 3.1eV to 1.6eV IR-LED ~750nm- 1m Eg = 1.65eV
Light Emitting Diodes (LEDs)
Candidate Materials
Materials with refractive index that could allow light to get out Readily doped n and p-types
Visible LEDs
LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390 - 770nm. This coincides with the energy value of 3.18eV - 1.61eV corresponding to different colors.
Violet Blue Green Yellow Orange Red
~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV
The band gap, Eg that the semiconductor must posses to emit each light
Light Emitting Diodes (LEDs)
Candidate Materials for LEDs
Light Emitting Diodes (LEDs)
Semiconductors of Interest for Visible-LEDs
Light Emitting Diodes (LEDs)
Conventional Materials for LEDs
Color Infrared Red Wavelength [nm] > 760 610 < < 760 Semiconductor material Gallium Arsenide (GaAs) Aluminium Gallium Arsenide (AlGaAs)
Green Blue
500 < < 570 450 < < 500
Gallium(III) Nitride (GaN) Gallium(III) Phosphide (GaP) Aluminium Gallium Phosphide (AlGaP) Zinc Selenide (ZnSe) Indium Gallium Nitride (InGaN) Diamond (235 nm) Boron Nitride (215 nm) Aluminium Nitride (AlN) (210 nm) Blue/UV diode with yellow phosphor
Ultraviolet
White
< 400
Broad spectrum
Light Emitting Diodes (LEDs)
Aluminium Gallium Arsenide (AlGaAs) Gallium Arsenide Phosphide (GaAsP) Gallium(III) Phosphide (GaP)
OLED Device Structures
Light Emitting Diodes (LEDs)
Typical Materials for OLEDs
Color Red Wavelength [nm] 610 < < 760 Semiconductor material DCM-doped Alq3 DCJTB-doped Alq3 DCJTB- and rubrene-doped Alq3
Light Emitting Diodes (LEDs)
Green
500 < < 570
Alq3 QA-doped Alq3 coumarin-doped Alq3 DMQA-doped Alq3 DSA-doped DPVBi BCzVB-doped CBP BCzVB-doped DPVBi DPVBi Salq DPVBi/Alq3 DCJTB-doped SAlq PAP-ph + Alq3 + DCM-doped Alq3
Blue
450 < < 500
White
Broad spectrum
Formulae Supplement for the Materials
Alq3 = tris(8-hydroxyquinolato)aluminium(III). BCzVB = 4, 4-(bis(9-ethyl-3-carbazovinylene)-1,1-phenyl. CBP = 4, 4-N,N-dicarbazole-biphenyl. DCJTB = 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran. DCM = 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran. DMQA = N,N-dimethylquinacridone. DPVBi = 4, 4-bis(2,2-diphenylvinyl)-1,1-biphenyl. DSA = distryrylarylene. PAP-ph = 1,7-diphenyl-4-biphenyl-3,5-dimethyl-1,7-dihydrodipyrazolo[3,4-b;4, 3-e]pyridine. QA = quinacridone. SAlq = bis(2-methyl-8-quinolato)-(triphenylsiloxy)aluminium(III).
Light Emitting Diodes (LEDs)
Light Emitting Diodes (LEDs)
Any Questions
Light Emitting Diodes (LEDs)
THANK YOU
