Opto Electronic Devices

Opto-electronic Devices A study of the devices and their characteristics….

LEDs are available in different shapes, sizes & colours.

Blue, green, and red LEDs. These can be combined to produce all colors and white. Infrared and ultraviolet (UVA) LEDs are also available. Like a normal diode, diode, the LED consists of a chip of semiconducting material impregnated, or doped, doped, with impurities to create a p-n junction. junction. As in other diodes, current flows easily from the p-side, or anode, anode, to the n-side, or cathode, cathode, but not in the reverse direction. Charge-carriers—electrons Charge-carriers—electrons and holes—flow holes—flow into the junction from electrodes with different voltages. voltages. When an electron meets a hole, it falls into a lower energy level, level, and releases energy in the form of a photon. photon.

Choice of LED material-1 Semiconductor material selection for p & n region depends upon the required colour of light to be emitted from the LED. For Infra-red & red light Aluminium gallium arsenide (AlGaAs). For green colour light use Aluminium gallium phosphide (AlGaP). For yellow, orange, red colour light use Gallium arsenide phosphide (GaAsP). Use Gallium nitride (GaN) — for green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier)

Choice of material-2 Indium gallium nitride (InGaN) — 450 nm - 470 nm — near ultraviolet, bluish-green and blue Silicon carbide (SiC) as substrate — blue Silicon (Si) as substrate — blue (under development) Sapphire (Al2O3) as substrate — blue Zinc selenide (ZnSe) — blue Diamond (C) — ultraviolet Aluminium nitride (AlN), aluminium gallium nitride (AlGaN), aluminium gallium indium nitride (AlGaInN) — near to far ultraviolet (down to 210 nm[13]) With this wide variety of colors, arrays of multicolor LEDs can be designed to produce unconventional color patterns.

Ultra-violet & blue LED Ultraviolet GaN LEDs. Blue LEDs are based on the wide band gap semiconductors GaN ( gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.

White LEDs A combination of red, green and blue LEDs can produce the impression of white light, though white LEDs today rarely use this principle. Most "white" LEDs in production today are modified blue LEDs: GaN-based, InGaN-active-layer LEDs emit blue light of wavelengths between 450 nm and 470 nm. This InGaN-GaN structure is covered with a yellowish phosphor coating usually made of cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia Corporation and has been used since 1996 for the manufacture of white LEDs.

Forward voltage drop in F.B. condition common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 milliamperes, a commonly found maximum value). Colour

Infrared

Potential .Diff.

1.6 V

red 1.8 to 2.1 V

orange

yellow

green

2.2 V

2.4 V

2.6 V

Blue 3 t0 3.5 V

Reverse break down voltage of many LED is of 5V .

White 3 to 3.5V

The wave length of emitted light The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. Flashlights and lanterns that utilise white LEDs in tarchs are becoming increasingly popular due to their durability and longer battery life.

Direct & Indirect Band gap Materials--Solution of schrodinger equation leads to the band structure of a semi-conductor. The top of the valence band of most of the semiconductor occures at wave vector k = 0 i.e., effective momentum having value zero. The bottom of the conduction band in some semiconductors occurs at k=0 such materials or optically active & are called direct band gap materials. As for example GaAs, InP, InGaAs etc. In Indirect band gap semiconductor the bottom of C.B. does not occur at k=0, but at certain non-zero value, as for example Si, Ge & AlAs etc..such materials are optically inactive. These materials have very weak photonic interactions and are unfit for optical devices.

E-k diagram of direct & Indirect B.G materials C.B.

E = Ec + h2k2/ 2me

Ec Direct B.G.

Photon of E = Eg

Indirect B.G.

EV Lattice vibration & Phonon heat liberation

E = Ev – h2 k2 / 2mh

VB E-k bands of Ga As Direct B.G. material

V.B. E-k bands of Si, Ge. Indirect B.G. material

At 0K temperature CB is empty & V.B. is totally filled by electrons, no transport possible. At room temperature some of V.B. e- moves to C.B. and adds to conductivity to pure semiconductor. Holes & electrons are equal in numbers. De-Boggle λ = h/p i.e. p = h / λ Wave vector, k = 2π/λ E = p2/2 m* = h2 / 2 λ2 m* = h2k2/8π2 m* Momentum p = h/λ = h k / 2π i.e. p = n . K, where n= h/2π. If wave number be 1/λ then E = h2k2/2 m* and velocity v = h .k /m* . Semiconductor Alloy Ax B(1-x) equivalent lattice constant, αalloy = x αA + (1-x) αB by vegard law.

Ex. Calculate the wavelength associated with a 1eV, i) photon ii) electron, iii) neutron, iv) corresponding wave vector k = 2π/λ for photon & electron. i) λph = hc/E = 6.6x10-34 3x108 / 1.6x10-19 = 1.24µm. ii) λe = h/(2m0E)1/2 = 6.6x10-34Js/[2x(0.91x10-30 kg)(1.6x1019)]1/2 λe = 12.3 A0 iii) λn = λe (mo/mn)1/2 = 12.3 A0 x (1/1824)1/2 = 0.28 A0. iv) kph = 2 x π / (1.24 x 10-6) = 5 x 106 /metre = 5 x 10-4 / A0 ke = 2x 3.14 / 12.3 A0 = 0.5 / A0 >>>>> kph hence electron of finite k value can not emit photon of energy Eg in indirect B.G semiconductors i.e. this event is very less probable & does not occur directly. It may occur with a third body help ie phonon or lattice vibration & heat emission process.

Energy & Momentum conservation Absorption: Emission

Ef = Ei + hω, hω is Energy of photon. Ef = Ei – hω, Ef & Ei are final & initial

energy of electron. E of photon hω≥ Eg. Momentum k of crystals having holes & electrons needs to be conserved. The photon kph = 2π/λ.Since 1ev photon have λ = 1.24µm, the corresponding k= 10-4 A0 = 0 as compared to k value for electrons. So initial and final electron states should have same k value for optical transition to be possible. Transition is impossible & very less probable otherwise. Indirect B.G. transitions have weak tendency & absorption coefficient is very less. It requires lattice vibration & phonon heat emission, instead of photon.

If e- in CB have same value of k as the hole has in C.B. then this transition causes spontaneous emission of photon, when EHP combines. This spontaneous emission is non-coherence or incoherent. Light emission in LED is of this type. So λ of various photons have some range of wave length say 540nm to 560nm for green LED. Spectral band width = 20nm or 200 A0.LED light output is not adequate for long distance communication. LASER diode must be used for it. Stimulated emission: If some photons of frequency ω are present in pn junction cavity it enhances the recombination, causing population inversion and stimulated photon emission. All photons have very narrow band of frequency and λ and emission is coherence. This light is LASER light. Emission may either be radiative (visible or non-visible), or nonradiative (Phononic heat). So designer must choose a semiconductor material for pn junction, which causes radiative emission to great proportion.

Mono Junction-LED LED when forward biased, emits light of colour, which depends on semiconductor material. Electrons are injected from n side to p side & holes are injected from p side to n side. LED should be designed so that photons are emitted close to the top layer i.e. p layer if be at the top side, rather than the berried layer other wise many of photon will be reabsorbed in lattice. Electron injection must be predominant, so we need pn+ diode with n side highly doped. i.e. Jn >> Jp. So np >> pn The injection efficiency, γinj = Jn /( Jn+ Jp + JGR) Diffusion current has three components: i) minority carrier electron diffusion current. ii) minority carrier hole diffusion current. iii) trap assisted recombination current in the depletion region of width w. Jn = e Dn np / Ln [exp {eV / (kB T)}-1] Jp = e Dp pn / Ln [exp {eV / (kB T)}-1] JGR = e ni w / 2 ζ [exp {eV / (2 kB T)}-1] ,, where ζ is the recombination time in the depletion region & depends upon trap density. For a perfect material of high purity trap centres & recombination centres are very few and injection efficiency is close to 1.

Radiative emission & non-radiative, via defect or via phonon

Vertical transistion concerned with same k value of electrons in C.B. & holes in V.B. are radiative. The photon energy & electron & hole energy are related by: hω – Eg = h2 k2/2 [ 1/ m*e +1/ m*h] = h2k2/2 m*r. where m*r is mass of EH system.

Pn+ mono junction LED p

Light photons

n+ Buried region +

R= 1.2 kΩ

--

5V dc

Un bised pn+ junction band diagram

Photons moving this side will get absorbed in this layer Forward bised case band diagram Very less hole injection

Hetro-junction LED Problems with single semiconductor LED 1) In homo-junction single semiconductor LED, photon emission volume is not close to surface & it causes photon re- absorption, reducing the emission efficiency. 2) Since surface has many defect states & is not clean and pure, it causes non-radiative recombination & reduces efficiency. Effective volume is large as e- are duffusing through a long length to reach to p side from n+ side.

Hetro-junction LED structure Ga As Active region

Alx Ga(1-x) As

p-

p Wide B.G. semiconductor material

n

Narrow B.G material. Wide B.G. material

Energy Band digram of hetro jn LED

E g > E” g

Alx Ga(1-x) As

electron injection

E” g

Photons of E = Eg

Hole injection

Active region confined in NBG region.

Features of hetro-junction LED-------Charges are injected in to a narrw B.G active region from a wide B.G. n+ region. Electrons & holes both are injected in two the middle active region of narrow B.G. The electrons can not enter in to wide gap p layer & donot suffer from poor surface condition. Photons emitted from active region do not move towards top or bottom side since the photon energy is smaller than the B.G. of n or p – region. Active region in confined to a rang of 0.1 to 0.2µm.

Recent LED In September 2003, a new type of blue LED was demonstrated by the company Cree, Inc. to provide 24 mW at 20 mA. This produced a commercially packaged white light giving 65 lumens per watt at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents.

Recent LED-2 In 2006 they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Also, Seoul Semiconductor has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents and better even than standard fluorescents.[20] Nichia Corporation has developed a white light LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.

It should be noted that high-power (≥ 1 watt) LEDs are necessary for practical general lighting applications. Typical operating currents for these devices begin at 350 mA. The highest efficiency high-power white LED is claimed by Philips Lumileds Lighting Co. with a luminous efficacy of 115 lm/W (350 mA). In early 2008, researchers at Bilkent University in Turkey demonstrated a new technique for producing white light from blue LEDs coated with nanocrystals. This approach was shown giving off "more than 300 lumens per watt".

Bicolour & flashing LEDs

Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red & green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.

LED Driver Circuit DC supply ON/OFF Indicator IF = 10mA

Vo

-

±VSat

+ VCC R1

Fig.1(a)

+

VF=1.6V

R

+ VCC + VCC

R

Fig.1(b)

D1 D1

R2 R1 Q1 VB

+ VB

Fig.2 a

Q1

Fig.2 (b) R1

D2

Conclusions Light emitting diodes are small size pn monojunction or hetro-junction device, which emits light if forward biased, by a suitable low dc source. Most of the LED have current rating typically ranging From 10mA to 20 mA so must be connected in series With a suitable resistor to limit the current through it. If forward drop across the LED is known & safe Value of current is known, then select the dc bias Voltage, & calculate the series resistor required. LEDs are being used in many display devices & Meter panels. Tri-cvolour LED array may generate Light of any colour, since brightness of emitted Light depends upon current flowing through the RGB tri-LED array.