Opto Electronic Devices - drmgriyearbtech.com · absorbed photons make electron hole pair in the depletion region. When an electron-hole pair is generated due to the incident radiation,

4.1 PHOTODETECTORSPhotodetectors or Photosensors are sensors of light or other electromagnetic

energy. Photodetectors has a PN junction that converts light photons into current. Theabsorbed photons make electron hole pair in the depletion region.

When an electron-hole pair is generated due to the incident radiation, the pairseparated at the junctions as a result of the existence of electric field. As a result, itgenerates open circuit voltage or short circuit current. This is the principle applied inthe photodiodes to produce both voltage and current in an external circuit.

(a) Photo-conducting material geometry of slab

(b) Photo-conductor detectorcircuit symbol

L

D

W

Electrodes

Incident radiation

Figure 4.1

The geometry of the slab of a photo conducting material is shown in figure 4.1.Let L W and D be respectively the length, width and thickness of the conducting material.It consists of two electrodes on opposite faces.

Opto Electronic Devices4

4.2 ENGINEERING PHYSICS - II

The incident radiation falls on the opposite surfaces of the materials. Thedetectorate the terminal devices and are used as light dependent resistors. The commonlyused symbol for a photo conductive detector consists of a resistance symbol with incidentarrows representing the radiations, as shown in figure. The detectors are available inthe form of photodiodes and photo transistors.

Photodetector Bias Circuit

The photodetector (P) is connected in series with a voltage source and a loadresistor RL as shown in figure. When a light radiation is incident on the photodetector,whose energy is greater than Eg, the increase in conductivity of the detector takesplace. As a result, the flow of current in the circuit leads to an increase in potentialacross the load resistor RL. The same can be measured using a high impedance Voltmeter.A blocking capacitor C is introduced in the output line to remove any dc componentswhile measuring the current in the circuit, due to time variations of the incident light.

Bias voltage

PC

RLOutputvoltage

Figure 4.2: Photodetector bias circuit

The Sensitivity and linearity of the detector depends on RL. One can obtain theoptium value of RL by a fractional change in the resistance of the photo detector whenit is under illumination. When the change in the fractional resistance is less than 5%.The sensitivity shows a large value at RL = RD, where RD is the resistance ofthe photodetector. On the other hand, if the change in the resistance is large,i.e., RL << RD, which is required for output voltage measurements.

Let Io and I be the intensities of the incident and transmitted radiations, respectively,on the semiconductors detector materials. Therefore, the average generations of chargecarries, i.e., electron-hole pairs per unit volume is

rg = /v

Where is the quantum efficiency of the absorption process and v is the frequency ofradiation (=c/).

OPTO ELECTRONIC DEVICES 4.3

Applications

i) Photoconductivity detectors in the form of single crystal or polycrystalmaterials are prepared in the form of Schottky barriers, n-p homojunctions,n-p hetrojunctions, n-p-n, or p-n-p transistors with gain greater than I.

ii) The television camera vidicon and electrophotography are the two applicationsin the device form.

iii) It is used to detect a variety of light and particle as a light controlled switch.

iv) The silver halide coated on the photographic film is also based onphotoconductivity.

4.2 PHOTO CONDUCTORSIf radiation falls upon a semiconductor, its conductivity increases. This

photoconductive effect is explained as follows: Radiant energy supplied to thesemiconductor ionizes covalent bonds: that is, these bonds are broken and hole-electronpairs in excess of those generated thermally are created. These increased current carriersdecrease the resistance of the material. Hence such a device is called a photoresistoror photoconductor.

Donor level

Acceptor level

(i)

(ii)

(iii)

Eg

Conduction band

Valence band

Figure 4.3

Figure shows the energy diagram of a semiconductor having both acceptor anddonor impurities. If photons of sufficient energies illuminate this specimen,photogeneration takes place.


The following three transistions are possible:

i) The electron-hole pair can be created by a high-energy photon. This is calledintrinsic excitation.

ii) A photon may excite a donorm electron into the conduction band.

iii) A valence electron may go into an acceptor state.

The last two transitions are called impurity excitations. The density of statesin the conduction and valence bands is much greater than the density of impuritystates. So photoconductivity is due principally to intrinsic excitation.

Cut-off wavelength and spectral response

The minimum energy of a photon required for intrinsic excitation is the forbidden-gap energy of the semiconductor material. Hence the minimum frequency vc causephotoconduction is given by

hvc = Eg

corresponding maximum wavelength or critical wavelength is given by

hc/c = Eg

If c is expressed in microns and Eg in electron volts,

c = 1.24/Eg

100

75

50

25

00.5 1.0 1.5 2.0

Si

Ge

Wavelength ( m)

Rel

ativ

e re

spon

se %

Figure 4.4


Figure 4.4 shows spectral response versus wavelength curve for Ge and Si.

If the wavelength of the radiation exceeds c, then the energy of the photon isless than Eg, such a photon cannot cause a valence electron to enter the conductionband. Hence c is called the critical, or cut-off wavelength, or long wavelength threshold,of the material. For Si, Eg = 1.1ev and c = 1.13m whereas for Ge, Eg = 0.72 ev andc = 1.73 m at room temperature.

4.3 PHOTO DIODEA photo diode is essentially a reverse biased P-N junction diode which is designed

to respond to photon absorption.

Principle

A reverse biases P-N junction diode has a reverse saturation current which ismainly due to the flow of the minority carriers. When light is incident on the depletionregion of the reverse biased pn junction, the concentration of minority carriers (electronsin p type and holes in n type) increases to a great extent.

But the change in majority carriers is too low. Consequently reverse currentincreases. The reverse current through the diode varies almost linearly with the intensityof light.

Construction

–+R

P

NLigh

t

Clear plastic

(a) (b)

Figure 4.5

A photo diode consists of a P-N junction embedded in a clear plastic capsule [seeFigure 4.5(a)]. The symbol of a photo diode is shown in Figure 4.5(b)].

Light is allowed to fall upon one surface across the junction. All the sides of theplastic capsule, excepting the illuminated one, are either painted black or enclosed in ametallic case.


Working and Characteristics

When photo diode is kept under dark condition and a sufficient reverse voltage isapplied, then an almost constant current, independent of magnitude of reverse bias, isobtained. This current corresponds to the reverse saturation current due to thermallygenerated minority carriers and is denoted by Id. Majority charge carries are not allowedto cross the junction by the potential hill under this reverse bias condition.

When light falls on the diode surface, additional electron-hole pairs are formed.But since the concentration of majority carriers is much greater as compared to that ofminority carriers, the percentage increase of majority carriers is much smaller than thepercentage increase of minority carriers. Hence, we can neglect the increase in majoritycarrier density and can consider the radiation entirely as minority carrier injector. Theseinjected minority carriers diffuse to the junction, cross it and contribute to the additiionalcurrent.

Thus under large reverse bias conditions, the totald reverse current is given by

I = Is + Id

Where Is is the short circuit current and is proportional to light intensity.

With any bias V, the reverse current and thermal electron-hole pairs i.e., darkcurrent is given by

Id = cv kT0I 1 e

Hence, the volt-ampere characteristic of photo diode is given by

I = cv kTs 0I I 1 e

Dio

de c

urre

nt (m

A)

Reverse Voltage V

2000 Lumen/m2

1500 Lumen/m2

1000 Lumen/m2

Dark current

Figure 4.6


The volt-ampere characteristic curve of a photodiode is as shown in figure. Fromthe curve it is seen that

i) The current increases with increase in the level of illumination for a givenreverse voltage.

ii) Only for the dark current at zero voltage the current is zero.

The photodiode finds extensive application in light detection systems, reading offilm sound track, light operated switches, high-speed reading of computer punched cardsand tapes.

4.3.1 p-i-n PhotodiodesA positive-intrinsic-negative (p-i-n) photodiode consists of p and n regions

separated by a very lightly n doped intrinsic region. Silicon pin photodiodes are used at0.8 m wavelength and InGaAS pin photodiodes are used at 1.3 m and 1.55 mwavelengths. In normal operation, the pin photodiode is under high reverse bias voltage.So the intrinsic region of diode is fully depleted of carriers. When an incident photonhas an energy greater than or equal to the bandgap energy of the photo diode material,the electron-hole pair is created due to the absorption of photon. Such photon generatedcarriers in the depleted intrinsic region where most of the incident light photons areabsorbed, are separated by the high electric field present in the depletion region andcollected across the reverse biased junction. This gives rise to a photocurrent flow inthe external circuit. The pin photodiode acts as a linear device such that the photocurrentis directly proportional to incident optical power. Thus

I = RP

where I = photocurrent

P = incident optical power

R = Responsibility of the photodiode =q

h v

. Thus R =IP

.

Here = quantum efficiency of the diode

=number of electron hole pair generated

number of incident photons

= I/q/P/h v

h v = energy of the incident photon

q = charge of electron


The responsivity of the photodiode depends on the bandgap of the material,operating wavelength, the doping and the thickness of the p, i and n regions of thediode. For example to get high quantum efficiency and hence the maximum sensitivity,the thickness of the depletion layer should be increased so that the absorption of photonwill be maximum. But it reduces the response speed of the photodiode. In the wavelength1.3 m and 1.55 m, InGaAS pin photodiode have high quantum efficiency and highresponsivity.

IP N

Ohmic contact

id CD RL

Detectoroutput voltage

(a) (b)

Figure 4.7: PIN photodiode with reverse bias and its equivalent circuit

Figure shows the photodiode with reverse bias and its equivalent circuit.

Analysing the above circuit, the bandwidth is inversely proprortional to rise time ‘tr’.

Thus f3dB =r L D

0.35 0.35t 2.19 R C

For high speed applications, CD should be small (–1 pF)

When tr is very large, the speed of response is limited. For PIN diodes, tr = 0.5 to10 ns. The PIN photodiodes have wide bandwidth and high sensitivity thanphotoconductors.

The different materials used in different regions of a PIN diode are given below:

Region Different materials

p In Al As,, Al Ga As

i In Ga As,, Ga As

n In Ga As,, Al Ga As

Substrate n ln P, n Ga As

PIN photodiodes are generally heterojunction type. These are mainly used in thewavelengths from 0.8 m to 1.7 m.


4.3.2 High Speed PhotodiodesPIN photodiodes have high speed response or fast response. Generally the response

time depends on

i) the transit time of the photocarriers in the depletion region.

ii) the diffusion time of the photo charge carriers generated outside the depletionregion and

iii) time constant RC of the photodiode circuit. The above three factors dependon the absorption coefficient, depletion layer width and junction capacitance.

To get high speed response, the time constant RC should be small particularly thecapacitance ‘C’ is about 1 pF. The diffusion time and transit time are also about fewnano seconds. Based on these facts, In Al, As, In GaAs, Al Ga As photodiodes havehigh speed response. Similarly In As, In Sb and Hg Cd Te photodiodes are have fastresponse (< 10 ns).

4.3.3 Long Wavelength PhotodiodesSilicon photodiodes are used to detect wavelength from 0.2 to 1.1 m. But

germanium photodiodes are used to detect wavelengths from 0.5 to 1.8 m. In the caseof In As photodiodes, the wavelengths are detected from 1 to 3.8 m.

i) In optical fiber communications, for the wavelength range from 1.3 m to1.7 m, In Ga As, In Al As, Al Ga As photodiodes are used. These have highspeed response, high optical absorption and high quantum efficiency and lowdark current.

ii) Germanium photodiodes are also used upto the wavelengths 2 m. These areused in high data (800 Mb/s) transmission link operating at longer wavelengths.

iii) Lead sulphide (PbS) is a well known near infrared photodiode material whichdetects wavelengths from 1 to 3.4 m.

iv) Indium Antimonide (In Sb) photodiodes are used to 7 m and have high speedof response (< 50 ns). Its performance can be further increased by cooling itto 77 K (liquid nitrogen temperature).

v) Mercury cadmium telluride (Hgx Cd1 – x Te) photodiodes consists of semimetalHg Te and the semiconductor Cd Te. These can detect wavelengths from 5 to25 m. To reduce noise, it should be cooled by 77 K.


4.4 PHOTO TRANSISTORPhototransistor is an npn or pnp transistor in which the forward current is controlled

by the incident light at the pn junction. Due to transistor action, the current is amplified.For the normal operation of the photo transistor (npn), the base is kept open. The emitterregion is given a negative potential while collector region is given a positive potential.Radiation is concentrated on the region near the collector junction JC.

n

n

p

E

+– VCE

JC

IC

JE

C

Radiation

Figure 4.8

The phototransistor is usually connected in a common-emitter configuration withthe base open. The junction JE is slightly forward biased. The junction JC is reversebiased. Assume first, that there is no radiant excitation. Under these circumstancesminority carriers are generated thermally. The electrons crossing from the base to thecollector, as well as the holed crossing from the collector to the base, constitute thereverse saturation collector current ICO.

10 mW/cm2

5.0

1.0

0 5.0 10 15 20 25

10

8.0

6.0

4.0

2.0

0

V , Collector - emitter voltage, VCE

I o C

olle

ctor

cur

rent

. mA

Figure 4.9


The collector current is given by

IC = ( + 1) (ICO + IL)

Due to transistor action, the current caused by the radiation multiplied by thelarge factor ( + 1).

The output volt-ampere characteristics of an npn silicon phototransistor are shownin figure for three different values of illumination intensities.

Us es

i) Phototransistors are used in light operated relays.

ii) They are also used to compare the intensities of different light sources madefor infrared detection.

4.5 LASER DIODESLaser diode or Diode Laser is a semiconductor device similar to LED in which

laser beam is created at diodes junction. These are used to convert the electrical signalto light signal.

Semiconductor Laser Materials

Among the semiconductors, only the direct band gap semiconductors are used.Direct band gap is the one in which a conduction band electron can recombine directlywith a hole in the valance band. So emission of light is possible only in the direct bandgap semiconductors. Most of the compound semiconductors have the direct gap.

These is no elemental direct band semiconductor formed by group III elements(Al, Ga, In) and group V elements (P, As, Sb). For example in the 800-900 nm wavelengthrange, Ga1_x , Alx As alloy is used where

x =Composition of aluminium arsenide

Composition of gallium arsenide

which determines the band gap of the alloy. Similarly in the 0.92 - 1.65 m wavelengthrange, In1–x, Gax Asy P1–y alloy is used where y = 2.2 x and 0 x 0.47. Here ydetermines the band gap of the alloy. Further the direct band gap semiconductors formedby group IV and VI such as Pb Te and Pb Sn Te have been developed to design lasersfor the middle infrared region.


P-N junction laser diodes

i) Homojunction laser

Homojunction laser means that a p-n junction is formed by a single crystallinematerial such that the basic material has been the same on both sides of the junction.For example, in GaAs laser, both the p-layer and n-layer are made up of Ga As only.

Drawbacks of homojunction lasers

1. Threshold current density is very large (400 A/mm2).

2. Only pulsed mode output is obtained.

3. Laser output has large beam divergence.

4. Coherence and stability are very poor.

5. Electromagnetic field confinement is poor.

ii) Heterojunction laser

Heterojunction means that the material on one side 'of the junction differs fromthat on the other side of the junction. In the. modern Ga As diode laser, a heterojunctionis formed between GaAs and GaAl As.

Mostly the heterojunction laser diodes are used as optical sources in the opticalfiber communication because they have so many advantages.

1. Threshold current density is small (10 A/mm2).

2. Continuous wave operation can also be possible.

3. Due to efficient waveguide structure, the beam divergence is small, carrierand optical confinement can be achieved simultaneously; high coherence andmonochromacity are obtained.

4. High output power (= 10 mW) can be achieved even with low threshold current.

5. These are highly stable with longer life.

Operation of a laser diode

i) Absorption

An electron in the valence band can make an upward transition to the conductionhand by absorbing a photon of energy (Ec - Ev) where Ev is the maximum energy of thevalence band and Ec is the minimum energy of the conduction band.


ii) Spontaneous emission

An electron in the conduction band can recombine with a hole in the valenceband by spontaneously emitting a photon of energy (Ec - Ev).

iii)Stimulated emission

An electron in the conduction band can recombine with a hole in the valenceband in the presence of a stimulating or inducing photon whose energy also equals to(Ec – Ev) giving stimulated emission, The emitted photon and inducing photon travel inthe same direction with the same energy. The emitted photon is fully coherent with theinducing photon.

Calculation of wavelength of emitted radiations

Now Ec – Ev = Eg where Eg is the band gap energy of the material.

Eg = h v = hC (or) =

g

hCE

Taking wavelength in m and Eg in eV, the above relation reduces to

(m) = g

1.24E eV

4.6 LIGHT EMITTING DIODES (LED)Light emitting diode (LED) emits light by injection luminescence. Here a p-n

junction diode is operated under forward bias. Under forward bias, majority carriersfrom both sides of the junction cross the internal potential barrier and enter the materialat the other side where they become the minority charge carriers and cause the localminority carrier population to be larger than normal. This is called minority carrier injection.The excess minority carriers diffuse away from the junction recombining with majoritycarriers as they do so (figure 4.10).

– –– –

++ +

+

h

hFermi level

P-typeN-type

C.BFermi level

V.B

Figure 4.10: Radiative recombination of the injected minority carrierswith the majority carriers in a forward biased p-n junction.


In LED, every injected charge carrier takes part in a radiative recombination andhence gives rise to an emitted photon. In reverse bias, no carrier injection takes placeand consequently no light is emitted.

The number of photons emitted is proportional to the carrier injection rate or thetotal current flowing. The wavelength of the emission is given by

ggvc E

hCEE–EhC

For example, in Ga As, Eg = 1.44 eV. This corresponds to a wavelength value = 0.86µm. Thus by adding phosphorus with Ga As the value of the bandgap is increasedand the wavelength of the emitted radiation is in the visible range.

Requirements for a suitable LED material

i) Energy gap more than 2 eV or equal to 2 eV.

ii) There should be both p and n type layers.

iii) Low resistivity.

iv) Efficient radiative path ways must exist.

Doping with B, Al, Sc and Be in Ga As P gives rise to yellow, blue, green and redemission reflectively. Further GaP, InP and CdS are also LED materials.

Table 4.1: Communication based LEDs and their properties

LED material Bandgap energy (eV) Wavelength range (µm)

In Ga As P 0.73 - 1.35 0.9 - 1.7

In Ga As 0.95 - 1.24 1 - 1.3

Ga Al As 1.40 - 1.55 0.8 - 0.9

Ga As 1.4 0.9

The above LEDs are used in the optical fiber communication as light sources.These have lifetime more than 105 hours.


To get high efficient emission of radiation or radiative recombination process,

nr

rnr r

nr

1

1

Here r - Radiative life time;

nr - Non radiative life time

To get high efficiency, either nr is increased or r is reduced.

Transient Response of LEDs

Usually hetero junction LEDs are widely used in the communication thanhomo-junction LEDs.

The rise time to the half current point (which is also the half power point) of theLED is given by

t1/2 = pS

p S

ICln ln 2

I I

and the 10 - 90 present rise time is given by

t10 – 90 = S

p

Cln 9

I

where Cs is the space-charge capacitance and it will be very small (350 pF). Further

= q

2 kT and Ip is the current used to drive the LED and is the minority carrier life

time. In the high current limit,

t1/2 = ln 2 and t10–90 = ln 9

Thus the rise time depends on minority carrier life time. If the carrier life isreduced, the rise time is also decreased as well as the band width of LED is increasedwhich results in the decrease of power output. The power bandwidth product,

r

1 hCP Jq

where J = Drive current density and

r = Radiative life time.


The P is constant for a given current injection level. Suppose doping in theactive layer is increased, then it reduces the radiative life time which increases theLED band width. If the band width is increased, then the power is decreased. Fasterresponse LEDs have small rise time and hence their radiative life time is reduced. Theband width is increased and the power is reduced. Otherwise, the faster LEDs generallyemit less power than slow response LEDs.

Hetero junction LED structures

There are high radiance surface emitting hetero junction LEDs and edge emittingdouble hetero junction LEDs.

i) Surface emitting LED

Metallization

Substrate

Confinementlayers

Metallizationheat sink

Active layer Metal contact

Fiber

Epoxy

SiOIsolation

2 SiOIsolation

2

n - GaAs

N - Ga Al As wide bandgap

p - Ga As narrow bandgap

P - Ga Al As wide band gap

Figure 4.11: Surface emitting LED

Construction:

N p P structure forms a double hetero junction layers. A layer of Ga As whichhas a narrow gap is sandwiched between two layers of GaAl As which has a widerbandgap and a lower refractive index (Fig. 4.11). These two layers of Ga Al As form asconfinement layers.


This dual confinement gives high efficiency and high radiance. The Ga As layeris the active layer or recombination region and is in the form of circular region. Thisarea is typically 20 to 50 m in diameter. These have high drive currents. The surfaceemitting LED has a. radiation pattern as in the lambertion source which is isotropic witha 1200 half power beam width. It we view the pattern from any direction, it is equallybright.

ii) Edge emitting LED

Stripe contact

SiO isolationlayer

2

Active area

LED output

Metallization

Confinementlayers

SubstrateMetallization

Heat sink

Figure 4.12: Edge emitting LED

Construction:

GaAs forms an active area which is in the form of circular region at the middle ofthe active layer. Ga AI AB layers form the optical confinement or light guiding layerswhose refractive index is lower than that of active region.

The output of the beam is highly incoherent. Lengths of the active regions rangefrom 100 to 150 m.

The emitted beam is lambertion (Fig. 4.12) so that radiance B = B0 cos where Bo is the value of radiance at the centre of the beam.

In the plane perpendicular to the junction, the half" power beam width is verysmall. The emission pattern of the edge emitting diode is more directional than that ofthe surface emitting LED. Drive current is about 100 mA with output optical powerabout 0.5 W to 10 W. Manufacturing of edge emitting LEDs is more expensive.Modulation bandwidth is about 500 kHz with rise time of 600 to 800 ns.


In Ga As P and Ga As P are used as LED materials at 1.3 m. Ga Al As is usedas a LED material at 0.85 m having smaller rise time about 2 to 20 ns and bandwidthis greater than 200 MHz. The spectral width is about 50 nm at 0.85 m wavelength and70 nm at 1.3 m wavelength.

General properties of LEDs

The life time of LED is about 105 hours. The edge emitters have lower drivecurrent than the surface emitters. If the drive current is 100 mA, the optical outputpower is in between 0.5 W to 10 W. The manufacturing of edge emitting LED ismore expensive than the surface emitting LED. The modulation band width of LED iscomparatively low (~ 500 kHz) with rise time of 600 to 800 ns.

But In Ga As P edge emitting LED at 1.3 m and Ga Al As edge emitting LED at1.3 m and Ga Al As edge emitting LED at 0.85 m have smaller rise time (2 to 20 ns),Thus the band width is greater than 200 MHz. The band width can be further increasedby reducing the optical output. The spectral width of surface emitting LEDs is about1.3 nm and so the fiber bandwidth is reduced. For edge emitting LEDs the spectralwidth is more such that it is 50 nm at 0.85 m wavelength and 70 nm at 1.3 urn wavelength.

LEDs in computers

LEDs are used in computers for display purposes. Mostly homo junction LEDsare used.

Construction of LED

A shallow p-n junction is formed and electrical contacts made to both regionswhilst leaving as much of the upper surface of the p material uncovered so that the flowof radiation from the device is impeded as little as possible. The p-n junction is surroundedby plastic encapsulation since the reflection losses at the semiconductor-plastic interfaceare lesser than for a corresponding semiconductor - air interface.

p

n

Substrate

Al

SiO2

Elec

trica

l con

tact

s p-njunction

Plasticencapsulation

Electrical contacts

Figure 4.13: LED Structure


If we take Ga As/air, when n1 = 3.6 and n2 = 1

Then fractional transmission, F = 13.0nnnn1

nn

41

2

21

212

1

2

But with Ga As / plastic where n1 = 3.6 and n2 = 1.5, F = 0.036

There is a three fold increase of light transmission.

LEDs are used extensively in segmental and dot matrix displays of numeric andalpha numeric characters. Several LEDs are used in series to form one segment whilea single LED may be used to form a decimal point.

4.7 SEVEN SEGMENT DISPLAYSeven segment displays are the most commonly used numeric display devices. A

number of LED’s may be grouped to form a display as illustrated in figure. Actuallyseven segment display consists of seven LED’s which can be switched ON and OFFindividually to display a range of pattern. From the figure, it is clear that when theLED’s a,b,g,e and d conducts, the number 2 is displayed. Similarly the combinationa,b,g,c and d corresponds to the number 3 and so on.

a

g

d

f

e

b

c

Figure 4.14

The seven segment display is the popular one used in calculator and other displaysystems such as clocks, wrist-watches, measuring instruments, etc.

4.8 ADVANTAGES OF LEDS IN ELECTRONIC DISPLAYS1. LEDs are miniature in size and any number can be stacked together in a small

space to form numerical display.

2. The light intensity of LEDs can be controlled easily by varying the currentflow.


3. LEDs are rugged and can therefore withstand shocks and vibrations.

4. They can be operated over a wide range of temperatures from 0 to 70ºC.

5. They are very fast in operation. They can be switched on and off in a timeless than 1 nanosecond.

6. LEDs are available such that they emit light in different colours like red, green,yellow and amber.

7. It has long life and has a high degree of reliability.

8. It has low drive voltage and low noise.

9. Unlike liquid crystal display, the angle of viewing is not limited.

10.The brightness of these displays is sufficient for any environment.

Disadvantages of LEDs in electronic displays

1. These are not suited for large area display because of their high cost.

2. When we compare with liquid crystal display, LEDs consume more energy.

Table 4.2: Different LEDs for visible light emission

Colour and Material Bandgap LED structurewave length (nm)

Blue, 450 Ga N direct heterojunction

Blue, 480 Si C indirect homojunction

Green, 555 Ga P indirect homojunction

Yellow green, 570 Al In Ga P direct heterojunction

Yellow, 585 Ga As P:N indirect homojunction

Amber, 595 Al In Ga P direct heterojunction

Red, 630 Ga As P:N indirect homojunction

Red, 650 Ga As P direct homojunction

Red 700 Ga P:Zn O indirect homojunction.


4.9 LIQUID CRYSTAL DISPLAY (LCD)Liquid crystal displays (LCDs) is one of the important passive types of display.

Liquid crystals are the intermediate phases between liquid and crystal. Liquids havefluidity and isotropic properties. Crystals have rigidity, lattice structure and anisotropicproperties. Liquid crystals have orientational order but lack of positional order withfluidity and anisotropy in dielectric constant. 4 Methoxy - 4’ n - Butyl BenzylideneAniline(MBBA) molecules are used as LC display material. It can act as a liquid crystal between21°C to 48°C only.

So we can operate LC displays only in this temperature range. It has elongatedrod like structure. There are three phases in liquid crystals: Smetic, Nematic andCholestric: Smetic phase molecules are cigar shaped and are arranged in layers; moleculescan move forward and backward and not up and down. i.e. Two dimensional motioncan be possible like a soap bubble. Nematic phase molecules have no layered structure.

But all the molecular axes are parallel to each other. This can be thought of asone dimensional liquid. Cholestric phase molecules are plate like shaped and there is astack of thin layers. As one moves down the stack, direction of orientation rotates in themanner of a screw. Crystals in this phase possess double refraction.

General properties

1. Liquid crystals are very sensitive to environmental conditions like temperature,mechanical stress and electric fields. Any change in these parameters willchange the optical properties of liquid crystals.

2. Cholestric phase can change the colour of radiation when the temperature ischanged. This fact is used to measure temperature of the children as well asused in thermography to detect breast cancer.

3. Nematic phase is widely used for displays. This is based on change of planeof polarisation during the application of electric field.

Another important liquid crystal is phenyl cyano biphenyl which has pnematic formbetween 18°C to 35°C. The liquid crystal displays are of two types:

1. Dynamic scattering display

2. Twisted nematic field effect display.

Now - a - days the dynamic scattering display is not used because it has shorterlife time, high power consumption and its operating voltage is around 15 V. It has beenused upto 1975. Let us see the twisted nematic field effect display system:


The liquid crystal material is kept in between two glass plates which are coatedwith transparent tin oxide and they can act as electrodes. The top glass plate is rotatedthrough 90°. Therefore the liquid crystal molecules are also twisted through 90° whenthere is no applied electric field (Figure 4.15(a)).

Above the top glass plate we have polarizer and below the bottom glass plate wehave analyser in crossed positions. Under no field conditions when light passes throughthe liquid crystal cell through the crossed polariser and analyser the cell appears brightdue to additional phase difference introduced by twisting.

AnalyserBright

Light

Polariser

Light

Polariser

AnalyserDark

Twisted nematicliquid crystalmolecules

(a) (b)

Figure 4.15: Twisted nematic field effect display

When we apply electric field, the liquid crystal molecules are all oriented parallelto the field direction and there is no twisting of molecules (Figure 4.15(b)). So the cellappears dark due to the crossed polariser and analyser. Normally we are usingtransparent type display in which the digits are appearing as black coloured ones inwhite back ground.

Advantages of Liquid Crystal Displays

1. It reflects light instead of generating light. So it can be viewed under a widerange of lighting conditions.

2. It requires relatively little power since it is not generating light. For example,in the case of LED display of 1 cm2 display area, the power consumption isabout 1 watt. But for liquid crystal display of 1 cm2 display area, the powerconsumption is less than 1 microwatt.


3. Since liquid crystal display can be in the form of thin films, the liquid crystaldisplay device has very small thickness.

4. It is very cheap.

Disadvantages

1. Angle of viewing is limited (maximum 45° only).

2. Contrast is poor when we compare it with LED.

3. It can be viewed only when there is some light. Otherwise it cannot be viewed.

4. It is highly sensitive to environmental conditions like temperature change,electromagnetic interference, etc.

5. Working temperature range is small.

Table 6.4 shows the comparison between the liquid crystal display and LED display.

Table: 4.4: Comparison between LCD and LED displays

S.N LCD LED display

1. Low power consumption (µW) High power consumption (m W)

2. Suitable for large area display Not suitable for large area display.

3. Lesser cost More cost

4. Angle of viewing is limited Angle of viewing is not limited.

5. At present colour displays are not Any colour display can be madeavailable

6. Moderate dynamic response (µs) Fast dynamic response (n s)

7. Operating temperature is limited Operating temperature is from(10°C to 45°C) 0°C to 70°C

8. Intensity of light can not be controlled Intensity of light can be controlled.


SOLVED PROBLEMS1. Calculate the wavelength of emission from Ga As whose bandgap is 1.44 eV.

Å8628106.144.1

10310626.6EhC

19

834

g

2. The In Ga As P diode laser has peak emission wavelength = 1.55 µm.What is its bandgap in eV?

eV8.055.124.1

)m(24.1)eV(Eg

3. If the quantum efficiency at 1.3 µm wavelength of light is 0.65 and the numberof photons incident at this wavelength is 5 × 105, what is the number of electron-hole pairs generated?

photonsincident ofnumber generated pairs holeelectron ofnumber

number of electron-hole pairs generated = 0.65 × 5 105

= 3.25 × 105

4. At 1.3 µm, the quantum efficiency of In Ga As photodetector is 0.6. Calculateits responsibility.

W/A628.010310625.6

103.1106.16.0hCq

hqR 834

619

5. Define multiplication factor of avalanche diode. A silicon avalanche photodiodehas quantum efficiency of 0.7 at wavelength 863 nm. Suppose 0.5 µW of opticalpower produces avalanche multiplied current of 10µA, calculate the avalanchemultiplication factor.

Multiplication factor = P

T

IIM

where IT = total multiplied output current and

IP = primary unmultiplied photo current.


Now IP = R P0 = 00 PhCqP

hq

= 834

6919

10310625.6105.010863106.17.0

= 0.243 µA.

M = 41A243.0

A10II

P

T

6. A silica optical fiber has a core refractive index of 1.50 and a cladding refractiveindex of 1.47. Determine, (a) the critical angle at the core cladding interface,(b) the N.A. for the fiber and (c) the acceptance angle in air for the fiber.

The critical angle, c = 5.7850.147.1sin

nnsin 1

1

21

The numerical aperture N.A. = 30.047.1–50.1n–n2/1222/12

221

The acceptance angle in air, max = sin–1 N.A. = 17.4°.

7. Calculate the total number of guided modes propagating in the multimodestep index fiber having diameter of 50µm and numerical aperture of 0.2 andoperating at a wavelength of 1 µm.

490101

2.010509.4.A.Nd9.4N2

6–

6–2

Hence the fiber can support approximately 490 guided modes. In the case of gradedindex fiber, the number of modes propagated inside the fiber = (Nstep/2) = 245 only.

8. Suppose a fiber has a diameter of 5µm and its core refractive index = 1.450and cladding refractive index = 1.447. If the wavelength of propagation is1 µm, how many modes can be propagated inside the fiber?

N = 4.9 × 1101

)447.1–450.1(1059.4.A.Nd2

6

2/12262

Therefore there is a single mode propagation.


9. The relative refractive index difference for an optical fiber is 0.05. If theentrance end of fiber is facing the air medium and refractive index of core is1.46, estimate its numerical aperture.

N.A = n1 (2)1\2 = 1.46 (2 × 0.05)1/2 = 0.46.

10. Calculate the Vnumber and number of modes propagating through the fiber havingn1 = 1.53, n2 = 1.50 and 0 = 1µm.

Vnumber = 2/122

21

0n–na2

= 72.941

)50.1–53.1(502 2/122

Maximum number of modes propagating through the fiber .44862

)72.94(2

V 22

11. A step index fiber has a diameter of 200 µm and N.A = 0.3. Compute thenumber of propagating modes at an operating wavelength of 850 nm.

Number of modes 222

22

)A.N(a22

V

modes 2458910850850

(0.3)10100(3.14)218

21222

The number of modes is doubled to account for the two possible polarisations.

Total number of modes = 24589 × 2 = 49178.

12. Find the cutoff wavelength of the step index multimode fiber havingcore radius = 25 µm, refractive index of core = 1.48, Vnumber = 25 and = 0.01.

V2na2 1

cutoff

= .mµ315.125

01.0248.11052 6

13. In the design of single mode fiber, find the value of maximum value of coreradius the fiber having N.A. = 0.05 and the operating wavelength and theoperating wavelength = 1.3 µm.

We know that Vnumber of single mode fiber = 2.405

µm95.905.02

3.1405.2.)A.N(2

Vamex

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Opto Electronic Devices - drmgriyearbtech.com · absorbed photons make electron hole pair in the depletion region. When an electron-hole pair is generated due to the incident radiation,