.ADVANCED ELECTRONICS

UNIT I - OPTOELECTRONIC DEVICES

INTRODUCTION

The ancient Greeks speculated on the nature of light from about 500 BC. The practical interest at that time centred, inevitably, on using the sun’s light for military purposes; and the speculations, which were of an abstruse philosophical nature, were too far removed from the practicalities for either to have much effect on the other. In fact, the fundamental atomic processes of nature are not describable in these same terms and it is only when we try to force them into our more familiar frame work. Such as the wave-particle duality of Electrons and Photons arise. Electrons and Photons are neither waves not particles but are entities whose true nature is somewhat beyond our conceptual powers.

In Optoelectronics devices the following properties are very important:

The wave nature of light:

The frequency of the wave described by equation F = ω2πAnd wave length by λ = 2π

kwhere

ω = angular frequency,k = propagation constant

Velocity of light

C = fλ = ωkThe wave nature of light will be analysed by Maxwell’s equations.

Polarization: It is customary to fix attention on the electric field for purposes of general

electromagnetic wave behaviour, primarily because the effect of the electric field on the electrical charges within atoms tends to be more direct than that of the magnetic field.

The symmetry which exists between the E and H fields of the electromagnetic wave means that conclusions arrived at for the electric field have close equivalence for the magnetic field.

It is simply convenient only to deal with one of them rather than two.

The electromagnetic spectrum:In practice, since electro – magnet wave sources cannot be markedly smaller than the wave length of the radiation.

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Visible radiation lies in the range 400 – 700 nm (1 nm = 10-9 m)The infrared region of the spectrum lies just beyond 700 nm to 300 000 nmThe ultraviolet region lies below 400 nm and begins at about 3 nm.

Classification of Opto Electronics Devices:

It is simplest form a p-n junction diode can be constructed as a homojunction using abrupt change between p and n-type regions in a single piece of semiconductor.

The p and n-regions might initially be considered to be separated slabs of material when these are placed in intimate contact. The difference in hole concentration between the p and n-type regions should result in the diffusion of holes. In the same way, electrons ought to diffuse in the reverse direction from the n-type material into the p-type. If the two carriers types meet near the interface, they will recombine. The effect should then be that a layer known as depletion region.

The band diagram for a p-n junction:

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Some features will be expected. The relative spacing’s of the Conduction band. The Fermi level The valence band

Inside the depletion region, the band structure will clearly be modified.

Heterojunction Diodes: Instead of metal-semiconductor junction, alternatively different semiconductors may be used. It must be feasible to grow the layered structure as a perfect crystal, which requires the two materials to be lattice matched. Despite these restrictions, it is entirely possible to fabricate diodes with different band-gaps on either side of the Junction. In this case the structure is known as a heterojunction.

Features of Heterojunction diodes:

The difference in refractive index between the two materials. The modification to the energy band diagram introduced by the variation may be used

to provide different potential barriers to the motion of electrons and holes.

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LASER DIODES:

Laser is acronym for Light Amplification by Stimulated emitted of radiation.

Require of light sources: Light output should be highly direction. It must emit light at required wavelengths. To couple large amount of power into an optical fibre, the emitting area should be

small. It must require very small power for its operation. The light source should have compact size and high efficiency. High optical output power and coupling efficiency. Ideal laser light is single-wavelength. This is related to molecular characteristics of

material being used in the Laser. For optical fibre systems the laser sources used for almost exclusively are

semiconductor Laser diodes. The output radiation is highly monochromatic and the light beam is very directional.

Principle of operation:

(i) Photon absorption(ii) Spontaneous emission(iii) Stimulated emission

(i) Photon absorption:When photon with energy E2-E1 is incident on the atom. The atom is initially in E1. The atom excited into the higher energy state E2 through absorption of photon.

(ii) Spontaneous emission:Spontaneous emission, an atom returns to the lower energy state in random manner. It gives incoherent radiation.

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(iii) Stimulated emission:By Stimulated emission, when a photon having equal energy to the difference between the two states (E2-E1) interacts with the atom causing it to the lower state with the creation of the second Photon. It gives the coherent radiation. Coherent mean, when an atom is stimulated to emit light energy by an incident wave, the liberated energy can be added to the wave.

LASER DIODE MODES:

Semiconductor Laser diodes are preferred over LED for the optical fibre communication systems requiring BW greater than approximately 200 MHz Laser diodes have

Response time less than 1hs. Optical BW of 2nm or less High coupling efficiency Laser diodes multi-layered. Smaller temperature dependence.

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Lasers are oscillators operating at optical frequency the oscillator is formed by a resonant Cavity providing Selective Feedback.

This cavity is much smaller being approximately 250-500 um long. 5-15um wide, 0.1-0.2um thick. These dimensions are referred to as the longitudinal lateral and transverse dimensions of the cavity.

The purpose of reflecting mirrors is to provide strong optical feedback in the longitudinal direction.

The two Heterojunctions provide carrier and optical confinement in a direction normal to the junction. The current at which lasing starts is the threshold current. Above this current the output power increases sharply.

Distributed feedback laser (DFB):

In DFB laser the lasing action is obtained by periodic variations of refractive index which are incorporated into the multiplayer structure along the length of the diode.

Modes:

Longitudinal modes Lateral modes Transverse modes

Lasing Conditions

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E(z,t) = I(z)ej(ωt-βz)

Where, ω is the optical radian frequency β is the propagation constant The condition to just reach the lasing threshold is the point at which the optical gain is equal to the total loss at in the Cavity.

Optical gain at threshold = Total loss in the Cavity (∝t)gth = β Jth

Jth = 1β ( ∝+ 1

2 Lln1/R1R2 )

Resonant Frequency:

Ej2βL = 1 ---------------------------------------------(1)

Here 2βL = 2πm (m --- integer)----------------(2)

β = 2πnλ ---------------------------------------------(3)

m = βLπ --------------------------------------------(4)sub (3) in (4)

m = Lλ/2n------------------------------------------(5)

λ = cv -------------------------------------------(6)

m = 2L nvc -------------------------------------------(7)

PHOTOCONDUCTIVE CELLS:

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Applications:Photoconductive cells are used in many different types of circuits and applications.

Analog Applications:• Camera Exposure Control• Auto Slide Focus - dual cell• Photocopy Machines - density of toner• Colorimetric Test Equipment• Densitometer• Electronic Scales - dual cell• Automatic Gain Control - modulated light source• Automated Rear View Mirror

Digital Applications:• Automatic Headlight Dimmer• Night Light Control• Oil Burner Flame Out• Street Light Control• Absence / Presence (beam breaker)

• Position Sensor

Pin-Photodiode:

The basic limitation of a p-n junction Photodiode – that the depletion layer is so thin that radiation of long wavelength.

A region of intrinsic or lightly doped material is introduced between two heavily doped p and n-type regions.

The effective depletion layer width may therefore be fixed at a value far greater than the natural one approximately the width of the intrinsic region.

The n- region is a mesa of GaInAs

In this device, light enters through the InP Substrate, which has an energy gap of 1.35ev and so is transparent to wavelength longer than about 0.92um.

The quantum efficiency is then almost uniform between 1.0 and 1.6 um just below GaInAs.

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Avalanche Photodiode:

In the Avalanche photodiode impact ionisation is used as a method of multiplying the photo current before it enters the circuit of any electrical amplifier.

RAPD is a device with p+ -i -p-n+ structure. Under reverse bias, most of the applied voltage is dropped across p-n junction.

As the voltage is increased, the depletion layer associated with this junction widens, until it just reaches through the intrinsic region.

In the depletion region, the field is extremely high. Normally the peak field is held at 10% below the value at which avalanche breakdown occurs.

The photo generated carriers drift in the moderate electric field, with electrons travelling towards the p-n+ junction.

Carrier multiplication then takes place inside the high field region by impact ionization.

However, since the avalanche process is a statistical one, there is a corresponding increase in the noise level.

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Photovoltaic cell (or) Solar cell:

Scientific base for solar pv electric power generation is solid state physics ofSemiconductor.

Silicon is a popular candidate material for a solar pv cells because, It is a semiconductor material Technology is well developed to make silicon to be +ve (or) –ve charge carriers –

essential elements for an electric cell (or) battery. Silicon is abundant in supply and relatively inexpensive in production

Micro and nano technologies have enhanced the opto-electricity conversion efficiency of silicon solar pv cells.

Working principle of Silicon Solar p-v cells:

Photovoltaic material of device converts light into electric energy. Silicon solar pv cells is a device made up of semiconductor materials that produce

electricity and light. A p-n junction is created in Silicon by a doping process.

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The Photons from the exposed light prompted electrons flowing from n-junction to the p-junction.

The Solar cell is composed of a p type semiconductor and n-type semiconductor. Solar light hitting the cell produces two types of electrons, (+ve) and (-ve) When we connect loads such as light electric current will flow between the two

electrodes.

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Energy Payback Time:

EPBT is the time necessary for a photovoltaic panel to generate the energy equivalent to that used to produce it.

A ratio of total energy used to manufacture a pv module to daily energy of a pv system.Common solar cell materials:

SINGLE CRYSTALLINE POLY CRYSTALLINE

Silicon (Si)GaAs Cadmium telluride (CdTe)

Copper indium diselenide(CIs)

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Types of Solar pv cells:

Flat plate systems: On rigid flat surfaceUsually from single wafers from 300 to 250 to 200 μmArea : 170 cm2 approximatelyOutput power : 1-2w approximatelyOutput voltage: 0.5v approximately

Concentrator systems: With optical components, eg: lenses to direct and concentrate sunlight on the pv cells of small areas.Involving tracking mechanisms for detecting the sunlightCan increase power flux of sunlight hundred of timesHeat dissipation required.

LASER RANGE FINDER:

A laser range finder is a range finder which uses a laser beam to determine the distance to an object.

The most common form of laser range finder operates on the time of light principle. Due to the high speed of light this technique is not appropriate for high precision sub

millimetre measurements. Some of the laser light might reflect of leaves (or) branches which are closer than the

object giving an early return and a reading which is too low.

Calculation:

D = ct2 between A and B pointc speed of lightt amount of time for round trip.

t = φω -----------------------(1)eq Phase delay ω angular frequency

D = 12ct

Sub eq 1 here, D = 1

2 cφω

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Technologies:

Time of light: This measures the time taken for a light pulse to travel to the target and back with the speed of light known and an accurate measurement of the time taken.

Multiple frequency phase shift: This measures the phase shift of multiple frequencies on reflection then solves some simulations equations.

Interferometry: The most accurate and most useful technique for measuring changes in distance rather than absolute distances.

Applications: Military 3-D modelling Forestry Sports Industrial production processes Laser measuring tools.

Light Activated SCR:

The Light Activated SCR is also known as Light triggered thyristor (LTT)It may be triggered with a light source (or) with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCRThe Light Intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage bias, lower the light intensity required.These devices are available up to the 6kv and 3.5kvThe symbol for a light Activated SCR(LASCR) is shown below

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Basic operation of LASCR:

i. When no light is present, the LASCR is OFF, no current will flow through the load.ii. However, when LASCR is illuminated, it turns ON, allowing current to flow through

the load.iii. The resistor in this circuit is used to set the triggering level of the LASCR.iv. Resistors can be added between the Gate(G) and Cathode(C) to reduce its

susceptibility to noise and dvdt effects, but this degrades its sensitivity to light

triggering.v. When photons from a light source collide with electrons within the p-type

semiconductor, they gain enough energy to jump across the p-n junction energy barrier.

vi. Even when the photons are eliminated, the LASCR will remain ON until the polarity of the anode and cathode are reversed or the power is cut.

Applications of LASCR:

High voltage current applications. High voltage anode-cathode circuits. High voltage Direct Current (HVDC) Transmission.

Optical Isolator:

An Optical isolator, or optical diode is an optical component which allows the transmission of light in only one direction

It is typically used to prevent unwanted feedback into an Optical oscillator. The operation of the device depends on the Faraday effect. The main component of the optical isolator is Faraday rotator. The magnetic field B, applied to the Faraday rotator causes a rotation in the

polarization of the light due to the Faraday effect. Angle of rotation β = vBd Faraday isolator is made up of three parts, they are

input polarizerFaraday rotatorO/P polarizer

it is also known as polarization dependent isolator. The most important optical element in an isolator is the Faraday rotator. The characteristics that one looks for in a Faraday rotator optic include a

Verdict constant Low absorption coefficient Low non-linear refractive index

For long distance fibre communication, typically at 1310nm (or) 1550nm yttrium iron garnet (YIG) crystals are used.

Optical isolators are different from ¼ wave plate, base isolators.

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The polarization rotation due to the Faraday rotator is always in the same relative direction.

In the forward direction the rotation is +45 ° In the reverse direction the rotation is -45 °.

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