UNIT-III

SOURCES AND DETECTORS

DIRECT AND INDIRECT BAND GAP SEMICONDUCTORS:

According to the shape of the band gap as a function of the momentum, semiconductors are

classified as

1. Direct band gap –semiconductors

2. Indirect band gap semi conductors.

(a) Direct band gap semiconductors

In direct band gap semiconductors, the electrons at the bottom of the conduction band and

the holes at the top of the valence band on the either side of the forbidden energy gap have the

same value of the crystal momentum. The direct recombination between the electrons in the

conduction band and holes in the valence band takes place. When the electron hole recombination

takes place, the momentum of electrons and holes, remains the same and band gap energy is

emitted. As light. Fig 3.1 shows the diagram of electron energy versus momentum for direct band

gap semiconductor.

(b) Indirect band gap semiconductor

In Indirect band gap semiconductor, the conduction band minimum energy level and valence band

maximum energy level occur at different values of momentum. When an electron recombines with a

hole, the electron must lose some momentum so that it has the same momentum corresponding to

the energy maximum of the valence band. The conservation of momentum during, the

recombination process requires the emission of .a third particle known as phonon with momentum

zK where zSK is the difference in momentum between the minimum energy level in conduction band

and maximum energy level in valence band.

Fig. shows the diagram of electron energy versus momentum for indirect band

gap semiconductor.

OPERATION OF LED :

LED is a p-n junction diode made from a semiconductor material such as aluminium gallium-

arsenide (Al GaAs) or gallium-arsenide-phosphide (Ga AS P). It emit light by spontaneous emission

i.e. light is emitted as a result of the recombination of electrons and holes. When the LED is forward

biased, electrons corss the pn junction from the n type material and recombine with holes the p type

material. These free electrons ire in the conduction band and at higher energy level than in the holes

in the valence band. When recombination takes place, the recombining electrons release energy in

the form of heat and light. A large surface area on one layer of the semiconductor material permits

the photons to be. emitted as visible light. Various impurities are added during the doping process to

establish the wavelength of emitted light.

DOUBLE HETEROJUNCTION LED

It is formed when a layer of material with. a particular band gap energy is sandwiched with a layer of

material having a higher band gap energy. This is called double hetrojunctions because there are two

hetrojunctions placed on each side of the active material.

The DHLED structure consists of a p type Ga As layer sand witched between a p type Al Ga As layer

and an type Al Ga As layer. When a forward bias voltage is applied the electrons from the n

layer move into the p type layer and combine with holes which are the majority carriers in the

central p-type GaAs layer

Due to the recombination of electrons and holes, photons with energy corresponding to bandgap

energy Eg of p type GaAS layers are produced. The injected electrons are not allowed to diffuse into

the p type Al Ga As, because of the presence of the potential barrier. Hence we get electro

luminescence in a Ga As function which gives good internal efficiency as well as high radiance.

Fig. (a) Layer structure for double hetrojunction LED with an applied forward bias.

SURFACE EMITTING LED (SLED) AND AN EDGE EMITTING LED (ELED):

Ans. Surface emitting LED (SLED) : SLED operates at 850 nm wavelength. SLED is a five layered

double hetrojunction on device consisting of a GaAs and GaA1As layers. The plane of the active light

emitting region is oriented perpendicularly to the axis of the fiber. From the substrate of the device,

a well is etched. Fibers are cemented in the well to accept the emitted light. The circular active area

in practical surface emitters is normally 50urn in diameter and up to 2.5 urn thick. SLED has a low

thermal impedence in the active region which allows high current densities and gives high radiance

emission into the optical fiber. The isotropic pattern from a surface emitter LED is lamberitian

pattern in which source is equally bright when viewed from any

Edge Emitter LED: It consists of an active junction region which is a source of incoherent light and

the two guiding layers. The refractive index of both guiding layers is lower than that of the active

region but higher than refractive index of the surrounding material. Length of the active regions

range from 100 to 150 4um. The emission pattern of edge emitter is more directional than that of

surface emitters. Most of the propagating light is emitted at ore and face due to a reflector on the

other end face and an antireflection coating on the emitting end face.

RADIATIVE AND NON RADIATIVE RECOMBINATION EFFECTS:

Radiative Recombination: In radiative recombination when an electron- hole pair recombine a

photon of energy liv which is approximately equal to band gap energy is emitted.

Non Radiative Recombination: In non radiative recombination when an electron hole pair recombine

the energy is released in the form of heat. In this optical absorption occurs in active region. Non-

radiative recombination includes recombination at traps or defects, surface recombination and the

Auger recombination. In Auger recombination, the released energy during electron hole

recombination is given to another electron or hole as kinetic energy rather than producing light.

INTERNAL QUANTUM EFFICIENCY OF LEDS AND LED POWER AND EXTERNAL QUANTUM

EFFICIENCY:

The internal quantum efficiency in the active region is the fraction of electron

hole pairs that recombine radiatively. It is the ratio of the radiative recombination rate to total

recombination rate

External quantum efficiency: It is defined as ratio of photons emitted from LED to number of

internally generated photons.

To find external efficiency take into account reflection effects at surface of LED.

ADVANTAGES AND DISADVANTAGES OF LASERS:

LASER is an acronym for “Light amplification by the stimulated emission of radiation”. It amplifies or

generates light by means of the stimulated emission c radiation.

Advantages

(1) Ideal Lasers generate light of single wavelength only.

(ii) Lasers can be modulated very precisely.

(iii) Lasers can produce relatively high power.

(iv) High percentage of laser light can be transferred into the fiber.

Disadvantages

(i) Lasers are expensive as compared to LEDs.

(ii) The wavelength that a laser produces is a characteristic of the material use to build it and of its

physical construction.

(iii) Amplitude modulation using an analog signal is difficult will most laser because laser output

signal power is generally non linear with input sign power.

SPONTANEOUS AND STIMULATED EMISSION OF RADIATION

(a) Absorption

(b) Spontaneous emission

(c) Stimulated emission.

Consider an atomic system having two energy state E1 and E2. Let an atom is initially in lower energy

state E1. When a photon with energy (E2 - E1) is incident on the atom it may be excited into the

higher energy state E2 through absorption of the photon.

Alternatively when the atom is initially in higher energy state E2 which is unstable

state, it can make transition to lower energy state E1 providing the emission of a photon in two

ways:

(a) By spontaneous emission

(b) By stimulated emission.

(a) Spontaneous emission: In spontaneous emission atom returns to its lower energy state in

random manner. In this emission photons are radiated in arbitrary directions. Very few photons

create light in the desired direction. Photons propagate within a wide cone thus yield widespread

radiated light. Radiation of photons is independent of other photons i.e. no phase correlation exists

between different photons and total radiated light is incoherent. Spontaneous emission occurs in

LED which has very wide spectral width.

(b) Stimulated emission: When a photon having an energy equal to the energy difference between

the two states (E2 — E1) interacts with the atom in the upper energy state, it causes the atom to

return to lower energy, state with the emission of second photon. In stimulated emission an external

photon stimulates the induced emission.

The stimulated photon propagates in the same direction as the photon that stimulated it so the

stimulated light will be well directed. The stimulated radiation is coherent as stimulated photon is in

time alignment with external photon. All stimulated photons propagate in the same direction and

contribute to output light so the sources having stimulated emission have high current to light

efficiency. LASER generally uses stimulated emission of radiation.

DISTRIBUTED FEEDBACK LASER (DFB) AND VERTICAL CAVITY SEMICONDUCTOR LASERS:

Ans. In case of a junction laser, the essential condition of feedback for lasing is obtained from the

cavity facets formed by cleaving. In DFB laser, feedback is not localized at facets but is distributed

throughout the cavity length. This is achieved by using an corrugated structure or grating which

leads to a periodic variation of the refractive index within the cavity along the direction of the wave

propagation. The feedback in the cavity occurs due to the energy propagating in the forward

direction is being continuously feedback into the opposite direction by Bragg diffraction at the

corrugation or grating. Laser using such corrugations as feedback elements are known as

“Distributed feedback” laser or Distributed “Bragg reflector” lasers.

In DFB’s, the main design objective is to generate a single line spectrum at the output, under high

data rates of modulation. A grating is etched along the cavity length on the surface of a cladding

layer. The grating leads to an effective spatial modulation of the refractive index which contributes

to the device feedback mechanism so that single mode is produced and undesirable modes are

suppressed.

(b) Distributed Bragg Reflectors (DBR)

The fundamental difference between DFB and DBR lasers in their grating mechanisms. In DFB, the

grating is along the cavity length while iii DBR, the grating is at both ends of the active region. When

it is at both ends, it can act as a perfect

Optical mirror because of the difference between the constant refractive index of the active layer

and the continuously changing refractive index of the grating layer. This provides a required

feedback mechanism for optical power generation and spectral purity. DBR devices require higher

threshold current than DFB structures.

Vertical cavity semiconductor Lasers (VCSELS)

VCSELS emit light from their surface instead of the edge as in DFB lasers. The difference between an

edge emitting laser and a VCSEL is that the laser cavities or the resonators are placed above or below

the active layer, so that the light is emitted

Perpendicular to the active layer. Several quantum wells are built into the active region and (DBRs)

act s a highly reflective mirrors which provides a positive feedback mechanism for lasing action. A

VCSEL operates in a single mode. Its small size resonant cavity yields lower power consumption and

higher switching speed. The light output beam from VCSEL is circular. It offers number of

advantages. It has lower threshold current, high light efficiency, easy manufacturing and packaging.

LASER P1 CHARACTERISTICS AND EXTERNAL QUANTUM EFFICIENCY

Laser P-I characteristics: The output power of the laser due to an applied current is given by

ADVANTAGES OF SEMICONDUCTOR INJECTION LASER.

Advantages of semi conduction injection laser are

(i) Injection lasers are able to provide several mill volt of optical power.

(ii) The line width of the radiation is considerably narrow of the order of mm or less.

(iii) It possess good spatial coherence which allows the output to be focused by an optical lens into a

spot which has a greater light intensity than a unfocused emission.

(iv) The, modulation capabilities extends up to several Mega Hertz.

Q. 2. Define quantum efficiency and responsively of a photo detector. How does the responsively

depend on the quantum efficiency of the device and wavelength of the incident radiation.

Ans. Quantum efficiency: Quantum efficiency is defined as the ratio of number of electrons collected

to the number of incident photons. It is also defined as the fraction of incident photons which are

absorbed by photo detector and generate electrons which are collected at detector terminals.

All the incident photons are not absorbed to generate electron hole pairs therefore quantum

efficiency is generally less than one. It depends on the absorption coefficient of the semiconductor

material used within the photo detector. Responsivity.

Responsivity represents the sensitivity of a photo detector. The function of photo detector is to

convert the optical signal into electrical signal. When the incident on semiconductor material has an

energy greater than band gap energy then an electron-hole pair is generated each time a photon is

absorbed by semiconductor. More photons that strike the photo detector, more charge carriers will

be produced i.e. greater will be the photo current I.,, i.e. photo current is directly proportional to

incident optical power Pm.

From above equation responsivity is directly proportional to quantum efficiency at particular

wavelength. An ideal responsivity versus wavelength quantum efficiency is shown in fig . for a silicon

photo diode having unit 4.1.

Responsivity of photo detector increases with wavelength because more photons are present for

same optical power. For photons having energy have less than band gap energy E, the linear

dependence of responsivity on wavelength does not continue because photon energy becomes too

small to generate electrons. The quantum efficiency drops to zero.

P-N PHOTODIODE:

P-N photodiode is a reverse biased P-N junction diode with light permitted

to fall one one surface of device across the junction keeping remaining sides unilluminated.

Fig. shows basic structure of a photo diode.

Fig. shows PN photodiode with depletion and diffusion region.

A reverse biased P-n junction consists of a region known as depletion region which is devoid of

free charge carriers. There is large electric field across depletion region which opposes flow of

electrons from n side to p-side and holes from P to side to n side. A reverse biased P-N junction

diode has small amount of reverse saturation current due to thermally generated electron-hole

pairs. When P-N junction is illuminated with light on one side, electron hole pairs are created

through absorption. The photo induced electrons in conduction band of P type will move across the

junction to n-side and holes produced in valence band of N type will flow across junction to P side.

This process of diffusion and rapid crossing of depletion region takes place so rapidly that there is

little possibility of recombination. The resulting flow of current is proportional to incident optical

power.

Fig. 4.4(a) Variation of Optical Power inside the photodiode. As shown in Fig. 4.4(a) optical power

decreases exponentially as the incident light is absorbed inside the depletion region.

Fig. 4.4( b ) shows the curves between the photo current versus reverse biased voltage with light

intensity as parameter

The bandwidth of a P-n photodiode is limited by the transit time

The width of depletion region depends on the concentration of acceptor and donor impurities.

Drift velocity depends on the applied voltage.

Disadvantages. When the light strikes P-N junction photodiode. Some of the photons enters into

the n region. These electron-hole pairs are not affected by the field across the junction and do not

contribute to the photo current. The conversion efficiency of P-N junction photodiode is low but

they respond very quickly to any change of light intensity.

Detection process in P-n junction photo diode

Consider a reverse biased P-N junction photodiode. Due to reverse biasing, a thick depletion

layer develops on the either side of the junction. The large potential

barrier across the depletion layer prevent the majority carriers to cross the junction. Suppose a

photon of light is incident in or near the depletion region. If the incident photon has energy hv equal

to or greater than the bandgap energy Eg of the semiconductor material of the P-N junction, the

photon will excite an electron from valence band to conduction band. This, process will generate an

“electron-hole” pair as shown in Fig. 4.5. This is known as photogeneration. The photogenerated

electron- hole pairs are separated in the depletion layer and are swept away by the electric field due

to the applied reverse biased votlage. In order to achieve maximum carrier pair generation, the

depletion region should be sufficiently thick so that large fraction of the incident light can be

absorbed.

P-I-N PHOTODIODE:

p-i-n photodiode consists of p and n regions separated by a very lightly deped intrinic (i) region. The

intrinic layer has only a very small amount of dopant and acts as a wide depletion layer. In normal

operation, a sufficiently large reverse bias voltage is applied across the device so that the intrinsic

region is fully depleted of carriers. At longer wavelengths, light penetrates more deeply into the

semiconductor material. To operate at lqnger wavelength, we must have a wider depletion region

which is obtamèd in P-i-n photo diode.

When an incident photon has an energy greater than or equal to the bandgap energy E of the

semiconductor material, the energy of the photon excites an electron from the valence band to the

conduction band. This process produces electron hole pairs. The generated carriers are called

photocarriers.

Light is incident on depletion region so photo generatd carriers are generated in the depletion

region. The high electric field developed across the depletjon region causes the carriers to separate

and to be collected by the reverse biased voltage. This causes a current to flow in the external circuit

which is referred to as photocurrent.

The performance of p-i-n photodiodes can be improved by using a double heterostructure design. In

this intrinsic layer is sandwiched between the p-type and n-type layers of a different semiconductor

whose band gap is chosen such that light is absorbed only in the middle i-layer. For light wave

applications a P-i-n photodiode using In GaAs for intrinsic layer and In P and 8 surrounding P-type

and n-type layers are used.

AVALANCHE PHOTODIODE :

Avalanche photo diodes amplify the signal during the detection process. They use a similar principle

to that of photo multiplier tubes. In APDS multiplication takes place within the semiconductor

material. An internal amplification of between 10 and 100 times takes place in APDS.

Avalanche photo diodes (APDs) internally multiply the primary signal photocurrent before it enters

the input circuitry of the following amplifier which increases receiver sensitivity. For carrier

multiplication to take place, the photogenerted carriers must traverse a region where a very high

electric field is present. Photo generated electron or whole get energy from high field region and

ionizes the bound electrons in the valence band. This carrier multiplication mechanism is known as

impact ionization. The newly generated carriers are again accelerated by, high electric field and gain

Enough energy to cause further impact ionization. This phenomenon is avalanche effect. Below

diode breakdown voltage, a finite number of carriers are created where as above breakdown infinite

no. of carriers is created.

Silicon Reach through Avalanche photo diode

Silicon Reach though avalanche photodiode is shown in figure 4.8. It is composed of high resistivity P

type material deposited as an epitaxial layer on p + (heavily deposed p type) substrate. A p type

diffusion or ion implant is then made in high resistivity material followed by the construction of a (n

+) layer. This configuration is referred to as reach through structure.

When a low reverse bias voltage is applied across APD, most of the potential drop is across pn+

junction. When we increase the reverse bias, the width of depletion layer increases which increases

the electric field across Pn+ for upto the point needed to cause avalanche breakdown. At this point,

the depletion layer just ‘reaches through’ to nearly intrinsic region.

Photons pass through the n+p junction and are absorbed in the layer. This absorption produces a

free electron in the conduction band and a hole in the valence band. The electric potential across

the layer is sufficient to attract the electrons towards one contact and the holes towards the

other. The potential gradient across the layer is not sufficient for the charge carriers to gain

enough energy for multiplication to takeplace Around the Junction between the n+ and P layers, the

electric field is so mtense that the charge carriers are strongly accelerated and pick up energy, when

these electrons collide with other atoms in the lattice they produce new electron hole pairs. The

newly released charge carriers are themselves accelerated in opposite directions and collide again.

Germanium Avalanche photodiode

Germanium avalanthe photo diodes are usçd to fabricate more sensitive and fast APDS. These are

used over wavelength range of . They have same n+p structure as that of

Si APDS. GeAPDS have dark current which is very much sensitive to temperature variations. They

have relatively high absorption coefficient at due to which they have quite low avalanche

breakdown voltages. Ge APD structures are fabricated to provide multiplication initiated by holes to

reduce excess noise factor in longer wavelength.

ADVANTAGES AND DISADVANTAGES OF AVALANCHE PHOTODIODE:

Advantages of avalanche photodiode:

(i) Avalanche photodiode are more sensitive to detection of optical signal.

(ii) APDS amplify the signal during the detection of optical signal.

Disadvantages of avalanche photodiode:

(i) APDS require high bias voltages which are wavelength dependent.

(ii) APDS have random nature of gain mechanism which gives an additional noise contribution.

(iii) APDS are costly as compared to other photodiodes.

(iv) APDS are difficult to fabricate.

SIGNAL TO NOISE RATIO IN P-I-N. AND APD RECEIVERS:

Receiver Noise

Optical Receivers convert incident optical power into electric current through photodiode. The

relationship assumes that such a conversion is noise free. But this is not the case even

for perfect receiver. Two fundamental noise mechanisms shot noise and thermal noise lead to

fluctuations in current even when incident optical signal has constant power. Electrical noise

induced by current fluctuations affects the receiver performance.

Noise Mechanisms

The photocurrent in photo diode depends of light power input . Even if input light

power is constant; the photo current does not remain constant as in reality it contains noise

components.

1. Shot Noise. Suppose input power is constant which means number of photons per unit of

time is constant. But the actual number of photons arrived at a particular time is unknown

and so it is a completely random variable. Hence, the no. of photo generated electrons at

any particular instant is a random variable. The number of electrons producing photocurrent

will also vary because of their random recombinations and absorptions even though the

average number of electrons is constant, the actual number of electrons will vary.

Deviation of actual is of electrons from the average number is known as shot noise.

Thermal Noise

Electron motion due to temperature occurs in random way. Thus the number c electrons flowing

through a given circuit at any instant is a random variable. The deviations of an instantaneous

number of electrons from their average value because of temperature change is called thermal

noise. Thermal noise is often called Johnson noise and Nyguist noise.

OR

At a finite temperature, electrons more randomly in any conductor. Random thermal motion of

electrons in a resistor manifests as a fluctuating current even in the absence of an applied voltage.

The load resistor in the front end of an optical receive ads such fluctuations to the current generated

by photodiode. This additional no component is referred to as thermal noise.

The photodiode current generated is given by

Noise generated in load resistor. An actual receiver contains many other electric components, some

of which add additional noise. If we consider amplifier noise figure Fn them

F represents the factor by which thermal noise is enhanced by various resistor used in pre and main

amplifiers.

Total Current Noise

Total noise is obtained by adding the contributions of shot noise and thermal noise.

As are independent random processes with approximately

Gaussian statistics total variance of current fluctuations

P-i-n Receivers

Thermal Noise limit

Load resistance. The effect of thermal noise is also quantified through a quantity noise equivalent

power (NEP). NEP is defined as minimum optical power per unit B required to produce SNR = 1.

NEP is given by

Short Noise limit:

The SNR of ADD receivers is worse than that of p-i-n receivers when shot noise dominates because

of excess noise generated inside APD.