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 galliumarsenide (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. Nonradiative 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.