optics for 4th sem b.tech

LEDs are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm2) with integrated optical components to shape its radiation pattern and assist in reflection.
LEDs present many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness, smaller size and faster switching. However, they are relatively expensive and require more precise current and heat management than traditional light sources.
Applications of LEDs are diverse. They are used as lowenergy
replacements for traditional light sources in wellestablished
applications such as indicators and automotive lighting. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in communications technology.
Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped, with impurities to create a pn
junction. As in other diodes, current flows easily from the pside,
or anode, to the nside,
or cathode, but not in the reverse direction. Chargecarriers—
electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the pn
junction. In silicon or germanium diodes, the electrons and holes recombine by a nonradiative
transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to nearinfrared,
visible or nearultraviolet
light.
LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with evershorter
wavelengths, producing light in a variety of colors.
LEDs are usually built on an ntype
substrate, with an electrode attached to the ptype
layer deposited on its surface. Ptype
substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
Most materials used for LED production have very high refractive indices. This means that much light will be reflected back in to the material at the material/air surface interface. Therefore Light extraction in LEDs is an important aspect of LED production, subject to much research and development.
Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts [mW] of electrical power.
Advantages
•Efficiency: LEDs produce more light per watt than incandescent bulbs.
•Color: LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
•Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.
•On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LEDs used in communications devices can have even faster response times.
•Cycling: LEDs are ideal for use in applications that are subject to frequent onoff
cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
•Dimming: LEDs can very easily be dimmed either by Pulsewidth
modulation or lowering the forward current.
•Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
•Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burnout
of incandescent bulbs.
•Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.[citation needed]
•Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
•Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
•Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.
Disadvantages
•High price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps.
•Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Overdriving
the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heatsinking
is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
•Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or currentregulated
power supplies.
•Light quality: Most coolwhite
LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under coolwhite
LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor based coolwhite
LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in stateofart
white LEDs.
•Area light source: LEDs do not approximate a “point source” of light, but rather a lambertian distribution. So LEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.
•Blue Hazard: There is increasing concern that blue LEDs and coolwhite
LEDs are now capable of exceeding safe limits of the socalled
bluelight
hazard as defined in eye safety specifications such as ANSI/IESNA RP27.105:
Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.
•Blue pollution: Because coolwhite
LEDs (i.e., LEDs with high color temperature) emit much more blue light than conventional outdoor light sources such as highpressure
sodium lamps, the strong wavelength dependence of Rayleigh scattering means that coolwhite
LEDs can cause more light pollution than other light sources. It is therefore very important that coolwhite
LEDs are fully shielded when used outdoors. Compared to lowpressure
sodium lamps, which emit at 589.3 nm, the 460 nm emission spike of coolwhite
and blue LEDs is scattered about 2.7 times more by the Earth's atmosphere. Coolwhite
LEDs should not be used for outdoor lighting near astronomical observatories.
LASER
A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, lowdivergence
beam, or can be converted into one with the help of optical components such as lenses. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum
("monochromatic" light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously. The coherence of typical laser emission is distinctive. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position.
A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
Principal components:1. Gain medium2. Laser pumping energy3. High reflector4. Output coupler5. Laser beam
Continuous wave operation
In the continuous wave (CW) mode of operation, the output of a laser is relatively constant with respect to time. The population inversion required for lasing is continually maintained by a steady pump source.
Pulsed operation
In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating 'on' and 'off' periods. In many applications one aims to deposit as much energy as possible at a given place in as short time as possible. In laser ablation for example, a small volume of material at the surface of a work piece might evaporate if it gets the energy required to heat it up far enough in very short time. If, however, the same energy is spread over a longer time, the heat may have time to disperse into the bulk of the piece, and less material evaporates. There are a number of methods to achieve this.
Qswitching
In a Qswitched
laser, the population inversion (usually produced in the same way as CW operation) is allowed to build up by making the cavity conditions (the 'Q') unfavorable for lasing. Then, when the pump energy stored in the laser medium is at the desired level, the 'Q' is adjusted (electroor
acoustooptically)
to favourable conditions, releasing the pulse. This results in high peak powers as the average power of the laser (were it running in CW mode) is packed into a shorter time frame.
Modelocking
A modelocked laser emits extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses are typically separated by the time that a pulse takes to complete one round trip in the resonator cavity. Due to the Fourier limit (also known as energytime
uncertainty), a pulse of such short temporal length has a spectrum which contains a wide range of wavelengths. Because of this, the laser medium must have a broad enough gain profile to amplify them all. An example of a suitable material is titaniumdoped,
artificially grown sapphire (Ti:sapphire).
The modelocked laser is a most versatile tool for researching processes happening at extremely fast time scales also known as femtosecond physics, femtosecond chemistry and ultrafast science, for maximizing the effect of nonlinearity in optical materials (e.g. in secondharmonic
generation, parametric downconversion,
optical parametric oscillators and the like), and in ablation applications. Again, because of the short timescales involved, these lasers can achieve extremely high powers.
Pulsed pumping
Another method of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flashlamps, or another laser which is already pulsed. Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high energy, fast pump was needed. The way to overcome this problem was to charge up large capacitors which are then switched to discharge through flashlamps, producing a broad spectrum pump flash. Pulsed pumping is also
required for lasers which disrupt the gain medium so much during the laser process that lasing has to cease for a short period. These lasers, such as the excimer laser and the copper vapour laser, can never be operated in CW mode.
Gas lasers
Gas lasers using many gases have been built and used for many purposes.
The heliumneon
laser (HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education because of its low cost.
Carbon dioxide lasers can emit hundreds of kilowatts at 9.6 μm and 10.6 μm, and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.
Argonion
lasers emit light in the range 351528.7
nm. Depending on the optics and the laser tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and 514.5 nm.
A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.
Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Heliumsilver
(HeAg) 224 nm and neoncopper
(NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),making them candidates for use in fluorescence suppressed Raman spectroscopy.
Chemical lasers
Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (27002900
nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were invented by George C. Pimentel.
Excimer lasers
Excimer lasers are powered by a chemical reaction involving an excited dimer, or excimer, which is a shortlived
dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. They typically produce ultraviolet light, and are used in semiconductor photolithography and in LASIK eye surgery. Commonly used excimer molecules include F2 (fluorine, emitting at 157 nm), and noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).
Solidstate
lasers
Solidstate
laser materials are commonly made by "doping" a crystalline solid host with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby (chromiumdoped
corundum). The population inversion is actually maintained in the "dopant", such as chromium or neodymium. Formally, the class of solidstate
lasers includes also fiber laser, as the active medium (fiber) is in the solid state. Practically, in the scientific literature, solidstate
laser usually means a laser with bulk active medium, while waveguide
lasers are caller fiber lasers.
"Semiconductor lasers" are also solidstate
lasers, but in the customary laser terminology, "solidstate
laser" excludes semiconductor lasers, which have their own name.
Neodymium is a common "dopant" in various solidstate
laser crystals, including yttrium orthovanadate (Nd:YVO4), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These lasers are also commonly frequency doubled, tripled or quadrupled to produce 532 nm (green, visible), 355 nm (UV) and 266 nm (UV) light when those wavelengths are needed.
Ytterbium, holmium, thulium, and erbium are other common "dopants" in solidstate
lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 10201050
nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmiumdoped
YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by waterbearing
tissues. The HoYAG
is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
Titaniumdoped
sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy as well as the most common ultrashort pulse laser.
Thermal limitations in solidstate
lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermooptic
coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diodepumped
solidstate
laser, the diodepumped
thin disk laser. The thermal limitations in this laser type are mitigated by using a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kilowatt levels of power.
Fiberhosted
lasers
Solidstate
lasers where the light is guided due to the total internal reflection in an optical fiber are called fiber lasers. Guiding of light allows extremely long gain regions providing good cooling conditions; fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. Erbium and ytterbium ions are common active species in such lasers.
Quite often, the fiber laser is designed as a doubleclad
fiber. This type of fiber consists of a fiber core, an inner cladding and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a singlemode
fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region, while still having a high numerical aperture (NA) to have easy launching conditions.
Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers.
Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of longliving
color centers.
Photonic crystal lasers
Photonic crystal lasers are lasers based on nanostructures
that provide the mode confinement and the density of optical states (DOS) structure required for the feedback to take place. They are typical micrometresized
and tunable on the bands of the photonic crystals.
Semiconductor lasers
Semiconductor lasers are also solidstate
lasers but have a different mode of laser operation.
Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths of over 3 μm have been demonstrated. Low power laser diodes are used in laser printers and CD/DVD players. More powerful laser diodes are frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm), are used in industry for cutting and welding. Externalcavity
semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelengthtunable
narrowlinewidth
radiation, or ultrashort laser pulses.
A 5.6 mm 'closed can' commercial laser diode, probably from a CD or DVD player.
Vertical cavity surfaceemitting
lasers (VCSELs) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes, and potentially could be much cheaper to manufacture. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,[19] and 1550 nm devices an area of research. VECSELs are externalcavity
VCSELs. Quantum cascade lasers are semiconductor lasers that have an active transition between energy subbands
of an electron in a structure containing several quantum wells.
The development of a silicon laser is important in the field of optical computing, since it means that if silicon, the chief ingredient of computer chips, were able to produce lasers, it would allow the light to be manipulated like electrons are in normal integrated circuits. Thus, photons would replace electrons in the circuits, which dramatically increases the speed of the computer. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However, recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are called hybrid silicon laser. Another type is a Raman laser, which takes advantage of Raman scattering to produce a laser from materials such as silicon.
Some of the other applications include:
•Medicine : Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry
•Industry : Cutting, welding, material heat treatment, marking parts
•Defense : Marking targets, guiding munitions, missile defence, electrooptical
countermeasures (EOCM), alternative to radar
•Research : Spectroscopy, laser ablation, Laser annealing, laser scattering, laser interferometry, LIDAR, Laser capture microdissection
•Product development/commercial: laser printers, CDs, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.
•Laser lighting displays : Laser light shows
•Laser skin procedures such as acne treatment, cellulite reduction, and hair removal.
PHOTO DIODE
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.
Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or Xrays)
or packaged with a window or optical fibre connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction.
Principle of operation
A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the builtin
field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.
[edit] Photovoltaic mode
When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. The diode becomes forward biased and "dark current" begins to flow across the junction in the direction opposite to the photocurrent. This mode is responsible for the photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is just an array of large area photodiodes.
Photoconductive mode
In this mode the diode is often (but not always) reverse biased. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. The photocurrent is linearly proportional to the illuminance.[1]
Although this mode is faster, the photovoltaic mode tends to exhibit less electronic noise.[citation needed] (The leakage current of a good PIN diode is so low – < 1nA – that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.)
Other modes of operation
Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each photogenerated
carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.
Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the basecollector
junction. The electrons that are generated by photons in the basecollector
junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfefe). Note that while phototransistors have a higher responsivity for light they are not able to
detect low levels of light any better than photodiodes.[citation needed] Phototransistors also have slower response times.
Materials
The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.
Materials commonly used to produce photodiodes include:
Material
Wavelength range (nm)
Silicon
190–1100
Germanium
400–1700
Indium gallium arsenide
800–2600
Lead sulfide
<10003500
Features
Critical performance parameters of a photodiode include:
responsivity
The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as a quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity.
dark current
The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.
noiseequivalent
power
(NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. The related characteristic detectivity (D) is the inverse of NEP, 1/NEP; and the specific detectivity () is the detectivity normalized to the area (A) of the photodetector, . The NEP is roughly the minimum detectable input power of a photodiode.
When a photodiode is used in an optical communication system, these parameters contribute to the sensitivity of the optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error ratio.
Applications
PN
photodiodes are used in similar applications to other photodetectors, such as photoconductors, chargecoupled
devices, and photomultiplier tubes.
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions.
In other consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators) or instruments to analyze samples (immunoassay). They are also used in blood gas monitors.
PIN diodes are much faster and more sensitive than ordinary pn
junction diodes, and hence are often used for optical communications and in lighting regulation.
PN
photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified chargecoupled
devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.
Applications
PN
photodiodes are used in similar applications to other photodetectors, such as photoconductors, chargecoupled
devices, and photomultiplier tubes.
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions.
In other consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators) or instruments to analyze samples (immunoassay). They are also used in blood gas monitors.
PIN diodes are much faster and more sensitive than ordinary pn
junction diodes, and hence are often used for optical communications and in lighting regulation.
PN
photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified chargecoupled
devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.
Theory
Simple explanation
1.Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
2.Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.
3.An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
Photogeneration of charge carriers
When a photon hits a piece of silicon, one of three things can happen:
1.the photon can pass straight through the silicon — this (generally) happens for lower energy photons,
2.the photon can reflect off the surface,
3.the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electronhole
pair and sometimes heat, depending on the band structure.
When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electronhole
pairs.
A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy.
Charge carrier separation
There are two main modes for charge carrier separation in a solar cell:
1.drift of carriers, driven by an electrostatic field established across the device
2.diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential).
In the widely used pn
junction solar cells, the dominant mode of charge carrier separation is by drift. However, in nonpnjunction
solar cells (typical of the third generation solar cell research
such as dye and polymer solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion.[20]
The pn
junction
Main articles: semiconductor and pn
junction
The most commonly known solar cell is configured as a largearea
pn
junction made from silicon. As a simplification, one can imagine bringing a layer of ntype
silicon into direct contact with a layer of ptype
silicon. In practice, pn
junctions of silicon solar cells are not made in this way, but rather, by diffusing an ntype
dopant into one side of a ptype
wafer (or vice versa).
If a piece of ptype
silicon is placed in intimate contact with a piece of ntype
silicon, then a diffusion of electrons occurs from the region of high electron concentration (the ntype
side of the junction) into the region of low electron concentration (ptype
side of the junction). When the electrons diffuse across the pn
junction, they recombine with holes on the ptype
side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates. The electric field established across the pn
junction creates a diode that promotes current in only one direction across the junction. Electrons may pass from the ntype
side into the ptype
side, and holes may pass from the ptype
side to the ntype
side, but not the other way around This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".
Connection to an external load
Ohmic metalsemiconductor
contacts are made to both the ntype
and ptype
sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the ntype
side, or have been "collected" by the junction and swept onto the ntype
side, may travel through the wire, power the load, and continue through the wire until they reach the ptype
semiconductormetal
contact. Here, they recombine with a hole that was either created as an electronhole
pair on the ptype
side of the solar cell, or are swept across the junction from the ntype
side after being created there.
The voltage measured is equal to the difference in the quasi Fermi levels of the minority carriers ie. electrons in the ptype
portion, and holes in the ntype
portion.
Equivalent circuit of a solar cell
The equivalent circuit of a solar cell
The schematic symbol of a solar cell
To understand the electronic behavior of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behavior is well known. An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.The resulting equivalent circuit of a solar cell is shown on the left. Also shown, on the right, is the schematic representation of a solar cell for use in circuit diagrams.
Characteristic equation
From the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor:
I = IL − ID − ISH
where
•I = output current (amperes)
•IL = photogenerated current (amperes)
•ID = diode current (amperes)
•ISH = shunt current (amperes)
The current flowing through these elements governed by the voltage across them:
Vj = V + IRS
where
•V = voltage across the output terminals (volts)
•I = output current (amperes)
•RS = series resistance (Ω)
By the Shockley diode equation, the current diverted through the diode is:
where
•I0 = reverse saturation current (amperes)
•n = diode ideality factor (1 for an ideal diode)
•q = elementary charge
•k = Boltzmann's constant
•T = absolute temperature
•For silicon at 25°C, volts.
By Ohm's law, the current diverted through the shunt resistor is:
where
•RSH = shunt resistance () Ω
Substituting these into the first equation produces the characteristic equation of a solar cell, which relates solar cell parameters to the output current and voltage:
An alternative derivation produces an equation similar in appearance, but with V on the lefthand
side. The two alternatives are identities; that is, they yield precisely the same results.
In principle, given a particular operating voltage V the equation may be solved to determine the operating current I at that voltage. However, because the equation involves I on both sides in a transcendental function the equation has no general analytical solution. However, even without a solution it is physically instructive. Furthermore, it is easily solved using numerical methods. (A general analytical solution to the equation is possible using Lambert's W function, but since Lambert's W generally itself must be solved numerically this is a technicality.)
Since the parameters I0, n, RS, and RSH cannot be measured directly, the most common application of the characteristic equation is nonlinear regression to extract the values of these parameters on the basis of their combined effect on solar cell behavior.