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Semi conductor diode


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Semi conductor diode

  1. 1. Question: What is P-N junction? Explain the formation of potential barrier and depletion layer? Withno external voltage.A p–n junction is a junction formed by joining p-type and n-type semiconductors together in very closecontact. The term junction refers to the boundary interface where the two regions of the semiconductormeet. If they were constructed of two separate pieces this would introduce a grain boundary, so p–njunctions are more often created in a single crystal of semiconductor by doping, for example by ionimplantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type ofdopant on top of a layer of crystal doped with another type of dopant).Formation of the Depletion Region.At the instant of the PN junction formation free electrons near the junction diffuse across the junctioninto the P region and combine with holes.Filling a hole makes a negative ion and leaves behind a positive ion on the N side.These two layers of positive and negative charges form the depletion region, as the region near thejunction is depleted of charge carriers.As electrons diffuse across the junction a point is reached where the negative charge repels any furtherdiffusion of electrons.The depletion region now acts as a barrier.Barrier Potential.The electric field formed in the depletion region acts as a barrier.External energy must be applied to get the electrons to move across the barrier of the electric field.The potential difference required to move the electrons through the electric field is called the barrierpotential.
  2. 2. Barrier potential of a PN junction depends on the type of semiconductor material, amount of doping andtemperature.This is approximately 0.7V for silicon and 0.3V for germanium.Question: Draw the energy level diagram of a PN junction and explain how a junction barrier isdeveloped across an unbiased junction.Energy Diagrams of PN Junction:The valence band and conduction band in N-Type material are slightly lower than that of P-Typematerial.As diffusion occurs the depletion region forms and the energy level of the N region conduction banddrops, causing alignment of the top of the N region conduction band and the bottom of the P regionconduction band. At this point the energy bands are at equilibrium.There is an energy gradient across the depletion region that a N region electron must climb to get to theP region.Biasing the PN Junction.Forward Bias.An external D.C. voltage is applied to a PN junction to bias it.Forward bias allows current through a PN junction.
  3. 3. Bias voltage imparts sufficient energy to the free electrons so they overcome the barrier potential andmove into the P region.Conduction occurs with electron current in the N-Type material and hole current in the P-Type material.As more electrons and holes flow in the depletion region the number of positive and negative ions isreduced, causing the depletion region to narrow.The energy loss in overcoming the barrier potential results in a voltage drop across the PN junctionequal to the barrier potential.( 0.7V for Si, 0.3V for Ge.)When a PN junction is forward biased the N region conduction band is raised to a higher level thatoverlaps with the P region conduction band.
  4. 4. Reverse BiasReverse bias prevents current flow through a PN junction. P NReverse bias potential attracts the majority carriers away from the PN junction widening the depletionregion.The potential across the depletion region is equal to the bias voltage.Reverse current can flow through the PN junction due to minority carriers.When a PN junction is reverse biased the energy hill increases.Reverse Breakdown.If the reverse bias voltage is increased to the breakdown voltage, the reverse current will drasticallyincrease.This is due to collisions of fast minority carriers with atoms, knocking out valence electrons. This processavalanches and results in a high reverse current damaging the PN structure.
  5. 5. Question: Explain the majority & minority carriers produces in n-type & t-type materials.Majority and Minority Carriers: In the intrinsic state, the number of free electrons in Ge or Si is due onlyto those few electrons in the valence band that have acquired sufficient energy from thermal or lightsources to break the covalent bond or to the few impurities that could not be re-moved. The vacanciesleft behind in the covalent bonding structure represent our very limited supply of holes. In an n-typematerial, the number of holes has not changed significantly from this intrinsic level. The net result,therefore, is that the number of electrons far outweighs the number of holes. For this reason In an n-type material (Fig. 1.13a) the electron is called the majority carrier and the hole the minority carrier. Forthe p-type material the number of holes far outweighs the number of electrons, as shown in Fig. 1.13b.Therefore: In a p-type material the hole is the majority carrier and the electron is the minority carrier.When the fifth electron of a donor atom leaves the parent atom, the atom remaining acquires a netpositive charge: hence the positive sign in the donor-ion representation. For similar reasons, thenegative sign appears in the acceptor ion. The n- and p-type materials represent the basic buildingblocks of semiconductor devices. We will find in the next section that the “joining “of a single n-typematerial with a p-type material will result in a semiconductor element of considerable importance inelectronic systems.Question: What is forward biasing & reverse biasing? Explain the PN Junction forward bias & reversebias.Forward biasing: Forward bias occurs when the P-type semiconductor material is connected to thepositive terminal of a battery and the N-type semiconductor material is connected to the negativeterminal, as shown below. This usually makes the p–n junction conduct.With a battery connected this way, the holes in the P-type region and the electrons in the N-typeregion are pushed towards the junction. This reduces the width of the depletion zone. Thepositive charge applied to the P-type material repels the holes, while the negative charge appliedto the N-type material repels the electrons. As electrons and holes are pushed towards thejunction, the distance between them decreases. This lowers the barrier in potential. Withincreasing forward-bias voltage, the depletion zone eventually becomes thin enough that thezones electric field cant counteract charge carrier motion across the p–n junction, consequentlyreducing electrical resistance. The electrons which cross the p–n junction into the P-type material(or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore,
  6. 6. the amount of minority diffusion in the near-neutral zones determines the amount of current that may flowthrough the diode.Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductorfor a macroscopic length. With this in mind, consider the flow of electrons across the junction. Theforward bias causes a force on the electrons pushing them from the N side toward the P side. Withforward bias, the depletion region is narrow enough that electrons can cross the junction and inject intothe P-type material. However, they do not continue to flow through the P-type material indefinitely,because it is energetically favorable for them to recombine with holes. The average length an electrontravels through the P-type material before recombining is called the diffusion length, and it is typically onthe order of microns.Although the electrons penetrate only a short distance into the P-type material, the electric currentcontinues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. Thetotal current (the sum of the electron and hole currents) is constant in space, because any variation wouldcause charge buildup over time (this is Kirchhoffs current law). The flow of holes from the P-type regioninto the N-type region is exactly analogous to the flow of electrons from N to P (electrons and holes swaproles and the signs of all currents and voltages are reversed).Therefore, the macroscopic picture of the current flow through the diode involves electrons flowingthrough the N-type region toward the junction, holes flowing through the P-type region in the oppositedirection toward the junction, and the two species of carriers constantly recombining in the vicinity of thejunction. The electrons and holes travel in opposite directions, but they also have opposite charges, so theoverall current is in the same direction on both sides of the diode, as required.Reverse Bias: Reverse biased usually refers to how a diode is used in a circuit. If a diode isreverse biased, the voltage at the cathode is higher than that at the anode. Therefore, no currentwill flow until the diode breaks down. Connecting the P-type region to the negative terminal ofthe battery and the N-type region to the positive terminal, corresponds to reverse bias. Theconnections are illustrated in the following diagram:Because the p-type material is now connected to the negative terminal of the power supply, theholes in the P-type material are pulled away from the junction, causing the width of thedepletion zone to increase. Similarly, because the N-type region is connected to the positiveterminal, the electrons will also be pulled away from the junction. Therefore the depletion regionwidens, and does so increasingly with increasing reverse-bias voltage. This increases the voltagebarrier causing a high resistance to the flow of charge carriers thus allowing minimal electriccurrent to cross the p–n junction.The strength of the depletion zone electric field increases as the reverse-bias voltage increases.Once the electric field intensity increases beyond a critical level, the p–n junction depletion zonebreaks-down and current begins to flow, usually by either the Zener or avalanche breakdownprocesses. Both of these breakdown processes are non-destructive and are reversible, so long as
  7. 7. the amount of current flowing does not reach levels that cause the semiconductor material tooverheat and cause thermal damage.This effect is used to ones advantage in zener diode regulator circuits. Zener diodes have acertain - low - breakdown voltage. A standard value for breakdown voltage is for instance 5.6V.This means that the voltage at the cathode can never be more than 5.6V higher than the voltageat the anode, because the diode will break down - and therefore conduct - if the voltage gets anyhigher. This effectively regulates the voltage over the diode.Another application where reverse biased diodes are used is in Varicap diodes. The width of thedepletion zone of any diode changes with voltage applied. This varies the capacitance of thediode. For more information, refer to the Varicap article.Question: What is break down? Explain zener break down and avalence break down?A Zener diode is a type of diode that permits current not only in the forward direction like anormal diode, but also in the reverse direction if the voltage is larger than the breakdown voltageknown as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener,who discovered this electrical property.A conventional solid-state diode will not allow significant current if it is reverse-biased below itsreverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, aconventional diode is subject to high current due to avalanche breakdown. Unless this current islimited by external circuitry, the diode will be permanently damaged. In case of large forwardbias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junctionbuilt-in voltage and internal resistance. The amount of the voltage drop depends on thesemiconductor material and the doping concentrations.A Zener diode exhibits almost the same properties, except the device is specially designed so asto have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with heconventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allowthe current to keep the voltage across the Zener diode at the Zener voltage. For example, a diodewith a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse biasvoltage applied across it is more than its Zener voltage. The Zener diode is therefore ideal forapplications such as the generation of a reference voltage (e.g. for an amplifier stage), or as avoltage stabilizer for low-current applications.
  8. 8. The Zener diodes operation depends on the heavy doping of its p-n junction allowing electronsto tunnel from the valence band of the p-type material to the conduction band of the n-typematerial. In the atomic scale, this tunneling corresponds to the transport of valence bandelectrons into the empty conduction band states; as a result of the reduced barrier between thesebands and high electric fields that are induced due to the relatively high levels of dopings on bothsides.The breakdown voltage can be controlled quite accurately in the doping process. Whiletolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200volts.Another mechanism that produces a similar effect is the avalanche effect as in the avalanchediode. The two types of diode are in fact constructed the same way and both effects are presentin diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominanteffect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalancheeffect becomes predominant and exhibits a positive temperature coefficient. In a 5.6 V diode, thetwo effects occur together and their temperature coefficients neatly cancel each other out, thusthe 5.6 V diode is the component of choice in temperature-critical applications. Modernmanufacturing techniques have produced devices with voltages lower than 5.6 V with negligibletemperature coefficients, but as higher voltage devices are encountered, the temperaturecoefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella termof "Zener diode".Avalanche breakdown: avalanche breakdown is a phenomenon that can occur in bothinsulating and semiconducting materials. It is a form of electric current multiplication that canallow very large currents to flow within materials which are otherwise good insulators. It is atype of electron avalanche.Explanation: Materials conduct electricity if they contain mobile charge carriers. There are twotypes of charge carrier in a semiconductor: free electrons and electron holes. A fixed electron ina reverse-biased diode may break free due to its thermal energy, creating an electron-hole pair. Ifthere is a voltage gradient in the semiconductor, the electron will move towards the positivevoltage while the hole will "move" towards the negative voltage. Most of the time, the electronand hole will just move to opposite ends of the crystal and stop. Under the right circumstances,however, (ie. when the voltage is high enough) the free electron may move fast enough to knockother electrons free, creating more free-electron-hole pairs (ie. more charge carriers), increasingthe current. Fast-"moving" holes may also result in more electron-hole pairs being formed. In afraction of a nanosecond, the whole crystal begins to conduct quite nicely.Avalanche breakdown usually destroys regular diodes, but avalanche diodes are designed tobreak down this way at low voltages and can survive the reverse current.The voltage at which the breakdown occurs is called the breakdown voltage. Once avalanchebreakdown has occurred, the material will continue to conduct if the voltage across it dropsbelow the breakdown voltage. This is different from a Zener diode because a Zener diode willstop conducting once the reverse voltage drops below the breakdown voltage.
  9. 9. Question: What is knee voltage?The point or area on a graph of diode current versus voltage where the forward current suddenlyincreases. It is approximately equal to the barrier potential of the diode.Question: Explain V-I Characteristics of P-N junction.Forward biasing: A p-n junction is said to be forward biased, if the positive terminal of the external battery B isconnected to p-side and the negative terminal to the n-side of the p-n junction. Here the forward bias opposes thepotential barrier VB and so the depletion layer becomes thin. The majority charge carriers in the P type and N typesare repelled by their respective terminals due to battery B and hence cross the junction. On crossing the junction,recombination process takes place. For every electron hole combination, a covalent bond near the +ve terminal ofthe battery B is broken and this liberates an electron which enters the +ve terminal of B through connecting wires.This in turn creates more holes in P-region. At the other end, the electrons from -ve terminal of B enter n-region toreplace electron lost due to recombination process. Thus a large current will flow to migration of majority carriersacross the p-n junction which is called forward current.Reverse biasing: A p-n junction is said to be reverse biased if the positive terminal of the battery B is connected toN-side and the negative terminal to p-side of the p-n junction. The majority carriers are pulled away from thejunction and the depletion region becomes thick. The resistance becomes high when reverse biased and so there isno conduction across the junction due to majority carriers. The minority carriers however cross the junction andthey constitute a current that flows in the opposite direction. This is the reverse current.The V-I characteristics of a p-n junction diode
  10. 10. Question: What are LED? How do they works? Explain Briefly, mention some of it’s application.A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in manydevices, and are increasingly used for lighting. Introduced as a practical electronic component in1962,[2] early LEDs emitted low-intensity red light, but modern versions are available across the visible,ultraviolet and infrared wavelengths, with very high brightness.The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electronsare able to recombine with holes within the device, releasing energy in the form of photons. This effectis called electroluminescence and the color of the light (corresponding to the energy of the photon) isdetermined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2),and integrated optical components are used to shape its radiation pattern and assist in reflection.[3]LEDs present many advantages over incandescent light sources including lower energy consumption,longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability.However, they are relatively expensive and require more precise current and heat management thantraditional light sources. Current LED products for general lighting are more expensive to buy thanfluorescent lamp sources of comparable output. They also enjoy use in applications as diverse asreplacements for traditional light sources in automotive lighting (particularly indicators) and in trafficsignals. 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 advanced communications technology.Application of LEDs fall into three major categories:  Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning.  Illumination where LED light is reflected from object to give visual response of these objects.  Generate light for measuring and interacting with processes that do not involve the human visual system.Question: How an ideal diode acts as a switch? The IDEAL DIODE serves as an electrical one-way valve. It conducts current much morereadily in one direction, called the FORWARD (BIAS) DIRECTION, than it does in the otherdirection called the REVERSE (BIAS) DIRECTION. IDEAL DIODE acts like a SWITCH that closes to allow current flow in the forwarddirection but opens to prevent current flow in the reverse direction.
  11. 11. Question: What is rectifier and rectification?Rectifier: Rectifier is an electrical device which can change an a.c current to d.c current.Rectification: The process of changes ac to d.c is called rectification.Question : What is half wave rectifier? Explain the use of a PN junction diode as a half wave rectifier?Find out the efficiency & ripple factor of a half wave rectifier?In half wave rectification, either the positive or negative half of the AC wave is passed, while the otherhalf is blocked. Because only one half of the input waveform reaches the output, it is very inefficient ifused for power transfer. Half-wave rectification can be achieved with a single diode in a one-phasesupply, or with three diodes in a three-phase supply. • Average value of the output voltage, Vdc • Average value of the output current, Idc • Output dc power, Pdc – Pdc = VdcIdc • rms value of the output voltage, Vrms • Output ac power, Pac – Pac = VrmsIrms • Efficiency, η – η = Pdc/Pac • Effective (rms) value of the ac component of the output voltage, Vac – Vac = Vrms2 – Vdc2 • Form factor, FF – FF = Vrms/Vdc • Ripple factor, RF
  12. 12. – RF = Vac/Vdc • Alternate form for ripple factor • Transformer utilization factor, TUF – TUF = Pdc/VsIs – Vs, Is are rms voltage and current of the transformer secondaryDetermine the Average Voltage, Vdc
  13. 13. .