1. 1 Atomic Structure Major parts of an atom. Proton: Protons are positively charged particles found in the atomic nucleus. Protons were discovered by Ernest Rutherford. Neutron: Neutrons are uncharged particles found in the atomic nucleus. Neutrons were discovered by James Chadwick in 1932. Electron: Electrons are negatively charged particles that surround the atom's nucleus. Electrons were discovered by J. J. Thomson in 1897. http://www.bmb.psu.edu/courses/bisci004a/chem/basechem.htm

2. 2 The nucleus was discovered by Ernest Rutherford in 1911 and is the central part of an atom. It is composed of protons and neutrons and contains most of an atom's mass. Electrons circle nucleus in defined shells K 2 electrons L8 electrons M18 electrons N32 electrons Within each shell, electrons are further grouped into subshells s 2 electrons p 6 electrons d10 electrons f 14 electrons electrons are assigned to shells and subshells from inside out Si has 14 electrons: 2 K, 8 L, 4 M Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. first shella maximum of 2 electrons second shella maximum of 8 electrons third shella maximum of 8 electrons

3. 3 Electronic Configuration Dot & Cross Diagrams With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Ca 40 20 2,8,8,2 B 11 5 2,3 With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; X X X X X X X X O O 8

4. 4 The goal of electronic materials is to generate and control the flow of an electrical current. Electronic materials include: 1-Conductors: have low resistance which allows electrical current flow.Good conductors have low resistance so electrons flow through them with ease (Copper, silver, gold, aluminum, nickel, steel). 2-Insulators: have high resistance which suppresses electrical current flow. Insulators have a high resistance so current does not flow in them (Glass, ceramic, plastics, wood).

5. 5 3-Semiconductors: can allow or suppress electrical current flow. Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between (carbon, silicon, and germanium). The atoms in a semiconductor are materials from either group IV of the periodic table, or from a combination of group III and group V (called III-V semiconductors), or of combinations from group II and group VI (called II-VI semiconductors). Silicon is the best and most widely used semiconductor. as it forms the basis for integrated circuit (IC) chips and is the most mature technology and most solar cells are also silicon based

6. 6 Grou p 123456789101112131415161718 Perio d 1 1H1H 2 He 2 3 Li 4 Be 5B5B 6C6C 7N7N 8O8O 9F9F 10 Ne 3 11 Na 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 4 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 5 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 6 55 Cs 56 Ba 57-71 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 7 87 Fr 88 Ra 89-103 104 Rf 105 Db 106 Sg 107 Bh 108 Hs 109 Mt 110 Ds 111 Rg 112 Cn 113 Uut 114 Fl 115 Uup 116 Lv 117 Uus 118 Uuo 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 71 Lu 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md 102 No 103 Lr

7. 7 All the elements used to make semiconductors appear in Column IV of the Periodic Table or are a combination of elements in columns at equal distance of Column IV on each side. http://enpub.fulton.asu.edu/widebandgap/NewPages/SCbasics.html

8. 8 Semiconductors are materials whose electrical properties lie between Conductors and Insulators. Ex : Silicon and Germanium The name “ Semiconductor ” implies that it conducts somewhere between the two cases (conductors or insulators) Conductivity : σ conductors ~10 10 /Ω-cm σ insulators ~ 10 -22 / Ω-cm The conductivity (σ) of a semiconductor (S/C) lies between these two extreme cases.

9. 9 http://www.electronics-tutorials.ws/diode/diode_1.html

10. 10 Drift and Diffusion Current Flow: Drift: charged particle motion in response to an electric field. Diffusion: Particles tend to spread out or redistribute from areas of high concentration to areas of lower concentration Recombination: Local annihilation of electron-hole pairs Generation: Local creation of electron-hole pairs Drift Direction of motion: Holes move in the direction of the electric field (from + to -) Electrons move in the opposite direction of the electric field (from - to +) Motion is highly non-directional on a local scale, but has a net direction on a macroscopic scale

11. 11 Drift and Diffusion It is well known that current is the rate of flow of charge, thus if the number density of charge carriers (electrons and holes) present in a semiconductor material are known currents flowing in such devices can be calculated using two current mechanisms: Drift and Diffusion Electron and holes will move under the influence of an applied electric field since the field exert a force on charge carriers (electrons and holes). These movements result a current of I d drift current number of charge carriers per unit volume charge of the electron Drift velocity of charge carrier area of the semiconductor

12. 12 Carrier Mobility mobility of charge carrier applied field is a proportionality factor

13. 13 Drift and Diffusion Average net motion is described by the drift velocity, vd with units cm/second Net motion of charged particles gives rise to a current http://users.ece.gatech.edu/~alan/ECE3080/Lectures/ECE3080-L-7-Drift%20-%20Diffusion%20Chap%203%20Pierret.pdf The other difference between drift current and diffusion current, is that the direction of the diffusion current depends on the change in the carrier concentrations, not the concentrations themselves

14. 14 Generation: Is the movement of an electron from the valence band to the conduction band. This will lead to the creation of an electron- hole pair. Recombination: Is the movement of an electron from the conduction band to the valence band. This will lead to the destruction of and electron- hole pair. The recombination processes can be reversed resulting in generation processes.

15. 15 SEMICONDUCTORS INTRINSICEXTRINSIC 1-Chemically very pure 2-Possesses poor conductivity 3-Has equal numbers of negative carriers (electrons) and positive carriers (holes) 1-Improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping 2-Alters the electrical properties of the semiconductor and 3-Improves its conductivity the positive charge conductor (p-type ) DOPING the negative charge conductor (n-type )

16. 16 The positive charge conductor (p-type ) DOPING The negative charge conductor (n-type ) Available as either elements( Si and Ge) or compounds ( GaAs, SiC, GaN, GaP ) Doping is the process by which small amounts of selected additives, called impurities, are added to semiconductors to increase their current flow. Semiconductors that undergo this treatment are referred to as Extrinsic Semiconductors. This type of semiconductor has a surplus of electrons, the electrons are considered the majority current carriers, while the holes are the minority current carriers. This type semiconductor holes are present in the greatest quantity they are majority current carriers while electrons are the minority current carriers.

17. 17 In the atoms, the larger the radius, the higher the electron potential energy. Hence, electron position can be described either by radius or by its potential energy. In the semiconductor crystal: the atom orbits overlap; radius-based description becomes impractical. Energy-based description works well: The highest orbit filled with electrons becomes the valence band, The higher orbit (nearly empty ) becomes the conduction band. ( Serway book) www.physics.qc.edu/.../10%20Semiconductors..ppt

18. 18 http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html#c3

19. 19 The term junction (formed by joining p-type anf n-type semiconductors together in a very close contact) means the boundary interface where the two regions of the semiconductor meet. Filling a hole makes a negative ion and leaves behind a positive ion on the N side. The positive and negative charges form the depletion region. The electric field formed in the depletion region acts as barrier and an external energy must be applied to get the electrons to move across the barrier of the electric field, such potential difference needed to move the electrons through the electric field is called barrier potential and depends on the type of semiconductor, the amount of doping and temperature. The value is around 0.7v for silicon and 0.3 v for germanium http://www.tpub.com/neets/book7/24h.htm

20. 20 Work function is the energy required to extract an electron from a solid in other words the work function is the energy required to remove an electron from the highest filled level in the Fermi distribution of a solid so that it is stationary at a point in a field-free zone just outside the solid, at absolute zero. An estimate of the work function can be obtained thermionically from Richardson's equation http://rsl.eng.usf.edu/Documents/Tutorials/TutorialsWorkFunction.pdf VBM: valence bands maximum CBM: conduction bands maximum Eg :band gap EF: Fermi energy

21. 21 http://www.tpub.com/neets/book7/24h.htm

22. 22 http://www.tpub.com/neets/book7/24h.htm

23. 23 http://www.tpub.com/neets/book7/24h.htm

24. 24 zener breakdown occurs due to strong electric field across the diode. When a lower voltage is given,higher electric field is generated.due to strong electric field,the covalent bonds breakdown and free electrons are generated when they are sufficient there is sharp rise in current and breakdown occurs. avalanche breakdown occurs due to higher velocity of minority carriers. due to less doping the width of depletion region is more. So when a high voltage is given, velocity of minority carriers increases towards opposite type semiconductor. When they collide with the walls of semiconductor, free electrons are generated. When free electrons are sufficient,then there is sharp rise in currents and breakdown occurs.

25. 25 http://www.electronics-tutorials.ws/diode/diode_3.html

26. 26 by Prof. Dr. Ali S. Hennache PN Junction ( 04 H ) : _Depletion region – Junction capacitance – Diode equation 1H _Effect of temperature on reverse saturation current – construction, working 1H, _ V-I characteristics and simple applications of :Junction diode, Zener diode 1H, _Tunnel diode and Varactor diode. Filter considerations 1H.

27. 27 Formation of a P-N junction P-N junctions are formed by joining n-type and p-type semiconductor materials, as shown below. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side. http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html When the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the N-type side, positive ion cores are exposed. On the P-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the N-type material and negative ion cores in the P-type material. This region is called the depletion region. Depleted region is thus, the part of a PN junction in which there are no electrons or holes and thus, this latter prevents current from flowing.

28. 28 Capacitor = Two conductors with a dielectric in between Diode in reverse bias mode = Two semiconductors with a depletion region in between PN junction: putting a P-type material next to N-type material to form the PN junction P-type is where we have more "holes"; N-type is where we have more electrons in the material. Initially, when we put them together to form a junction, holes near the junction tends to "move" across to the N-region, while the electrons in the N-region drift across to the p-region to "fill" some holes. This current will quickly stop as the potential barrier is built up by the migrated charges. So in steady state no current flows.

29. 29 When we put a potential different across the terminals we have two cases: 1. Positive end to P-type, Negative end to N-type: The electric field from the external potential different can easily overcome the small internal field (in the so-called depletion region, created by the initial drifting of charges): usually anything bigger than 0.6V would be enough. The external field then attracts more e- to flow from n-region to p-region and more holes from p-region to n-region and we have a forward biased situation. the diode is ON. 2. Positive end to N-type, Negative end to P-type: in this case the external field pushes e- back to the n-region while more holes into the p-region, as a result we get no current flow. Only the small number of thermally released minority carriers (holes in the n-type region and e- in the p-type region) will be able to cross the junction and form a very small current, but for all practical purposes, this can be ignored (the diode is somehow off) If the reverse biased potential is large enough we get avalanche break down and current flow in the opposite direction. In many cases, except for Zener diodes, we most likely will destroy the diode.

30. 30 Diode Junction Capacitance The concept of junction capacitance is a diode acting as a capacitor. Forward bias: Putting the power source such that charge is able to flow through the diode Reverse bias: Putting the power source such that charge is not able to flow through the diode As it is well known that capacitance is two conductors separated by an insulator/dielectric. In this part of the lecture we will be focusing on reverse biasing when talking about junction capacitance. http://tymkrs.tumblr.com/post/6976329612/diode-junction-capacitance a depletion region a dielectric Capacitor = Two conductors with a dielectric in between Diode in reverse bias mode = Two semiconductors with a depletion region in between

31. 31 The diode equation gives an expression for the current through a diode as a function of voltage. The Ideal Diode Law, expressed as: where: I = the net current flowing through the diode; I 0 or (I s )= "dark saturation current", the diode leakage current density in the absence of light; V = applied voltage across the terminals of the diode; q = absolute value of; electron charge k = Boltzmann's constant; and T = absolute temperature (K). The "dark saturation current" (I 0 ) is an extremely important parameter which differentiates one diode from another. I 0 is a measure of the recombination in a device. A diode with a larger recombination will have a larger I 0. Non-Ideal Diode Ideal Diode For actual diodes, the expression becomes: where: n = ideality factor, a number between 1 and 2 which typically increases as the current decreases.

32. 32 Temperature effects on p-n diode characteristics. Temperature can have a marked effect on the characteristics of a silicon semiconductor diode as shown in Figure below. It has been found experimentally that the reverse saturation current Io (Is) will just about double in magnitude for every 10°C increase in temperature. http://www.griet.in/ece/qna/EDCQNAUNITI.pdf

33. 33 Looking at the above equation (given in the previous lecture) it would appear that the current should decrease as the temperature increases. The exact opposite is what really occurs. The reverse saturation current, I S, is a strong positive function of temperature as discussed below. The increase in I S with temperature more than offsets the effect of T in the exponential above. Forward Bias: These curves show the characteristics of diode for different temperatures in the forward bias. As it can be seen from the figure given above, that curve moves towards left as we increase the temperature. We know with increase in temperature, conductivity of semiconductors increase. The intrinsic concentration (ni) of the semiconductors is dependent on temperature as given by: When temperature is high, the electrons of the outermost shell take the thermal energy and become free. So conductivity increases with temperature. Hence with increase in temperature, the forward curve would shift towards left i.e. curve would rise sharply and the breakdown voltage would also decrease with increase in temperature.

34. 34 Reverse Bias: This curve shows the characteristics of diode in the reverse biased region till the breakdown voltage for different temperatures. We know n i concentration would increase with increase in temperature and hence minority charges would increase with increase in temperature. The minority charge carriers are also known as thermally generated carriers and the reverse current depends on minority carriers only. Hence as the number of minority charge carriers increase, the reverse current would also increase with temperature as shown in the figure given on the previous page. The reverse saturation current gets double with every 10° C increase in temperature. In a PN junction diode, the reverse current is due to the diffusive flow of minority electrons from the p-side to the n-side and the minority holes from the n-side to the p-side. Hence I S, reverse saturation current depends on the diffusion coefficient of electrons and holes. The minority carriers are thermally generated so the reverse saturation current is almost unaffected by the reverse bias but is highly sensitive to temperature changes.

35. 35 IV characteristics for forward bias Point A corresponds to zero-bias condition. Point B corresponds to where the forward voltage is less than the barrier potential of 0.7 V (For Silicon). Point C corresponds to where the forward voltage approximately equals the barrier potential and the external bias voltage and forward current have continued to increase. The diode DC or static resistanceforward biased reverse biased

36. 36 The dynamic. resistance of a diode is designated r d

37. 37 Determine the dc resistance for a diode with the following operating point: A) I D =2 mA and V D = 0.5 V B) I D =20 mA and V D = 0.8 V C) I D =-1 μA and V D = -10 V Example

38. 38

39. 39 The p-n junction plays an important role as the basic device structure for fabricating a wide variety of electronic and photonic devices and is specially used in more advanced electronic technologies such as in Information and Communication Technology (ICT). For example, p-n junction structures have been used in fabricating the switching diodes, diode rectifiers, solar cells, light emitting diodes (LEDs), laser diodes (LDs), photo detectors, bipolar junction transistors (BJTs), heterojunction bipolar transistors (HBTs), and junction field-effect transistors (JFETs), metal–semiconductor field-effect transistors (MESFETs), high- electron mobility transistors (HEMTs), and tunnel diodes. The p-n heterojunctions can be formed from a wide variety of elemental and compound semiconductors such as n-Si/p-SiGe, n-ZnSe/p-GaAs, p-AlGaAs/n- GaAs, p-Ge/n-GaAs, n-InGaAs/n-InP, p-InAlAs/n-InGaAs, p-GaN/n-InGaN, and p-AlGaN/n-InGaN semiconductor electronic and photonic devices.

40. 40 Zener diodes are semiconductor diodes which have been manufactured to have their reverse breakdown occur at a specific, well-defined voltage (its “Zener voltage”), and that are designed such that they can be operated continuously in that breakdown mode. Commonly available Zener diodes are available with breakdown voltages (“Zener voltages”) anywhere from 1.8 to 200 V. Zener Diode Allows current flow in one direction, but also can flow in the reverse direction when above breakdown voltage http://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/

41. 41. Uses of Zener Diodes Since the voltage dropped across a Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in electric circuits. Using a resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the Zener voltage of the diode.

42. 42 Zener Breakdown Avalanche breakdown 1.This occurs at junctions which being heavily doped have narrow depletion layers 2. This breakdown voltage sets a very strong electric field across this narrow layer. 3. Here electric field is very strong to rupture the covalent bonds thereby generating electron hole pairs. So even a small increase in reverse voltage is capable of producing large number of current carriers. Ie why the junction has a very low resistance. This leads to Zener Breakdown. 1. This occurs at junctions which being lightly doped have wide depletion layers. 2. Here electric field is not strong enough to produce Zener breakdown. 3. Her minority carriers collide with semi conductor atoms in the depletion region, which breaks the covalent bonds and electron-hole pairs are generated. Newly generated charge carriers are accelerated by the electric field which results in more collision and generates avalanche of charge carriers. This results in avalanche breakdown. Breakdown voltage is a term used to describe the level of AC or DC voltage that results in the failure of a semiconductor device Avalanche Breakdown is A process that occurs in a diode when high voltage causes free electrons to travel at high speeds, colliding with other electrons and knocking them out of their orbits. The result is a rapidly increasing amount of free electrons.

43. 43 Tunnel diode structure The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Impurity concentration is 1 part in 10³ as compared to 1 part in 10 ⁸ in p-n junction diode. Also the depletion region, the area between the p-type and n-type areas, where there are no carriers is very narrow. Typically it is in the region of between five to ten nano-metres - only a few atom widths. The tunnel diode is generally made up of Ge and GaAs. Circuit symbol of tunnel diode is : It was introduced by Leo Esaki in 1958

44. 44 Under Forward Bias Step 1: At zero bias there is no current flow Step 2: A small forward bias is applied. Potential barrier is still very high – no noticeable injection and forward current through the junction. Step 3: With a larger voltage the energy of the majority of electrons in the n-region is equal to that of the empty states (holes) in the valence band of p-region; this will produce maximum tunneling current Step 4: As the forward bias continues to increase, the number of electrons in the n side that are directly opposite to the empty states in the valence band (in terms of their energy) decrease. Therefore decrease in the tunneling current will start. Step 5: As more forward voltage is applied, the tunneling current drops to zero. But the regular diode forward current due to electron – hole injection increases due to lower potential barrier.

45. 45 Because of heavy doping depletion layer width is reduced, reverse breakdown voltage is also reduced to a very small value resulting in appearance of the diode to be broken for any reverse voltage and a negative resistance section is produced in V-I characteristic of diode. Reduced depletion region can result in carrier ‘punching through’ the junction with the velocity of light even when they do not possess enough energy to overcome the potential barrier. The result is that large current (forward) is produced relatively low forward voltage (< 100 mV). Such a mechanism of conduction in which charge carriers punch through a barrier directly instead of climbing over it is called tunneling. That is why these diodes are tunnel diodes. Because of heavy doping, it can conduct in both forward as well as reverse direction. - Ve Resistance Region Vf Vp IpIp Vv Forward Voltage Reverse voltage IvIv VPVP Reverse Current Forward Current Ip:- Peak Current, Iv :- Valley Current Vp:- Peak Voltage, Vv:- Valley Voltage, Vf:- Peak Forward Voltage

46. 46 Varicap Diode (Variable Capacitor Diode) Varicap is a p-n junction with a special impurity profile, and its capacitance variation is very sensitive to reverse-biased voltage. Thus its reactance can be varied in a controlled manner with a bias voltage. Varactor diode symbol Varactor diodes are always operated under reverse bias conditions, and in this way there is no conduction. They are effectively voltage controlled capacitors.

47. 47 The transmission capacitance (C t ) established by the isolated uncovered charges is determined by C T =  ______ A WdWd Where  is the permitivity of the semiconductor materials, A the p-n junction area and W d the depletion width. C(pF) V R (V) (V R = applied reverse bias)

48. 48 They are widely used in parametric amplification, harmonic generation, mixing, detection, and in electronic tuning, voltage controlled oscillators in radio,TV, cellular and wireless receivers. commons.bcit.ca/cbennie/files/1207ch03.ppt

49. 49 Half wave rectifier 1H, full wave and bridge rectifiers 1H Power, efficiency and ripple factor for half wave and full wave rectifiers 1H Regulation, Harmonic components in rectified output 1H Rectifiers ( 04 H ):

50. 50 A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers are semiconductor diodes that conduct in only one direction. Today, most rectifier diodes are made of silicon. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. https://ssel.montana.edu/downloads/general/GC4.6.ppt

51. 51 Rectifier circuit are divided into three types 1- Uncontrolled rectifiers make use of diodes. The output D.C. voltage is fixed and is decided by the amplitude of the A.C. input voltage. The direction of the power flow is only from the source to the load. 2- The half controlled rectifier circuits comprise of diodes and Silicon- Controlled Rectifiers (SCRs). The D.C. load voltage can be controlled by changing the firing angle of the SCR. The control is limited in comparison with the fully controlled rectifier circuits. 3- The fully controlled rectifiers make use of SCRs only as the rectifying elements. The control of output voltage is obtained by changing the firing angle. as in the half controlled rectifiers. One important difference between This type of rectifier and half controlled type rectifier is that in fully controlled rectifier circuits direction of flow of power can be reversed i.e. it can be made to flow from D.C. to A.C. side. Types 2 and 3 are beyond the scope of this course

52. 52 Center-tapped Full-Wave Rectifier Full-Wave Bridge Rectifier Center-Tapped Full-Wave Rectifier Half-Wave Rectifier (HWR)Full-Wave Rectifier (FWR)

53. 53 Full-wave rectification A type of current conversion that uses both parts of the AC sine wave, both positive and negative, to produce a DC output with a single polarity. Half-wave rectification A type of current conversion that uses only one half of an AC waveform to convert into intermittent DC. This can be the positive half or negative half of an AC wave, depending on how the diode is connected to the circuit.

54. 54 HALF WAVE RECTIFIER The primary of the transformer is connected to ac supply. This induces an ac voltage across the secondary of the transformer. During the positive half cycle of the input voltage the polarity of the voltage across the secondary forward biases the diode. As a result a current I L flows through the load resistor, R. The forward biased diode offers a very low resistance and hence the voltage drop across it is very small. Thus the voltage appearing across the load is practically the same as the input voltage at every instant. During the negative half cycle of the input voltage the polarity of the secondary voltage gets reversed. As a result, the diode is reverse biased. Practically no current flows through the circuit and almost no voltage is developed across the resistor. All input voltage appears across the diode itself. https://www.classle.net/book/half-wave-rectifier

55. 55 Hence we conclude that when the input voltage is going through its positive half cycle, output voltage is almost the same as the input voltage and during the negative half cycle no voltage is available across the load. This explains the unidirectional pulsating dc waveform obtained as output. The process of removing one half the input signal to establish a dc level is called half wave rectification. A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in very small power supplies. A full wave rectifier achieves two peaks per cycle and this is the best that can be done with single-phase input. In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output

56. 56 In order to produce steady DC from a rectified AC supply, a smoothing circuit, sometimes called a filter, is required. In its simplest form this can be what is known as a reservoir capacitor, filter capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed. Peak Inverse Voltage When the input voltage reaches its maximum value V m during the negative half cycle the voltage across the diode is also maximum. This maximum voltage is known as the peak inverse voltage. Thus for a half wave rectifier http://www.visionics.ee/curriculum/Experiments/HW%20Rectifier/Half%20Wave%20Rectifier1.html

57. 57 PIV -+ + - PIV is the maximum (peak) voltage that appears across the diode when reverse biased. Here, PIV = V m. www.faculty.umassd.edu/xtras/catls/.../1851.ppt

58. 58 A Full Wave Rectifier is a circuit, which converts an ac voltage into a pulsating dc voltage using both half cycles of the applied ac voltage. It uses two diodes of which one conducts during one half cycle while the other conducts during the other half cycle of the applied ac voltage. During the positive half cycle of the input voltage, diode D1 becomes forward biased and D2 becomes reverse biased. Hence D1 conducts and D2 remains OFF. The load current flows through D1 and the voltage drop across R L will be equal to the input voltage.

59. 59 An ideal half-wave rectifier only "uses" half of the AC waveform (hence the name half-wave). An ideal full-wave bridge rectifier will use the entire AC waveform. An ideal full-wave rectifier (with a center-tapped transformer) will also use the entire AC waveform.

60. 60 A full-wave rectifier uses a diode bridge, made of four diodes

61. 61 A half wave rectifier removes one of the positive or the negative half cycle of the wave and only either half of the cycle appears in the output Where V max is the maximum or peak voltage value of the AC sinusoidal supply, and V S is the RMS (Root Mean Squared) value of the supply. Half Wave The outcome of the above equation is given in more details in appendix 1

62. 62 Full Wave In the full wave rectifier both the cycles appear in the positive or negative cycle of the output. The efficiency of a full wave rectifier (81.2%) is double of a half wave rectifier(40.6%) because the r.m.s. value in case of a full wave rectifier is Maximum current divided by 1.41 (under root of 2) whereas in case of a half wave rectifier the r.m.s current is half of maximum current during the wave cycle. The outcome of the above equation is given in more details in appendix 2

63. 63 If diode is ideal and/or R>>Rf then

64. 64 If diode is ideal and/or R>>Rf then