Electronic Fundamentals - Diodes PDF

Module 4 Electronic Fundamentals Topic B1- 4.1.1: Diodes  INTRODUCTION On completion of this topic you should be able to: 4.1.1.1 Identify symbols for the following semiconductor types: Diode Silicon...

Module 4 Electronic Fundamentals Topic B1- 4.1.1: Diodes  INTRODUCTION On completion of this topic you should be able to: 4.1.1.1 Identify symbols for the following semiconductor types: Diode Silicon Controlled Rectifier (thyristor) Light Emitting Diode Photo Conductive Diode Varistor 4.1.1.2 Describe the characteristics and properties of Diodes. 4.1.1.3 Describe the effects of connecting diodes in series and parallel. Continued… 30-03-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 4.1.1.4 Describe the main characteristics of the following semiconductors and describe their uses: Rectifier Diodes Silicon Controlled Rectifier (thyristor) Light Emitting Diode Photo Conductive Diode Varistor 4.1.1.5 Describe functional testing of Diodes. 30-03-2024 Slide No. 3  THE DIODE A diode is a device that allows current to flow in one direction but not the other. A diode can be compared to a check valve in a hydraulic system. A check valve is a one way gate for fluids A diode is a one way gate for electrons. 30-03-2024 Slide No. 4  SEMICONDUCTORS, CONDUCTORS, AND INSULATORS All materials are made up of atoms. Atoms contribute to the electrical properties of a material, including its ability to conduct electrical current. An atom can be represented by the valence shell and a core that consists of all the inner shells and the nucleus. The carbon atom has four electrons in the valence shell and two electrons in the inner shell. The nucleus consists of six protons and six neutrons so the +6 indicates the positive charge of the six protons. The core has a net charge of +4 (+6 for the nucleus and −2 for the two inner-shell electrons). 30-03-2024 Slide No. 5  CONDUCTORS A conductor is a material that easily conducts electrical current. The best conductors are single-element materials, such as copper, silver, gold, and aluminum. These are characterised by atoms with only one valence electron very loosely bound to the atom. These can easily break away from their atoms and become free electrons. Therefore, a conductive material has many free electrons. When they move in the same direction, they make up the current. 30-03-2024 Slide No. 6  INSULATORS An insulator is a material that does not conduct electrical current under normal conditions. Most good insulators are compounds rather than single-element materials. Valence electrons are tightly bound to the atoms. Therefore, there are very few free electrons in an insulator. 30-03-2024 Slide No. 7  SEMICONDUCTORS A semiconductor is between conductors and insulators in its ability to conduct electrical current. A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. The most common single-element semiconductors are silicon, germanium, and carbon. Compound semiconductors such as gallium arsenide are also commonly used. The single-element semiconductors are characterized by atoms with four valence electrons. 30-03-2024 Slide No. 8  COMPARISON OF A SEMICONDUCTOR ATOM TO A CONDUCTOR ATOM Why is silicon a semiconductor and copper a conductor? The core of the silicon atom has a net charge of + 4 (14 protons - 10 electrons). The core of the copper atom has a net charge of + 1 (29 protons - 28 electrons). 30-03-2024 Slide No. 9  COMPARISON OF A SEMICONDUCTOR ATOM TO A CONDUCTOR ATOM The valence electron in the copper atom “feels” an attractive force of +1. A valence electron in the silicon atom which “feels” an attractive force of +4. So, there is four times more force trying to hold a valence electron to the atom in silicon than in copper. 30-03-2024 Slide No. 10  COMPARISON OF A SEMICONDUCTOR ATOM TO A CONDUCTOR ATOM The valence electron in copper has less force holding it to the atom than the valence electron in silicon. The valence electron in copper has more energy than the valence electron in silicon. It is easier for valence electrons in copper to escape from their atoms and become free electrons. 30-03-2024 Slide No. 11  SILICON AND GERMANIUM Silicon and germanium are both semi-conductors. Silicon is the most widely used material in diodes, transistors, integrated circuits, and other semiconductor devices. Both silicon and germanium have the characteristic four valence electrons. 30-03-2024 Slide No. 12  SILICON AND GERMANIUM The valence electrons in germanium are in the fourth shell while those in silicon are in the third shell. The germanium valence electrons are at higher energy levels than those in silicon. They therefore, require a smaller amount of additional energy to escape from the atom. This property makes germanium more unstable at high temperatures, and this is a basic reason why silicon is the most widely used semi conductive material. 30-03-2024 Slide No. 13  COVALENT BONDS Each silicon atom positions itself with four adjacent silicon atoms to form a silicon crystal. A silicon atom with its four valence electrons shares an electron with each of its four neighbors. This creates eight valence electrons for each atom and produces a state of chemical stability. This sharing of valence electrons produces the covalent bonds that hold the atoms together; each shared electron is attracted equally by two adjacent atoms which share it. 30-03-2024 Slide No. 14  COVALENT BONDS The covalent bonding in an intrinsic silicon (one with no impurities) crystal. Covalent bonding for germanium is similar because it also has four valence electrons. 30-03-2024 Slide No. 15  CONDUCTION ELECTRONS AND HOLES Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously. While some electrons recombine with holes. 30-03-2024 Slide No. 16  N-TYPE AND P-TYPE SEMICONDUCTORS Semi-conductive materials do not conduct current well and are of limited value in their intrinsic state. This is because of the limited number of free electrons in the conduction band and holes in the valence band. Intrinsic silicon (or germanium) must be modified by increasing the number of free electrons or holes to increase its conductivity. This is done by adding impurities to the intrinsic material. There are two types of extrinsic (impure) semi-conductive materials: N-type P-type They are the key building blocks for most types of electronic devices. 30-03-2024 Slide No. 17  DOPING The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semi-conductive material. This process is called doping. Doping increases the number of current carriers (electrons or holes). The two categories of impurities are: N-type P-type 30-03-2024 Slide No. 18  N-TYPE SEMICONDUCTOR To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb). Each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms. 30-03-2024 Slide No. 19  N-TYPE SEMICONDUCTOR Four of the antimony atom’s valence electrons are used to form the covalent bonds with silicon atoms. This leaves one extra electron. This extra electron becomes a conduction electron. Because the pentavalent atom gives up an electron, it is often called a donor atom. The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. A conduction electron created by this doping process does not leave a hole in the valence band. 30-03-2024 Slide No. 20  MAJORITY AND MINORITY CARRIERS Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an N-type semiconductor. The N stands for the negative charge on an electron. The electrons are called the majority carriers in N-type material. Although the majority of current carriers in N-type material are electrons. Holes in an N-type material are called minority carriers. 30-03-2024 Slide No. 21  P-TYPE SEMICONDUCTOR To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga). Each trivalent atom (boron, in this case) forms covalent bonds with four adjacent silicon atoms. All three of the boron atom’s valence electrons are used in the covalent bonds. Since four electrons are required, a hole results when each trivalent atom is added. 30-03-2024 Slide No. 22  P-TYPE SEMICONDUCTOR Because the trivalent atom can take an electron, it is often referred to as an acceptor atom. The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. 30-03-2024 Slide No. 23  MAJORITY AND MINORITY CARRIERS Since most of the current carriers are holes, silicon (or germanium) doped with trivalent atoms is called a P-type semiconductor. Holes can be thought of as positive charges. This is because the absence of an electron leaves a net positive charge on the atom. The holes are the majority carriers in P-type material. A few free electrons are also created when electron-hole pairs are thermally generated. Electrons in P-type material are the minority carriers. 30-03-2024 Slide No. 24  THE DIODE A P-type material consists of silicon atoms and trivalent impurity atoms such as boron. The boron atom adds a hole when it bonds with the silicon atoms. Since the number of protons and the number of electrons are equal throughout the material, there is no net charge in the material and so it is neutral. An N-type silicon material consists of silicon atoms and pentavalent impurity atoms such as antimony. An impurity atom releases an electron when it bonds with four silicon atoms. Since there is still an equal number of protons and electrons (including the free electrons) throughout the material, there is no net charge in the material and so it is neutral. 30-03-2024 Slide No. 25  DIODE SYMBOL This is the symbol for a diode. It is used in circuit diagrams: The cathode (negative side) side looks like a K The anode (positive side) side looks like an ‘A’ on its side When the cathode is connected to a negative source – current flows. When connected with arrow pointing to negative source – current flows. 30-03-2024 Slide No. 26  THE DIODE If a piece of intrinsic silicon is doped so that part is N-type and the other part is P-type. A PN junction forms at the boundary between the two regions and a diode is created. The P region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers). The N region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers). 30-03-2024 Slide No. 27  THE DIODE The free electrons in the N region are randomly drifting in all directions. At the instant of the PN junction formation free electrons near the junction in the N region begin to diffuse across the junction into the P region. Here they combine with holes near the junction. 30-03-2024 Slide No. 28  FORMATION OF THE DEPLETION REGION When the PN junction is formed, the N region loses free electrons as they diffuse across the junction, creating a layer of positive charges near the junction. As the electrons move across the junction, the P region loses holes as the electrons and holes combine, creating a layer of negative charges near the junction. These two layers of positive and negative charges form the depletion region. 30-03-2024 Slide No. 29  PN JUNCTION After the initial surge of free electrons across the PN junction. The depletion region will expand to a point where equilibrium is established. There is no further diffusion of electrons across the junction. 30-03-2024 Slide No. 30  BARRIER POTENTIAL In the depletion region there are many positive charges and many negative charges on opposite sides of the PN junction. The forces between the opposite charges form a “field of forces” called an electric field. This electric field is a barrier to the free electrons in the N region. Energy must be expended to move an electron through the electric field. External energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region. 30-03-2024 Slide No. 31  BARRIER POTENTIAL The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts. The barrier potential of a PN junction depends on several factors, including the type of semi-conductive material. The typical barrier potential is approximately 0.7 V for silicon and 0.3 V for germanium. 30-03-2024 Slide No. 32  FORWARD BIAS To forward bias a diode (allow current to flow through it), apply a DC voltage across it. Forward bias is the condition that allows current through the PN junction. A DC voltage (VBIAS) source is across a diode in the direction to produce forward bias. The negative side of VBIAS is connected to the N region of the diode and the positive side is connected to the P region. 30-03-2024 Slide No. 33  FORWARD BIAS -VE pushes conduction-band electrons in N region toward PN junction. + VE pushes holes in P region toward PN junction – (Recall, like charges repel). When external voltage overcomes barrier potential, it provides N region electrons with enough energy to penetrate depletion region and cross the junction. These electrons then combine with the P region holes. As electrons leave N region, more flow in from - VE supply. 30-03-2024 Slide No. 34  FORWARD BIAS Current in N-region is movement of conduction electrons (majority carriers) toward the junction. Once conduction electrons enter P region and combine with holes, they become valence electrons. They then move as valence electrons from hole to hole toward + VE source. 30-03-2024 Slide No. 35  FORWARD BIAS Movement of valence electrons is the same as movement of holes in opposite direction. Current in P region is movement of holes (majority carriers) toward junction. As the electrons flow out of the P region through the external connection they leave holes behind in the P region. 30-03-2024 Slide No. 36  FORWARD BIAS EFFECT ON DEPLETION REGION As more electrons flow into the depletion region, the number of positive ions is reduced. As more holes effectively flow into the depletion region, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow. 30-03-2024 Slide No. 37  PN JUNCTION PN junction is forward biased with +ve on Anode and –ve on Cathode. When barrier potential is overcome, current will flow. Depletion region P-type material N-type material + – + – 30-03-2024 Slide No. 38  PN JUNCTION Depletion region P-type material N-type material + – + – 0.7 volts for silicon and 0.3 volts for germanium. Depletion region is effectively eliminated. 30-03-2024 Slide No. 39  REVERSE BIAS Reverse bias is the condition that essentially prevents current through the diode. Connect the VBIAS with the positive to the N region and the negative to the P region. The depletion region is shown much wider than in forward bias or equilibrium. 30-03-2024 Slide No. 40  REVERSE BIAS The positive side of the bias-voltage source “pulls” the free electrons, which are the majority carriers in the N region, away from the PN junction. As the electrons flow toward the positive side of the voltage source, additional positive ions are created. This results in a widening of the depletion region and a depletion of majority carriers. 30-03-2024 Slide No. 41  REVERSE BIAS In the P region, electrons from the negative side of the voltage source enter as valence electrons. They move from hole to hole toward the depletion region where they create additional negative ions. This results in a widening of the depletion region and a depletion of majority carriers. The flow of valence electrons can be viewed as holes being “pulled” toward the positive side. 30-03-2024 Slide No. 42  REVERSE BIAS The initial flow of charge carriers is transitional and lasts for only a very short time. As the depletion region widens, the availability of majority carriers decreases. As more of the N and P regions become depleted of majority carriers, the electric field between the positive and negative ions increases in strength. This keeps occurring until the potential across the depletion region equals the bias voltage, VBIAS. At this point, the transition current essentially ceases except for a very small reverse current that can usually be neglected. 30-03-2024 Slide No. 43  REVERSE BIAS Very small leakage current (IR) is produced by minority carriers during reverse bias. A small number of thermally produced electron-hole pairs exist in depletion region. Under influence of external voltage, some electrons diffuse across PN junction before recombination. This process establishes a small minority carrier current throughout material. 30-03-2024 Slide No. 44  REVERSE CURRENT IR is dependent primarily on junction temperature - NOT on amount of reverse biased voltage. A temperature increase causes an increase in leakage current. Germanium has a greater leakage current than silicon (typically µA or nA range). 30-03-2024 Slide No. 45  REVERSE BREAKDOWN If the external reverse bias voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase. The high reverse-bias voltage imparts energy to the free minority electrons so that as they speed through the P region. They collide with atoms with enough energy to knock valence electrons out of orbit and into the conduction band. The newly created conduction electrons are also high in energy and repeat the process. If one electron knocks only two others out of their valence orbit during its travel through the P region, the numbers quickly multiply. 30-03-2024 Slide No. 46  REVERSE BREAKDOWN As these high energy electrons go through the depletion region, they have enough energy to go through the N region as conduction electrons, rather than combining with holes. The multiplication of conduction electrons just discussed is known as avalanche. This avalanche results in a very high reverse current that can damage the diode because of excessive heat dissipation. 30-03-2024 Slide No. 47  V-I CHARACTERISTIC FOR FORWARD BIAS With zero voltage across the diode, there is no forward current. Gradually increasing the forward-bias voltage will gradually increase: Forward current Forward voltage A portion of the forward-bias voltage is dropped across the limiting resistor. When the forward-bias voltage is increased to where the voltage across the diode reaches approximately 0.7 V, forward current increases rapidly. 30-03-2024 Slide No. 48  V-I CHARACTERISTIC FOR FORWARD BIAS Increase the forward-bias voltage: Current increase very rapidly Voltage across the diode increases only gradually above 0.7 V This small increase in the diode voltage above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semi conductive material. 30-03-2024 Slide No. 49  GRAPHING THE V-I CURVE By plotting the results of the type of measurements, shown previously, on a graph: The V-I characteristic curve for a forward-biased diode is derived. The diode forward voltage (VF) increases to the right along the horizontal axis. The forward current (IF) increases upward along the vertical axis. 30-03-2024 Slide No. 50  GRAPHING THE V-I CURVE The forward current increases very little until the forward voltage across the PN junction reaches approximately 0.7 V, at the knee of the curve. After this point, the forward voltage remains at approximately 0.7 V, but IF increases rapidly. There is a slight increase in VF above 0.7 V as the current increases due mainly to the voltage drop across the dynamic resistance. Normal operation for a forward-biased diode is above the knee of the curve. The IF scale is typically in mA. 30-03-2024 Slide No. 51  DYNAMIC RESISTANCE The resistance of the forward-biased diode is not constant over the entire curve. Because the resistance changes along the V-I curve, it is called dynamic or AC resistance. Below the knee of the curve the resistance is greatest because the current increases very little for a given change in voltage. The resistance begins to decrease in the region of the knee of the curve. It becomes smallest above the knee where there is a large change in current for a given change in voltage. 30-03-2024 Slide No. 52  GRAPHING THE V-I CURVE When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR). Gradually increasing the reverse-bias voltage will produce a very small reverse current and the voltage across the diode increases. When the applied bias voltage is increased to a the breakdown value (VBR). 30-03-2024 Slide No. 53  GRAPHING THE V-I CURVE The reverse current begins to increase rapidly. Continuing the reverse bias past this point increases current (Avalanche). But the voltage drop across the diode doesn’t vary much. This NOT a normal mode of operation. 30-03-2024 Slide No. 54  V-I CHARACTERISTIC CURVE The combined curves for both forward bias and reverse bias produces the complete V-I characteristic curve for a diode: IF scale is in mA IR scale in uA. 30-03-2024 Slide No. 55  TEMPERATURE EFFECTS ON V-I CHARACTERISTIC A forward-biased diode, for a given value of forward voltage as: Temperature is increased Forward current increases 30-03-2024 Slide No. 56  TEMPERATURE EFFECTS ON V-I CHARACTERISTIC Also, for a given value of forward current, the forward voltage decreases. The blue curve is at room temperature (25°C). The red curve is at an elevated temperature (25°C + ΔT). For a reverse-biased diode, as temperature is increased, the reverse current increases. 30-03-2024 Slide No. 57  DIODE STRUCTURE AND SYMBOL A diode is a single PN junction device. It has conductive contacts and wire leads connected to each region. A diode has two parts: An N-type semiconductor a P-type semiconductor The schematic symbol for a general-purpose or rectifier diode, is shown: The P region is called the anode The N region is called the cathode The “arrow” in the symbol points in the direction of conventional current flow (opposite to electron flow). 30-03-2024 Slide No. 58  FORWARD-BIAS CONNECTION A diode is forward-biased when a voltage source is connected as shown. The positive terminal of the source is connected to the anode through a current-limiting resistor. The negative terminal of the source is connected to the cathode. The forward current (IF) is from cathode to anode as indicated. The forward voltage drop (VF) due to the barrier potential is from positive at the anode to negative at the cathode. 30-03-2024 Slide No. 59  REVERSE-BIAS CONNECTION A diode is reverse-biased when a voltage source is connected as shown. The negative terminal is connected to the anode side of the circuit. The positive terminal is connected to the cathode side. The reverse current is negligible. The entire bias voltage (VBIAS) appears across the diode. 30-03-2024 Slide No. 60  THE IDEAL DIODE MODEL The ideal model of a diode is a simple switch: A forward-biased diode acts like a closed (on) switch A reverse-biased diode acts like an open (off) switch The barrier potential, the forward dynamic resistance, and the reverse current are all neglected. 30-03-2024 Slide No. 61  THE IDEAL DIODE MODEL The ideal V-I characteristic curve graphically depicts the ideal diode operation. Since the barrier potential and the forward dynamic resistance are neglected. The diode is assumed to have a zero voltage across it when forward- biased. 30-03-2024 Slide No. 62  THE PRACTICAL DIODE MODEL The practical model adds the barrier potential to the ideal switch model. When the diode is forward biased, it is equivalent to a closed switch in series with a small equivalent voltage. The equivalent voltage source is equal to the barrier potential (0.7 V). This equivalent voltage source represents the fixed voltage drop (VF) produced across the forward-biased PN junction. When the diode is reverse-biased, it is equivalent to an open switch just as in the ideal model. 30-03-2024 Slide No. 63  THE PRACTICAL DIODE MODEL The characteristic curve for the practical diode model is shown. Since the barrier potential is included and the dynamic resistance is neglected. The diode is assumed to have a voltage across it when forward-biased. This is indicated by the portion of the curve to the right of the origin. 30-03-2024 Slide No. 64  THE COMPLETE DIODE MODEL The complete model of a diode consists of the barrier potential, the small forward dynamic resistance and the large internal reverse resistance. When the diode is forward-biased, it acts as a closed switch in series with the barrier potential voltage and the small forward dynamic resistance. When the diode is reverse-biased, it acts as an open switch in parallel with the large internal reverse resistance. 30-03-2024 Slide No. 65  THE COMPLETE DIODE MODEL A forward-biased diode is assumed to have a voltage across it when the diode has barrier potential and the forward dynamic resistance. This voltage (VF) consists of the barrier potential voltage plus the small voltage drop across the dynamic resistance. The curve slopes because the voltage drop due to dynamic resistance increases as the current increases. 30-03-2024 Slide No. 66  TYPICAL DIODES The anode and cathode are indicated on a diode in several ways. The cathode is usually marked by: A band A tab Or some other feature On those packages where one lead is connected to the case, the case is the cathode. 30-03-2024 Slide No. 67  RECTIFIER DIODE The term diode and rectifier are often used interchangeably. Diode – a small signal device with current capacity typically in milliamp range. Rectifier – a power device, conducting from1 to 1000 amps or even higher. The cathode of a diode is marked by a band or similar indicating method. The primary uses of a rectifier diode include: Half Wave Rectifiers Full Wave Rectifiers DC Blockers 30-03-2024 Slide No. 68  DIODES IN SERIES HALF WAVE RECTIFIERS Recall a diode in series with a load acts like a one way valve. Current will only flow in one direction. In series with an AC power source, the diode will be forward and reverse biased every cycle. Rectification is accomplished. 30-03-2024 Slide No. 69  DIODES IN SERIES HALF WAVE RECTIFIERS The simplest rectifier circuit is a half-wave rectifier which consists of a diode, an AC power source, and a load resistor. A diode in parallel with an AC power source will short ½ of each cycle to earth – still results in half wave rectification. 30-03-2024 Slide No. 70  DIODES IN SERIES FULL WAVE RECTIFIERS A full wave rectifier has diodes in series with the applied EMF, but in parallel with each other. When one diode is forward biased, the other is reverse biased. This arrangement rectifies both alternations of the AC cycle. Current flows through the load in one direction only – the applied AC has become pulsating DC. The pulsating DC is further regulated to produce a pure DC source without any AC ripple. 30-03-2024 Slide No. 71  DIODES IN SERIES BRIDGE RECTIFIERS A bridge rectifier is a full wave rectifier. All 4 diodes are in series with the applied EMF, but are parallel to each other. Two diodes conduct simultaneously in a bridge rectifier. A bridge rectifier has twice the power rectification capacity of a conventional full wave rectifier. 30-03-2024 Slide No. 72  SERIES CONNECTED DIODES With diodes in series, combined effect is to increase reverse blocking capability. When forward biased, diodes conduct same current & have similar voltage drops. However, reverse voltages across each individual diode could vary drastically. Dependent on characteristic of each diode. It can be seen that voltage drop across D2 will not cause breakdown however, avalanche breakdown will occur in diode D1. Simplest protection is to connect high value resistors in parallel with each diode. 30-03-2024 Slide No. 73  PARALLEL CONNECTED DIODES Connecting diodes in parallel will increase the current carrying capability. Theoretically, if exact characteristics of each diode are known it would be possible to design resistors so that exact current sharing is achieved. Practically however, this is not possible and a simple design can be used. 30-03-2024 Slide No. 74  PARALLEL CONNECTED DIODES If maximum average current in to be rectified is 300A and diodes are not capable. VR1 = VR2 = VR3 = 1V then R1 = R2 = R3 = 1V/100A = 10 mΩ (10 milliohm) Note that a higher series resistance increases the on-state losses. Select resistance of standard available value that is slightly higher than calculated. 30-03-2024 Slide No. 75  THE DMM DIODE TEST POSITION Many digital multimeters (DMMs) have a diode test position. It provides a convenient way to test a diode. When set to diode test, the meter provides an internal voltage sufficient to forward-bias and reverse-bias a diode. The meter provides a voltage reading or other indication to show the condition of the diode under test. 30-03-2024 Slide No. 76  WHEN THE DIODE IS WORKING The red (positive) lead of the meter is connected to the anode. The black (negative) lead is connected to the cathode to forward-bias the diode. If the diode is good, you will get a reading of between approximately 0.5 to 0.7 V being typical for forward bias. 30-03-2024 Slide No. 77  WHEN THE DIODE IS WORKING The diode is turned around to reverse-bias it. If the diode is good, you will get a voltage reading based on the meter’s internal source. The 2.6 V shown in the figure represents a typical value and indicates that diode has an extremely high reverse resistance. Essentially all of the internal voltage appearing across it. 30-03-2024 Slide No. 78  WHEN THE DIODE IS DEFECTIVE When a diode has failed open, you get an open circuit voltage reading (2.6 V is typical). “OL” indication for both the forward-bias and the reverse-bias condition Fail-Open: In a fail-open scenario, if a system or device fails, it automatically opens or allows access. This is usually used in systems where availability is prioritized over security. For instance, in a firewall setting, if the firewall fails, all network traffic would be allowed through.. 30-03-2024 Slide No. 79  WHEN THE DIODE IS DEFECTIVE If a diode is shorted, the meter reads 0 V in both forward and reverse bias tests. 30-03-2024 Slide No. 80  CHECKING A DIODE WITH THE OHMS FUNCTION On DMM, set to ’resistance’ or ‘Ω’ function. Red (+ve) lead to Anode and Black (-ve) to Cathode. Meter should indicate a very low resistance – typically much less than 100 ohms. Reverse connection, should show a very high resistance – ‘OL’ on DMM. 30-03-2024 Slide No. 81  SUMMARY OF DIODE BIAS Forward Bias: Permits Majority-carrier Current. Bias voltage connections: positive to P region; negative to N region Bias voltage connections: positive to P region; negative to N region The bias voltage must be greater than the barrier potential Barrier potential: 0.7 V for silicon Majority carriers flow toward the PN junction Majority carriers provide the forward current The depletion region narrows 30-03-2024 Slide No. 82  SUMMARY OF DIODE BIAS Reverse bias: Prevents majority-carrier current Bias voltage connections: positive to N region; negative top region The bias voltage must be less than the breakdown voltage Majority carriers flow away from the PN junction during short transition time Minority carriers provide the extremely small reverse current There is no majority carrier current after transition time The depletion region widens 30-03-2024 Slide No. 83  LIGHT EMITTING DIODE (LED) When the LED device is forward-biased, electrons cross the PN junction from the N-type material and recombine with holes in the P-type material. When combination takes place, the recombining electrons release energy in the form of heat and light. 30-03-2024 Slide No. 84  LIGHT EMITTING DIODE (LED) A large exposed surface area on one layer of the semi-conductive material permits the photons to be emitted as visible light. This process, called electroluminescence. Various impurities are added during the doping process to establish the wavelength of the emitted light. The wavelength determines the color of the light and if it is visible or invisible (infrared). 30-03-2024 Slide No. 85  LIGHT EMITTING DIODE LEDs are made of: Gallium Arsenide (GaAs), Gallium Arsenide Phosphide (GaAsP) Gallium Phosphide (Ga)P Silicon and germanium are not used because they are essentially heat-producing materials and are very poor at producing light. 30-03-2024 Slide No. 86  LIGHT EMITTING DIODE Red is the most common. GaAs LEDs emit infrared (IR) radiation, which is nonvisible GaAsP produces either red or yellow visible light GaP emits red or green visible light 30-03-2024 Slide No. 87  LIGHT EMITTING DIODE The forward voltage across an LED is considerably greater than for a silicon diode. Typically the maximum VF for LEDs is between 1.2 V and 3.2 V, depending on device. Reverse breakdown for an LED is much less than for a silicon rectifier diode (3V to 10V typical). The LED emits light in response to a sufficient forward current. An increase in IF corresponds proportionally to an increase in light output. 30-03-2024 Slide No. 88  LIGHT EMITTING DIODE The wavelength of light determines whether it is visible or infrared. A LED emits light over a specified range of wavelengths as indicated by the spectral output curves. The wavelength (λ) is expressed in nanometers (nm). The normalized output of the visible red LED peaks at 660 nm, the yellow at 590 nm, green at 540 nm, and blue at 460 nm. 30-03-2024 Slide No. 89  LIGHT EMITTING DIODE Cathode (lead on right looking from front) Anode (lead near tab) Anode (longer lead) 30-03-2024 Slide No. 90  ORGANIC LED Substrate - The substrate supports the OLED. Anode - The anode removes electrons (adds electron "holes") when a current flows through the device. Organic layers - These layers are made of organic molecules or polymers. Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline. 30-03-2024 Slide No. 91  ORGANIC LED Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. One polymer used in the emissive layer is polyfluorene. Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device. 30-03-2024 Slide No. 92  ORGANIC LED Vacuum Deposition 30-03-2024 Slide No. 93  ORGANIC LED OVPD 30-03-2024 Slide No. 94  ORGANIC LED Inkjet Technology 30-03-2024 Slide No. 95  ORGANIC LED Optimus Maximus is a full-sized 113-keys keyboard with colour OLED screens located inside each key. The screens are 10.1mm large and have 48x48 pixels resolution, according to its developers. Specially designed software will be able to change images on the colour screens depending on the program running. 30-03-2024 Slide No. 96  THE PHOTODIODE The photodiode is a device that operates in reverse bias, where I is the reverse current. The photodiode has a small transparent window that allows light to strike the PN junction. An alternate photodiode symbol is: 30-03-2024 Slide No. 97  THE PHOTODIODE A photodiode has a very small reverse leakage current. When a photodiode’s PN junction is exposed to light, the reverse current increases with the light intensity. When there is no incident light, the reverse current, Iλ, is almost negligible and is called the dark current. An increase in the amount of light intensity, expressed as irradiance (mW/cm2), produces an increase in the reverse current. 30-03-2024 Slide No. 98  THE PHOTODIODE The photodiode allows essentially no reverse current (except for a very small dark current) when there is no incident light. When a light beam strikes the photodiode, it conducts an amount of reverse current that is proportional to the light intensity (irradiance). 30-03-2024 Slide No. 99  SILICON CONTROLLED RECTIFIER – SCR Thyristors – a group of semiconductor devices that act as open or closed switches. SCR is one type of thyristor (others are Shockley diode, SCS, Diac and Triac). 4-layer semiconductor device – Consists of alternating P and N type materials (PNPN). Has 3 terminals: Anode Cathode Gate (control electrode) 30-03-2024 Slide No. 100  SILICON CONTROLLED RECTIFIER – SCR When forward biased, NO current flows until a pulse is applied to gate. SCR then conducts, and continues to conduct even after gate voltage is removed. Large amounts of power can be switched using small triggering current or voltage. 30-03-2024 Slide No. 101  SILICON CONTROLLED RECTIFIER – SCR To switch off SCR: Reverse bias the cathode and anode Reduce the current below a specified minimum value A common method used to switch off SCRs is to short them out – this reduces the current through them below specified minimum value and it switches off. Suitable for latching circuits – for example, Intruder alarm – remains on until reset. 30-03-2024 Slide No. 102  VARISTOR Voltage dependent devices which behave similar to back to back zener diodes. Used as surge protection device – protects against transient spikes (lightning and so on.) Connected directly across AC input to electronic components or circuits. Rest state – high impedance (several MΩ) in relation to component being protected. 30-03-2024 ☻Slide No. 103  VARISTOR When a power surge or voltage spike is sensed – varistor conducts (virtual short). Symmetrical, sharp breakdown characteristics enable varistor to provide excellent transient suppression performance. As with zener diodes – only conduct above breakdown. Creates instant shunt path for over-voltage – protects sensitive components. Because shunt path creates short circuit - fuse/breaker subject to damage in process. 30-03-2024 Slide No. 104  CONCLUSION Now that you have completed this topic, you should be able to: 4.1.1.1 Identify symbols for the following semiconductor types: Diode Silicon Controlled Rectifier (thyristor) Light Emitting Diode Photo Conductive Diode Varistor 4.1.1.2 Describe the characteristics and properties of Diodes. 4.1.1.3 Describe the effects of connecting diodes in series and parallel. Continued… 30-03-2024 Slide No. 105  CONCLUSION Now that you have completed this topic, you should be able to: 4.1.1.4 Describe the main characteristics of the following semiconductors and describe their uses: Rectifier Diodes Silicon Controlled Rectifier (thyristor) Light Emitting Diode Photo Conductive Diode Varistor 4.1.1.5 Describe functional testing of Diodes. 30-03-2024 Slide No. 106  This concludes: Module 4 Electronic Fundamentals Topic B1- 4.1.1: Diodes