Chapter1-Diode (1).pdf

Page 1 ECE_1071: Basic Electronics ECE_1071: BASIC ELECTRONICS TABLE OF CONTENTS Part - I: Analog Electroni...

Page 1 ECE_1071: Basic Electronics ECE_1071: BASIC ELECTRONICS TABLE OF CONTENTS Part - I: Analog Electronics Chapter – 1: Diodes and Applications Module-1: PN Junction Diodes 1.1.1 Introduction 2 1.1.2 Concept of PN junction 3 1.1.3 PN junction under bias 4 1.1.4 V-I Characteristics of diode 7 1.1.5 Static and dynamic Resistance of Diode 11 1.1.6 Ideal and Practical diodes 13 1.1.7 Equivalent circuit of diode 13 1.1.8 Break down phenomenon in diodes 17 1.1.9 Zener diode characteristics 18 1.1.10 Diode as a capacitor 20 Module-2: Applications of Diodes 1.2.1 Introduction 23 1.2.2 Basic Block diagram of DC power supply 24 1.2.3 Half wave rectifier 28 1.2.4 Full Wave Rectifier 32 1.2.5 Full wave bridge rectifier 35 1.2.6 Comparison of Rectifiers 38 1.2.7 Rectifier with filter 39 Module-3: Voltage Regulators 1.3.1 Zener Voltage Regulator 46 1.3.2 IC Voltage Regulators 54 Module-4: Special purpose diodes 1.4.1 Light Emitting Diodes 56 1.4.2 Photo diodes and applications 57 1.4.3 Optocoupler 58 1.4.4 Solar Cell 60 Department of Electronics and Communication Engineering, MIT Bengaluru. 1  Page 2 ECE_1051: Basic Electronics Part –I ANALOG ELECTRONICS Chapter -1: Diodes and Applications The term diode is used to represent a device or element which has two electrodes. These devices are characterized by the fact that they allow electric current to flow in one direction, and block flow of current in the opposite direction. This unilateral behaviour is predominantly used in switching and rectification. Diode is in fact, the very first electronic device invented. Initially, for several years vacuum tube version was in use, and were bulky in size, required higher power for operation, and were slow. Today, we have semiconductor diodes, which are very small in size, requires relatively low power and operates at higher speeds. Semiconductor diodes are available in various forms and are used in wide variety of applications. In this unit, we shall look at the operating behaviour and characteristics of semiconductor diodes along with their typical applications such as rectifiers, voltage regulators and some special purpose applications. Module – 1 : Diodes Learning Outcomes: At the end of this module, students will be able to: 1. Explain the operation of PN junction diode under different biasing conditions. 2. Plot the I-V characteristics of the diode. 3. Define static and dynamic resistance of the diode. 4. Explain the breakdown phenomenon observed in diodes. 5. Describe the working of Zener diode and plot its I-V characteristics. 6. Explain the operation of diode as a capacitor 1.1.1 Introduction Materials are broadly classified as metals, insulators, and semiconductors. A semiconductor like Germanium or Silicon has electrical conductivity lying between conductor and insulator. Department of Electronics and Communication Engineering, MIT Bengaluru. 2  Page 3 ECE_1051: Basic Electronics Semiconductors are the basic materials used in modern electronics. For example, Diodes, Transistors, Solar cells, Light-emitting diodes (LEDs), and integrated circuits. Self-Reading: 1. Crystal structure of Germanium and Silicon 2. Intrinsic and extrinsic semiconductors 3. N-type and P-type semiconductors and concept of minority and majority cariiers 4. Diffusion and drift current 1.1.2 Concept of PN Junction As you know, P-type semiconductor has large number of holes while, N-type semiconductor has large number of free electrons. When P-type and N-type materials are joined together, a gradient of charge carrier densities is created at the junction. This will cause the electrons to move from N-type material to the P-type material and holes to move from P-type material to the N-type material. This process of movement of charge carriers from the region of higher concentration to a lower concentration in the absence of external electric field is called diffusion. Diffusion of charge carriers across the junction will continue until the equilibrium condition is established. Also at the junction, N-type material will have positively charged immobile ions and P-type material have negatively charged immobile ions. Thus, the regions on either side of p–n interface lose their charge neutrality and become charged. For this reason, it is called space charge region. As the region is devoid or depleted of mobile charge carriers it is also called depletion region and is as shown in Figure. 1.1.1. Department of Electronics and Communication Engineering, MIT Bengaluru. 3  Page 4 ECE_1051: Basic Electronics Figure 1.1.1. Schematic of PN junction The space charge on either side of the junction causes a potential difference across the P-N junction and it is called the barrier potential. This is the minimum amount of voltage required to initiate flow of charge carriers across the junction. Doped germanium has a barrier potential of about 0.3 volts whereas, doped silicon has a barrier voltage of about 0.7 volts. 1.1.3 P-N junction under bias Application of external voltage across the diode is called biasing. Depending upon the polarity and magnitude of voltage applied, we can have three biasing conditions, as listed below. a) No bias or Zero bias b) Forward bias c) Reverse bias Figure 1.1.2. Circuit symbol of a diode a) Zero Bias: In the absence of any bias voltage, the net flow of charge carriers in any one direction for a semiconductor diode is zero. This occurs because minority carriers (holes) in the N-type material will encounter barrier in the depletion region to cross the junction and move to the P-type region. Same is the case for electrons in P-type material. This results in depletion region with high impedance, and hence no current flows through the diode. The built-in potential varies from 0.3 to 0.7 V depending upon the type of semiconductor material. A diode operated without any biasing is shown in Figure 1.1.3. Department of Electronics and Communication Engineering, MIT Bengaluru. 4  Page 5 ECE_1051: Basic Electronics Figure. 1.1.3. P-N diode with zero bias. b) Forward Bias: When a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material, the diode is said to be in a Forward Bias condition. Figure 1.1.4. shows the diode with forward bias. If the external voltage applied is greater than the value of the barrier potential, the carriers start crossing the junction and hence there will be a forward current. The diode is said to be in the ON condition. Figure.1.1.4. Forward biasing of P-N junction diode. [http://www.imagesco.com/articles/photovoltaic/photovoltaic-pg3.html]. Department of Electronics and Communication Engineering, MIT Bengaluru. 5  Page 6 ECE_1051: Basic Electronics c) Reverse Bias: When a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material, the diode is said to be in a reverse biased condition, as shown in Figure. 1.1.5. The positive voltage applied to the N-type semiconductor attracts electrons towards the positive electrode and hence away from the junction. At the same time, the holes in the P-type semiconductor are attracted towards the negative electrode. This results in widening of depletion layer due to a lack of electrons and holes near the junction and presents a high impedance path for the majority carriers. The height of potential barrier is increased, which prevents the flow of forward current through the diode. However, the applied potential favours the movement of minority carriers across the junction causing flow of current in the reverse direction. This current is called reverse saturation current and is represented by I0 or IS. Figure. 1.1.5. Reverse biasing of P-N junction diode. [http://www.imagesco.com/articles/photovoltaic/photovoltaic-pg3.html]. Self test: 1. The arrow direction in the diode symbol indicates a. Direction of electron flow. b. Direction of hole flow (Direction of conventional current) c. Opposite to the direction of hole flow d. None of the above 2. When the diode is forward biased, it is equivalent to a. An OFF switch Department of Electronics and Communication Engineering, MIT Bengaluru. 6  Page 7 ECE_1051: Basic Electronics b. An On switch c. A high resistance d. None of the above 3. The barrier potential voltage of Si diode is a. 0.2 V b. 0.7 V c. 0.8 V d. 1.0 V 4. List the methods available for testing the diode. How cut-in voltage of diode is measured in practice? 1.1.4 I-V characteristics of diode I-V characteristics of practical diode is shown in Figure 1.1.6. When the forward biased voltage is applied to diode, current is initially zero and then increases sharply after crossing the cut-in voltage. In this case, the diode behaves like a closed switch. Similarly, in reversed biased condition, the diode behaves like an open switch and very small current flows due to minority charge carriers, which is known as reverse saturation current. In the reverse biased condition, beyond a particular reverse voltage, a sudden rise of current will be observed, and this voltage is called breakdown voltage. Figure. 1.1.6. I-V characteristics of practical diode [http://www.learningaboutelectronics.com/Articles/Ideal-diode.php] Department of Electronics and Communication Engineering, MIT Bengaluru. 7  Page 8 ECE_1051: Basic Electronics a) Forward biased Characteristic: The application of a forward bias voltage to the junction diode results in the depletion layer becoming very thin and narrow which represents a low resistance path through the junction thereby aiding flow of current through the diode. The point at which this sudden increase in current takes place is called “knee” point and is represented on the static I-V characteristics as shown in Figure 1.1.7. Figure 1.1.7. I-V characteristics of P-N junction diode under forward biased condition. [http://www.electronics-tutorials.ws/diode/diode_3.html] b) Reverse biased Characteristic: In this case, PN junction offers high resistance value and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small leakage current flows through the junction which is in the order of microamperes (μA ) for ordinary rectifier diodes. Department of Electronics and Communication Engineering, MIT Bengaluru. 8  Page 9 ECE_1051: Basic Electronics Figure. 1.1.8. I-V characteristics of P-N junction diode under reversed biased condition. [ http://www.electronics-tutorials.ws/diode/diode_3.html] If the reverse bias voltage VR applied to the diode is increased beyond certain limits there will a large current due to avalanche effect and cause breakdown. This is shown in figure 1.1.8. c) The Diode Current: The current flowing through the diode is given by the following equation. ID= Diode current Io = Reverse saturation current. VD= Applied bias voltage (Positive for forward and negative for reverse bias) VT=T/11600 VT = Volt equivalent of temperature (T is in degree Kelvin) for Germanium η =1 and for Silicon η =2 For large forward bias, equation 1.1.1 approximates to Department of Electronics and Communication Engineering, MIT Bengaluru. 9  Page 10 ECE_1051: Basic Electronics For large reverse bias, equation 1.1.1 approximates to d) Effect of Temperature on the Reverse current: The reverse saturation current is a temperature dependent parameter. It doubles for every 10o C rise in temperature. Let I01 be the reverse saturation current at temperature T1 and I02 be the reverse saturation current at temperature T2, where T2 > T1. Thus, the rise in reverse saturation current can be modelled as Department of Electronics and Communication Engineering, MIT Bengaluru. 10  Page 11 ECE_1051: Basic Electronics 1.1.5 Static and Dynamic Resistance of Diode There are two types of diode resistance namely, DC and AC resistance. a) DC or Static Resistance The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. The resistance of the diode at the operating point can be found by the corresponding values of VD and ID as shown in Figure 1.1.9. The equation for the DC resistance can then be written as, Note that the DC or static resistance of a diode does not depend on the curve shape, it depends only on the operating point or the values of diode voltage and current. Department of Electronics and Communication Engineering, MIT Bengaluru. 11  Page 12 ECE_1051: Basic Electronics Figure. 1.1.9. Static resistance of a diode [http://www.freewebs.com] b) AC or Dynamic Resistance: To determine the dynamic resistance of a diode, a a tangent is drawn to the curve through the operating point as shown in Figure 1.1.10. The dynamic resistance of the diode is found using the following equation: Department of Electronics and Communication Engineering, MIT Bengaluru. 12  Page 13 ECE_1051: Basic Electronics Figure 1.1.10. Dynamic resistance [http://www.freewebs.com] Also, dynamic resistance is found by the derivative of the diode equation; where: 1.1.6 Ideal and Practical diode In an ideal diode, current flows freely through the device when forward biased, offering no resistance. An ideal diode is simply a P-N junction where the change from P-type to N-type material is assumed to occur instantaneously, also referred to as an abrupt junction. The simplified diode model ignores the effect of diode resistance in comparison with values of other elements of the circuit. The voltage drop across the diode is zero. A practical diode does offer some resistance to current flow when forward biased. Since there is some resistance (built-in potential) , there will be some power dissipated when current flows through a forward biased diode. Therefore, there is a practical limit to the amount of current a diode can conduct without damage. A reverse biased diode has very high resistance and excessive reverse bias can cause the diode to damage. 1.1.7 Equivalent circuit of diode: Diode is often replaced by its equivalent circuit during circuit analysis and design. For DC diode model, characteristics of an ideal diode and the modifications that were required due to Department of Electronics and Communication Engineering, MIT Bengaluru. 13  Page 14 ECE_1051: Basic Electronics practical considerations has been considered for following cases: (i) Ideal Diode (ii) Second approximation of diode (iii) Practical diode (i) For an Ideal diode Vγ = 0, RR = ∞ and RF = 0 as shown in Figure 1.1.14. In other words, the ideal diode is a short in the forward bias region and an open in the reverse bias region. Figure 1.1.11 Equivalent circuit of diode for second approximation. (ii) In Practical diode (silicon) Vγ = 0.7 V, RR < ∞ (typically several MΩ), RF ≈ rd (typically < 50 Ω) as shown in Figure 1.1.12. Department of Electronics and Communication Engineering, MIT Bengaluru. 14  Page 15 ECE_1051: Basic Electronics Figure 1.1.12 Equivalent circuit of practical diode. Exercise Problem 1: 1. A Silicon diode has a saturation current of 1pA at 200C. Determine (a) Diode bias voltage when diode current is 3mA (b) Diode bias current when the temperature changes to 1000C, for the same bias voltage. Ans. Given: The diode current ID=3mA, Reverse saturation current I0=1x10-12 A, Temperature T=20°C = 273+20 = 293K The diode is silicon η=2 The equation for the diode current ID is given by (b) The diode current when the temperature is 1000 C Department of Electronics and Communication Engineering, MIT Bengaluru. 15  Page 16 ECE_1051: Basic Electronics 2. Find the static and dynamic resistance of a P-N junction germanium diode if the temperature is 27°C and I0=1μA for an applied forward bias of 0.2V. Department of Electronics and Communication Engineering, MIT Bengaluru. 16  Page 17 ECE_1051: Basic Electronics 1.1.8 Break down phenomenon in diodes Breakdown voltage is the largest reverse voltage that can be applied without causing an exponential increase in the current in the diode. As long as the current is limited, exceeding the breakdown voltage of a diode does no harm to the diode. Two breakdown mechanisms exist in diode. They are a) Zener breakdown b) Avalanche breakdown a) Zener breakdown: In Zener breakdown, the electric field established due to the reverse voltage capable of getting the electrons out of their covalent bonds and away from their parent atoms as shown in Figure 1.1.13. Electrons are transferred from the valence to the conduction band. In this situation, the Department of Electronics and Communication Engineering, MIT Bengaluru. 17