Lab+31+Diode+Characteristics

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31 Diode Characteristics Name _NEELMANI BHARDWAJ___________________ Date ___________________ Class ___________________ READING Text, Sections 16–1 through 16–3 NOTE: Text references in Experiments 31–44 are only for Electronics Fundamentals: Circuits, Devices, and Applications . OBJECTIVES After performing this experiment, you will be able to: 1. Measure and plot the forward- and reverse-biased IV characteristics for a diode. 2. Test the effect of heat on a diode’s response. 3. Measure the ac resistance of a diode. MATERIALS NEEDED Resistors: One 330 Ω; one 1.0 MΩ One signal diode (1N914 or equivalent) SUMMARY OF THEORY Semiconductors are certain crystalline materials that can be altered with impurities to radically change their electrical characteristics. The impurity can be an electron donor or an electron acceptor. Donor impurities provide an extra electron that is free to move through the crystal at normal temperatures. The total crystal is electrically neutral, but the availability of free electrons in the material causes the material to be classified as an N-type (for negative) semiconductor. Acceptor impurities leave a “hole” (the absence of an electron) in the crystal structure. These materials are called P-type (for positive) semiconductors. They conduct by the motion of shared valence bond electrons moving between the atoms of the crystal. This motion is referred to as hole motion because the absence of an electron from the crystal structure can be thought of as a hole. When a P-type and an N-type material are effectively made on the same crystal base, a diode is formed. The PN junction has unique electrical characteristics. Electrons and holes diffuse across the junction, creating a barrier potential , which prevents further current without an external voltage source. If a dc voltage source is connected to the diode, the direction it is connected has the effect of either increasing or decreasing the barrier potential. The effect is to allow the diode to either conduct readily or to become a poor conductor. If the negative terminal of the source is connected to the N-type material and the positive terminal is connected to P-type material, the diode is said to be forward-biased, and it conducts. If the positive terminal of the source is connected to the N- Circuit Application 2015 Fall Page 1

type material and the negative terminal is connected to P-type material, the diode is said to be reverse-biased, and the diode is a poor conductor. While the actual processes that occur in a diode are rather complex, diode operation can be simplified with three approximations. The first approximation is to consider the diode as a switch. If it is forward-biased, the switch is closed. If it is reverse-biased, the switch is open. The second approximation is the same as the first except it takes into account the barrier potential. For a silicon diode, this is approximately 0.7 V. A forward-biased silicon diode will drop approximately 0.7 V across the diode. The third approximation includes the first and second approximations and adds the small forward (bulk) resistance that is present when the diode is forward-biased. PROCEDURE 1. Measure and record the resistance of the resistors listed in Table 31– 1 . Then check your diode with the ohmmeter. Select a low ohm range and measure the forward and reverse resistance by reversing the diode. The diode is good on this test if the resistance is significantly different between the forward and the reverse directions. If you are using an autoranging meter, the meter may not produce enough voltage to overcome the barrier potential. You should select a low ohm range and hold that range. Consult the operator’s manual for specific instructions. Record the data in Table 31–1 . Table 31–1 Component Listed Value Measured Value R 1 330 Ω 320 Ω R 2 1.0 MΩ 990 Ω D 1 forward resistance 146 Ω D 1 reverse resistance 0 Ω 2. Construct the forward-biased circuit shown in Figure 31–1 . The line on the diode indicates the cathode side of the diode. Set the power supply for zero volts. Figure 31–1 Circuit Application 2015 Fall Page 2

Circuit Application 2015 Fall Page 3

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Table 31–2 V F (measured) V R 1 (measured) I F (computed) 0.45 V 0.01V 0.0303mA 0.50 V 0.02V 0.061mA 0.55 V 0.07V 0.21mA 0.60 V 0.22V 0.67mA 0.65 V 0.74V 2.24mA 0.70 V 1.78V 5.39mA 0.75 V 4.45V 13.5mA 3. Monitor the forward voltage drop, V F , across the diode . Slowly increase V S to establish 0.45 V across the diode. Measure the voltage across the resistor, V R 1 , and record it in Table 31–2 . 4. The diode forward current, I F , can be found by applying Ohm’s law to R 1 . Compute I F and enter the computed current in Table 31–2 . 5. Repeat steps 3 and 4 for each voltage listed in Table 31–2 . 6. With the power supply set to the voltage that causes 0.75 V to drop across the diode, bring a hot soldering iron near the diode. Do not touch the diode with the iron. Observe the effect of heat on the voltage and current in a forward-biased diode. If you have freeze spray available, test the effect of freeze spray on the diode’s operation. Describe your observations. 7. The data in this step will be accurate only if your voltmeter has a very high input impedance. You can find out if your meter is high impedance by measuring the power supply voltage through a series 1.0 MΩ resistor. If the meter reads the supply voltage accurately, it has high input impedance. Connect the reverse-biased circuit shown in Figure 31–2 . Set the power supply to each reverse voltage listed in Table 31–3 , ( V R ). Measure and record the voltage across R 2 ( V R 2 ). Use this voltage and Ohm’s law to compute the reverse current in each case. Enter the computed current in Table 31–3 . Figure 31–2 Table 31–3 V R (measured) V R 2 (measured) I R (computed) 5.0 V 0.46V 0.00546 mA Circuit Application 2015 Fall Page 4

10.0 V 0.91V 0.01091mA 15.0 V 1.37V 0.01637mA 8. Graph the forward- and reverse-biased diode curves on Plot 31–1 . The different voltage scale factors for the forward and reverse curves are chosen to allow the data to cover more of the graph. You need to choose an appropriate current scale factor that will put the largest current recorded near the top of the graph. Plot 31–1 9. With the power supply set to 15 V, bring a hot soldering iron near the diode. Do not touch the diode with the iron. Observe the effect of heat on the voltage and current in the reverse-biased diode. If you have freeze spray available, test the effect of freeze spray on the diode’s operation. Describe your observations. CONCLUSION Circuit Application 2015 Fall Page 5

In this lab I learned about different types of diode and their characteristics. I also learned about the function of diode in the reverse and forward biases after going through the different types of voltage and current. I also plotted a graph for the diode showing its current during different types of voltage. EVALUATION AND REVIEW QUESTIONS 1. Does the diode’s reverse resistance stay constant? Explain your answer. Ans – No, the reverse resistance of the diode does not remain constant. However, it is considered as infinite but it is constant because of the leakage of small current. 3. From the data in Table 31–2 , compute the maximum power dissipated in the diode. The maximum power dissipated by the diode will be 6W , when 0.75V has been given. 4. Based on your observations of the heating and cooling of a diode, what does heat do to the forward and reverse resistance of a diode? ANS-So, whenever the temperature is increased. The forward dais increases as it cut off the depletion area. However, it does opposite in reverse biased. 5. Explain how you could use an ohmmeter to identify the cathode of an unmarked diode. Why is it necessary to know the actual polarity of the ohmmeter leads? Diode has a very less resistance when connected in forward bias. The positive lead will be connected with anode and the negative one with cathode. It is just because they conduct current in one direction and they will show no reading when connected in different direction. 6. A student measures the resistance of an unmarked diode with an ohmmeter. When the (+) lead of the ohmmeter is connected to lead 1 of the diode and the (−) lead of the ohmmeter is connected to lead 2 Circuit Application 2015 Fall Page 6

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of the diode, the reading is 400 Ω. When the ohmmeter leads are reversed, the reading is ∞. Which lead on the diode is the anode? ANS – THE LEAD 1 is the anode because it shows the finite readings. Circuit Application 2015 Fall Page 7