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# MODERN LAB METHODS I PHYS 3322

OK State

GPA 3.82

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This 0 page Class Notes was uploaded by Kendrick Wilderman on Sunday November 1, 2015. The Class Notes belongs to PHYS 3322 at Oklahoma State University taught by Staff in Fall. Since its upload, it has received 8 views. For similar materials see /class/232923/phys-3322-oklahoma-state-university in Physics 2 at Oklahoma State University.

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Date Created: 11/01/15

PHYS 3322 Modern Laboratory Methods I Diodes Purpose In this experiment you will investigate the currentvoltage characteristic of semiconductor diodes and determine the parameters of nonideal diodes Equipment 0 Two DMMs o 1 k9 resistor o Diodes 0 Adjustable DC power supply 0 Capacitors 0 lN9l4 Si switching o Oscilloscope o 22uF o lN34 Ge 0 120 VAC and transformer o 220 uF 0 1N4733 Si regulator board lab equipment for recti er circuits 0 lN4007 Si recti er Ten turn 1 k9 potentiometer O ares llght39emmmg with attached resistor network 10 6 lab equipment Background A simpli ed model for the IV characteristics of an ideal semiconductor diode relates the current and voltage by 1IoquAkET 1 1 I0 is the reverse saturation current VA is the applied voltage across the diode and is positive for the forward bias condition T is the temperature k3 is Boltzmann s constant and q 1602 X 103919 C Usually the magnitude of the electronic charge is represented by a lower a case e but I have written it as q in order to prevent confusion in the above equation ideal diode 1 Iowankquot 71 Figure l The current characteristics of an ideal diode Revised 16 January 2003 Diodes Figure 1 shows the characteristics of an ideal diode under both forward and reverse bias In forward bias the current increases exponentially or nearly so and in reverse bias the current is relatively constant and is equal to 10 Equation 1 can be approximated since the argument in the exponential of Equation 1 at 300 K or room temperature is large enough such that the effect of subtracting 1 from the exponential term is negligible at sufficiently high forward bias voltages At 300 K the argument is i3876V 1 T300K 2 kBT 00258eV Consequently for positive applied voltages forward bias Equation 1 can be approximated by I m IOquAkET in forward bias VA gtgt 0 3 How large VA must be in order to use this approximation can be garnered from Table l where the term in braces in Equation 1 and the exponential term in Equation 2 are compared A value of VA01 V results in 2 difference between the expressions with the percent difference decreasing at even larger positive values of VA Table 1 Forward bias comparison of Equations 1 and 2 at 300 K VA quAkBT l quAkET difference 005 594474 694474 168216 01 4722941 4822941 211732 02 2325076 2326076 004301 03 1121843 1121853 000089 04 5410628 5410629 18E05 05 261 E08 261E08 38E07 In reverse bias VAlt0 the current is negative and at room temperature or N300 K modest values of V A result in the exponential term being much smaller than one For VA0l V the exponential term has the value 148 at room temperature and the current is within 2 of 7L for the ideal diode Consequently Equation 2 becomes in reverse bias I 10 reverse bias VA ltlt 0 4 At larger negative values of V A the approximation of equation 4 is even better Hence under reverse bias conditions the current does not change under sufficient applied voltage it is said to saturate and hence I0 is termed the reverse saturation current With Equation 3 in mind the characteristics of an ideal diode are straightforward in forward bias Taking the common log of equation 3 gives logILVA logIO 5 2303 kBT Figure 2 shows a semilog plot of the current versus voltage for an ideal diode in forward bias For sufficient applied voltage the curve is linear with an intercept equal to log 10 Hence this is an additional and perhaps a better method to determine 10 Revised 16 January 2003 Diodes log logao VA Figure 2 Plot of log I versus VA forward bias for an ideal diode Less than ideal diodes For real diodes not ideal the current versus applied voltage curves deviates from the ideal case This deviation is dependent on the applied voltage To quantify the behavior of real diodes the ideal diode equation Equation 1 is modi ed to include an ideality factor in the exponential For a real diode 11quAquotkaT 1 6 where n is the ideality factor and is equal to one for an ideal diode Again under sufficient forward bias this expression can be written I m IOquAquotkET in forward bias VA gtgt 0 7 Taking the common log of this expression as before yields 1 v 10 1 8 2303nkBT A g 0 0 logI Equation 8 shows that under sufficient forward bias a semilog plot of current versus applied voltage for real diodes is a straight line The ideality factor n is not constant for all forward bias voltages but changes depending on the forward bias voltage and the characteristics of real diodes For many diodes at low forward currents n approaches 2 and at intermediate currents n m l and at high currents n m 2 Also apparent in many diodes is a region at high currents where the resistance of the bulk diode material decreases the current from the ideal to such a degree that a specific value for n cannot be determined Figures 3 and 4 show semilog IV plots for real diodes with the values of II listed in the plots were obtained from least squares fitting 36 Revised 16 January 2003 Diodes V A V A Figure 4 Plot of In I as a function of VA in forward bias for a real diode Revised 16 January 2003 Diodes Procedure Exercise 1 I V curves of diodes Measure the exact resistance of the 1 k9 resistor For each diode Construct the circuit shown in Figure 1 using the 1 k9 resistor and the DC power supply Attach one multimeter so that it measures the voltage drop across the diode and attach the other multimeter so that the current through the diode can be monitored As it is the bipolar power supply will work only when the diode is biased a certain way Why Note You may have to construct a different circuit to obtain all the necessary data for this experiment Measure the voltage across the diode and the current through the diode in both the forward and reverse bias conditi ns As yo vary the power supply voltage monitor the power dissipated in the resistor P V2 R Do not let the power dissipated in the resistor exceed its power rating Repeat the above measurements using each diode For each diode on separate graphs On linear scales plot the current yaxis versus the bias voltage On semilog scales plot the current versus the forward bias voltage For each diode determine 10 V0 and the ideality factor R IA l AAAAAAA vvvvvvvv I u 2 AAA vvvv V9 Figure 1 Circuit network for measuring the forward and reverse bias current in a diode Questions Discuss and compare the IV curves for the different diodes Revised 16 January 2003 56 Diodes Exercise 2 The half wave recti er Construct a halfwave recti er using a lN4007 diode as shown in Figure 2 with R470 Q The transformer and 120 VAC input are lab equipment and come as a single unit In this circuit the oscilloscope monitors the output of the diode Double check your circuit before applying power if the circuit is incorrect you will likely blow a fuse Sketch the output waveform using the oscilloscope with 1 no capacitor 2 a 22 HF capacitor and 3 a 220 uF capacitor in the circuit 120 VAC C 3 vvvv pa I ltgt Figure 2 The halfwave recti er circuit Exercise 3 The Full Wave Recti er Now convert the circuit into a fullwave recti er by adding a second lN4007 diode as shown in Figure 3 Double check your circuit before applying power if the circuit is incorrect you will likely blow a fuse Sketch the output waveform with the oscilloscope with 1 no capacitor 2 a 22 HF capacitor and 3 a 220 uF capacitor in the circuit 120 VAC Figure 3 The full wave recti er circuit Questions Explain the shapes of the output waveforms in the halfwave and fullwave recti er circuits Revised 16 January 2003

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