David L. Senasack June, 2006 Dale Jackson Career Center, Lewisville Texas. The PN Junction

Transcription

1 David L. Senasack June, 2006 Dale Jackson Career Center, Lewisville Texas The PN Junction Objectives: Upon the completion of this unit, the student will be able to; name the two categories of integrated circuits, cite the difference between dc and ac resistance, cite the difference between an analog signal and a digital signal, name the two steps required to convert an analog signal into a digital signal, calculate the values of a sinusoidal wave, define harmonics, describe the diode IV characteristic curve, and explain the spectrum for a repetitive waveform differ from that of a nonrepetitive waveform. Appropriate Grade Levels: Physics, Math or Electronics TEKS covered in this lesson plan: Science (c) (7) (D & E); (8) (D), (8) (A-C) Math (b)(4)(a-c) Electronics (c) (3) (A-G); (c) (3) (A-D) I. Lesson plan Introduction: SAY: Since September 12, 1958 when Jack Kilby, at Texas Instruments, made the first integrated circuit, it literally created the modern computer age and brought about sweeping changes in medicine, communication, manufacturing, and the entertainment industry. ASK: How is this modern invention produced? SAY: The answer lies in the lesson for today, the PN junction. Background connected to the Semiconductor Industry : In the state of Texas, this industry thrives on the production of integrated circuits used for many applications. Can you name some applications? Many people help design, develop, test and manufacture electrical and electronics equipment. This industry is called the semiconductor industry. Lecture Outline: A. Terms and Definitions: a. Analogb. Digitalc. Electron Shellsd. Electron Orbitse. Valence electronsf. Conduction electronsg. Ionsh. Metallic bondsi. Covalent bondsj. Intrinsic-

2 k. Dopingl. Forward biasm. Reverse biasn. Anodeo. Cathodep. Barrier potentialq. Dioder. Rectifiers. Zener diode- B. The Atomic Structure of Semiconductors a. Electron Shells & Orbits b. Valence electrons, conduction electrons, and ions c. Metallic bonds d. Covalent bonds e. Free electrons and holes i. positive ii. negative C. The PN Junction a. Doping b. The PN junction i. diode ii. the depletion region D. Biasing the Semiconductor Diode a. Forward Bias b. Reverse Bias i. Peak Inverse Voltage (PIV) ii. Reverse Breakdown E. Diode Characteristics a. Diode symbol b. Diode Characteristic curve c. Testing Diodes with an Ohmmeter or a Multimeter d. The Diode Offset Model e. Types of Diodes F. Rectifier Circuits a. Half-wave rectifiers b. Full-wave rectifiers c. Bridge rectifiers II. Job Description: In the design and manufacturing of integrated circuit, these are the basic practices needed by the engineers to accomplish some of the internal components. Engineers take this practice to heart in their every day design, develop, test, and manufacturing of their products. III. Activities and Resources: a. Time needed for the activity: one week or five school days?

3 b. Suggested group size: divide the class into groups of two to partner up on the lab activity after the lecture c. Materials needed: local companies that use this technology are: DRS, Texas Instruments, Micron, Semi, Raytheon, ST, TriQuint Semiconductor and Maxim (Dallas Semiconductor), access to internet, and industry magazines such as; Nuts & Volts, Popular Science, Popular Mechanics, Make: technology on your time, PC World, PC Magazine, and Windows IT Pro. IV. Activity or working problem Equipment Needed: an assortment of diodes, multimeter, graph paper, solderless breadboards, and an assortment of resistors (330 Ω & 1.0 MΩ), oscilloscope, power supply. When a p-type material and an n-type material are made on the same crystal base, a diode is formed from the pn junction. A pn junction has unique electrical characteristics. When it is formed, electrons and holes diffuse across the junction, creating a barrier potential that 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 conduct readily in one direction but not the other. If the negative terminal of the source is connected to the n-type material and the positive terminal is connected to the 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-type material and the negative terminal is connected to the p-type material, the diode is said to be reversebiased and the diode is a poor conductor. The schematic symbol for a diode is shown below. The arrow shows the direction of the conventional current (positive to negative).for many applications, thinking of a diode as a one-way valve is sufficient. Other applications require a better approximation. The ideal diode considers the diode as a one-way valve or as an open or closed switch. If it is forward-biased, the switch is closed; if it is reversedbiased the switch is open. The diode-offset model adds the forward-biased diode drop needed to overcome the barrier potential. For a silicon diode, this is approximately 0.7 V; for germanium, the drop is approximately 0.3 V.

4 In this experiment, you will take data on a forward- and a reverse-biased diode and plot the IV characteristic curve. You will also set up a circuit to plot the diode response directly on an oscilloscope. Procedure 1. Measure and record the values of the resistors used in this experiment. Then check the diode with an ohmmeter by measuring the forward bias and reverse bias resistance by reversing the meter leads across the diode. The diode passes this test if the resistance is significantly different between the two measurements. If you are using an autoranging meter, the ohmmeter may not produce enough voltage to overcome the barrier potential; in this case, consult the operator s manual for specific instructions to down range the meter. Record the data on the table below. 2. Construct the forward-biased circuit shown below. The line on the diode indicates the cathode side of the diode (with forward bias, this is the negative side). Set the power supply for zero volts. 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 R1, and record it in the table above. 4. Find the diode forward current, I F, by applying Ohm s law to R1. Compute I F and enter the computed current in the table above.

5 5. Repeat steps 3 and 4 for each voltage listed in the table above. 6. The data in this step will be accurate only if your voltmeter has a 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 below. Set the power supply to each voltage listed in the table. Measure the voltage across R2 and record in Table below. Apply Ohm s law to compute the reverse current, IR2, in each case. Enter the computed current in the Table below. 7. Graph the forward- and reverse-biased diode curves on the Plot below. 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. 8. You can plot the diode s forward characteristic or an oscilloscope by connecting the circuit as shown below. Channel 1 senses the voltage drop across the diode; channel 2 shows a signal that is proportional to the current. The scope is placed in the X-Y mode. The signal generator ground must not be the same as the scope ground. Channel 2 must be inverted to display the signal in the proper orientation. Set up the circuit and observe the signal.

6 Observations: 1. V. Conclusion and/or solution: 1. What factors affected the accuracy of the measurements in this experiment? (Consider both the forward-biased and reverse-biased cases) 2. How can you use an ohmmeter to identify the cathode of an unmarked diode? Why is it necessary to know the actual polarity of the ohmmeter leads? VI. Assessment: How does the teacher grade the lesson plan? 1. Use the experiment KEY and grade the lab 2. Develop and administer a unit exam 3. Have a discussion about what the students experienced in this unit