Prelab 3: Bipolar Junction Transistor Characterization

Transcription

1 Prelab 3: Bipolar Junction Transistor Characterization Name: Lab Section: 1. For the NPN device shown in Figure 1, label I C, I B, and I E next to their respective current arrows. Figure 1: A simple NPN device 2. What is β in terms of I C and I B? What is α in terms of I C and I E? Express α in terms of β. β (I C, I B ) = α (I C, I E ) = α (β) = 3. Given that you are a circuit designer working with an NPN device and you want the device to have a g m of 1 ms, what V BE value would you need to properly bias the NPN? Assume I S is A. 4. SPICE Write a SPICE netlist for the BJT test circuit shown in Figure 2. Refer to the HSPICE Tutorial if you have trouble with SPICE. Use the 2N4401 SPICE model provided on the course website. 1

2 2 I CC V BB + V CC + Figure 2: Circuit to simulate in SPICE Using the.dc command, sweep V CC from 0 V to 5 V in 0.01 V increments and step V BB from 0.6 V to 0.7 V in V increments. Run the simulation and check the output file for any errors. If there are no errors, plot I CC versus V CC and print out a copy of the plot. Note: If your I CC is negative, use Awaves to plot the absolute value of I CC. I CC appears to be negative because SPICE defines I CC to be going out of the BJT. 5. The configuration shown below in Figure 3 is known as the Darlington pair. Assume Q 1 has a DC current gain of β 1 and Q 2 has a DC current gain of β 2. Derive the overall current gain, β tot = I C2 /I B1, as a function of β 1 and β 2. Do not neglect any currents. I B1 Q 1 I C2 Q 2 Figure 3: Darlington configuration β tot = c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review. NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT.

3 1 Objective Experiment 3: Bipolar Junction Transistor Characterization The BJT was invented in 1948 by William Shockley at Bell Labs, and became the first mass-produced transistor. Having a good grasp of the physics of the BJT is key to understanding its operation and applications. In this lab, we will explore the BJT s four regions of operation and also determine its characteristic values, such as DC current gain β and Early voltage V A. The transistor used in this lab is the 2N4401, an NPN device. It is strongly recommended that you read and understand the section on BJT physics before beginning this experiment. 2 Materials You will need the components listed in Table 1. Note: Be sure to answer the questions on the report as you proceed through this lab. The report questions are labeled according to the sections in the experiment. CAU- TION: FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY HOT!!! Component Quantity 1 MΩ resistor kω resistor Ω resistor 4 2N4401 NPN BJT 2 Table 1: Components used in this lab 3 Procedure 3.1 Determining the Region of Operation 1. Set up the circuit shown in Figure 1, with R B = 1 MΩ, R C = 5.1 kω, R E = 100 Ω, and V CC = 5 V. If you need help identifying the terminals of the discrete BJT, please refer to Figure 2 2. Now gradually ramp up the value of V BB until I C = 0.5 ma. Measure V BE and V BC. The transistor is in which region of operation? Warning: Never set V BE higher than 5 V for any of the transistors used in the labs. Doing so will permanently damage the transistor and may cause personal injury. 3. Now measure I B. What is the value of β? 1

4 3 PROCEDURE 2 R C I C I B R B V CC + V BB + R E I E Figure 1: BJT measurement setup for this lab. Figure 2: For your reference, this is an illustration of the mapping between the terminals of a discrete NPN BJT and those of its circuit symbol counterpart. 4. From the value found above, calculate α and use it to calculate I E. Afterwards, measure I E and check if the calculated and measured values agree. 5. Now take a look at the 2N4401 datasheet. Does your calculated β value agree with the value given in the datasheet? Hint: β is called h FE in the datasheet, and there is a plot of h FE versus I C under Typical Characteristics. If the values do not agree, explain why there is a discrepancy. 6. On another note, let us examine the temperature dependence of the collector and base currents: Put two fingers around Q 1 to heat it up, and then, measure I B and I C (have your partner heat the BJT while you measure the currents if you are having trouble doing both at the same time). How do the values of I C and I B compare to the values you measured before you heated the transistor (i.e. do I C and I B increase or decrease)? 7. Explain, using the equation for the collector current, how you would expect I C to vary with temperature. Does this agree with your experimental results? If not, explain why this might be the case. (Hint: I S depends on the intrinsic carrier concentration n i and the diffusion coefficients D n and D p. Intuitively, how would n i, D n, and D p change with temperature? How would I S change with temperature as a result? Note: You do not need to explicitly answer the questions in the hint but do think about them.) 8. Let us now explore the different operation regions of a BJT. Set V BB to 4 V and V CC to 2 V. Measure I B, I C, V BE, and V BC. What is the region of operation for the BJT? 9. Set V BB to 3 V and V CC to 5 V. Measure I B, I C, V BE, and V BC. What is the region of operation for the BJT?

5 3 PROCEDURE Now swap the emitter and the collector connections of the BJT in the circuit (you can do this by physically rearranging the BJT to face the opposite direction). Set V BB to 4 V and keep V CC at 5 V. Measure I B, I C, V BE, and V BC. What is the region of operation? 3.2 Determining the Early Voltage Using the Parameter Analyzer Increasing the collector-base bias widens the depletion region at the collector-base interface. As a result, recombination decreases because the base is more depleted of available mobile holes, which are the main recombination source for electrons injected from the emitter. The widened depletion region also provides a greater electric field to sweep the injected electrons to the collector. Both of these effects result in an additional dependence of I C on V CE. The Early voltage is used to model this dependence. 1. Connect a BJT to the parameter analyzer s test fixture (without any resistors). Use ICS, the software accompanying the parameter analyzer, to bias the emitter at 0 V and the base at 0.6 V. Sweep the collector from 0 V to 5 V using 101 datapoints. Measure the current through the collector terminal. 2. Next, plot I C versus V C, the collector voltage. What two regions of operation are shown, and where is the boundary? 3. Use this plot to determine the Early voltage, V A. Hint: The parameter analyzer tutorial has instructions on the trend line tool found in Excel and thus, can help you calculate the Early voltage. 4. Repeat your calculation of V A for base voltages of V, V, V, V, and V (you can step the base voltage in ICS to get this data). Does V A depend on the base voltage V B? Why? 3.3 The BJT as a Diode 1. Connect a diode-connected BJT (i.e. the base and collector are shorted together) to the parameter analyzer s test fixture. Use ICS to ground the emitter and sweep the base/collector from 0 V to 0.7 V. Measure the current through the base/collector. (Teh base/collector acts as the P side of a diode). 2. Run the measurement and plot the base/collector current, I C versus V BE. Which other semiconductor device has this kind of I-V curve? 3.4 The Darlington Pair (Super High β) V CC = 3 V 100 Ω 100 Ω V BB Ω 1.2 V Q Ω Q 2 Figure 3: Darlington configuration for measurement 1. Construct the Darlington pair as shown in Figure Measure I B1, I C1, I B2, and I C2. Calculate β 1 = I C1 /I B1 and β 2 = I C2 /I B2.

6 3 PROCEDURE 4 3. What is the overall current gain, β tot = I C2 /I B1, based on your measured values of I C2 and I B1? Now, use the formula you derived in the prelab to calculate the overall current gain from β 1 and β 2. Compare this calculated β tot value to your measured β tot value. c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review. NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT.

7 Report 3: Bipolar Junction Transistor Characterization Name: Lab Section: For this lab, please record the values of current and voltage in the designated boxes. 3.1 Use this table to record and summarize relevant results as you proceed through the lab experiment: for each region of operation, fill in the appropriate entries in Table 1 and compute the resulting β and α values. Parameters Forward Active Saturation Cutoff Reverse Active V BE V BC I B I C β N/A N/A α N/A N/A Table 1: Regions of operations and measurements Measure V BE and V BC. What is the region of operation? V BE = V BC = Measure I B and compute β. I B = β = 1

8 Calculate α and use the result to calculate I E. What is the measured value of I E. Do the two values for I E agree? α = (Calculated) I E = (Measured) I E = Does your calculated β value agree with the value listed in the datasheet? If not, explain why there is a discrepancy Heat up the BJT with you fingers and record the respective values of I C and I B. How do the values of I C and I B change as you heat up the BJT? I B = I C = Explain, using the equation for the collector current, how you would expect I C to vary with temperature. Does this agree with your experimental results? If not, explain why this might be the case. (Hint: I S depends on the intrinsic carrier concentration n i and the diffusion coefficients D n and D p. Intuitively, how would n i, D n, and D p change with temperature? Also, how would I S change with temperature as a result? Note: You do not need to explicitly answer the questions in the hint but do think about them.)

9 Set V BB to 4 V and V CC to 2 V. Measure I B, I C, V BE, and V BC. What is the region of operation? I B = I C = V BE = V BC = Set V BB to 3 V and V CC to 5 V. Measure I B, I C, V BE, and V BC. What is the region of operation? I B = I C = V BE = V BC = Swap the emitter and collector connections of the BJT. Set V BB to 4 V and keep V CC at 5 V. Measure I B, I C, V BE, and V BC. What is the region of operation? I B = I C = V BE = V BC = Remember to fill out Table 1 using all the data you have collected thus far Attach the plot of the I-V curve to this worksheet. Label the two regions of operation and draw the boundary between them Use the I-V curve to determine V A. V A = Repeat your calculation of V A for base voltages of V, V, V, V, and V (you can step the base voltage in ICS to get this data). Does V A depend on V B? Why?

10 4 V B V V V V V V A Table 2: Early voltage calculations Attach the plot of the I-V curve to this worksheet. Which other semiconductor device has this kind of I-V curve? Measure I B1, I C1, I B2, and I C2. Calculate β 1 and β 2. I B1 = I C1 = I B2 = I C2 = β 1 = β 2 = What is the overall current gain, β tot = I C2 /I B1, based on your measured values of I C2 and I B1? Now, use the formula you derived in the prelab to calculate the overall current gain from β 1 and β 2. Compare this calculated β tot value to your measured β tot value. (Measured) β tot = (Calculated) β tot =

11 5 c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review. NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT.