ObjectiveMaterialsProcedureDetermining the Region of OperationDetermining the Early Voltage Using the HP4155The BJT as a DiodeThe Darlington Pair (Super High )UNIVERSITY OF CALIFORNIA AT BERKELEYCollege of EngineeringDepartment of Electrical Engineering and Computer SciencesEE105 Lab ExperimentsExperiment 3: Bipolar Junction TransistorCharacterization1 ObjectiveThe BJT was invented in 1948 by William Shockley at Bell Labs, and b e c ame the first mass-producedtransistor. Having a good grasp of the physics of the BJT is key to understanding its operation andapplications. In this lab, we will explore the BJT ’s four operation regions and determine its characteristicva lues: DC current gain β and Early voltage VA. The transistor we will use is the 2N4401, an NPN device.It is strongly sug gested that you read and understa nd the section on BJT physics before beginning thisexp eriment.2 MaterialsComponent Quantity2N4401 NPN BJT 21 MΩ resistor 15 kΩ resis tor 1100 Ω resisto r 1Table 1: Components used in this lab3 Procedure3.1 Determining the Region of Operation1. Set up the circuit shown in Figure 1, with RB= 1 MΩ, RC= 5 kΩ, a nd RE= 100 Ω. Set VCCto 5 V.2. Increase VBBuntil IC= 0.5 mA. Measure VBEand VBC. What is the region of operation of thetransistor?Warning: Never set VBEhigher than 5 V for any of the transistors we us e. Doing so will permanentlydamage the transistor.3. Now measure IB. What is the value of β?4. From the value found above, calculate α. Use α to calc ulate IE, then measure IEand check if theva lues agree.5. Let’s examine the temperature dependence of collector current. Put two fingers around Q1to heat it,then measure IBand IC(have your partner hea t the BJ T while you measure the currents if you’rehaving trouble doing b oth at the same time). How does ICcompare to the value you mea sured beforeyou heated the transistor?13 PROCEDURE 2−+VBBRBIBRCIC−+VCCREIEFigure 1: BJT measurement setup for this lab6. Explain, using the equation you know for collector current, how you’d exp ect ICto var y with tempera-ture. Does this agree with your experimental r esults? If not, explain why this might be the case. Hint:ISdepends on the intrinsic carrier concentration niand the diffusion coefficients Dnand Dp. Intu-itively, how would ni, Dn, and Dpchange with temperature? How would ISchange with temperatureas a result?7. Look at the datasheet for the 2N4401. Does β (called hF Ein the datasheet) agree with the valuesgiven in the datasheet (Hint: A plot of hF Eversus ICis given under “Typical Characteristics”)? Ifthe values do not agree, explain why you might see discrepancies .8. Set VBBto 4 V and VCCto 2 V. Measure IB, IC, VBE, and VBC. What is the region of operation ofthe BJT ?9. Set VBBto − 3 V and VCCto 5 V. Measure IB, IC, VBE, and VBC. What is the region of operationof the BJT?10. Swap the emitter and the co llec tor of the BJT in the circuit (you can do this by physically turning thedevice to face the opposite directio n). Set VBBto 4 V and keep VCCat 5 V. Measure IB, IC, VBE,and VBC. What is the region of operatio n of the BJT?3.2 Determining the Early Voltage Using the HP4155Increasing the collector-base bias widens the depletion r egion at the interface. As a result, recombinationdecreases because the base is more depleted in mobile holes, which are the main recombination source forinjected electrons from the emitter. The widened depletion region also provides a greater electric field tosweep the injected electrons to the collector. Both of these effects result in an additiona l dependence of ICon VCE. The Ear ly voltage is used to model this depe ndence.1. Connect a BJT to the parameter analyzer’s test fixture (without any resistors). Use ICS to bia s theemitter at 0 V and the base at 0.6 V. Sweep the collector from 0 V to 5 V. Measure the currentthrough the collector terminal.2. Run the measurement and plot ICversus VC, the collector voltage. What two r e gions of operation areshown and where is the boundary?3. Use this plot to determine the Early voltage, VA. Hint: The HP4155 Tutorial has instructions thatshould help you calculate the Early voltage using Excel.4. Repeat your calculation of VAfor base voltages of 0.625 V, 0.65 V, 0.675 V, and 0.7 V (you can stepthe bas e voltage in ICS to get this data). Does VAdepend on the base voltage VB? Why?3 PROCEDURE 33.3 The BJT as a Diode1. Connect a diode-connected BJT (i.e. the base and collector are shorted) to the parameter analyze r’stest fixture. Use ICS to ground the emitter and sweep the base/ c ollector from 0 V to 0.7 V. Measurethe current through the base/collector (acting as the P side of the diode).2. Run the measurement and plot the base/collector current ICvs. VBE. What semico nductor devicedoes this I-V curve look like?3.4 The Darlington Pair (Super High β)−+VBB1.2 VQ1VCC= 3 VQ2Figure 2: Darlington configuratio n for measure ment1. Construct the Darlington pair with your second BJT as shown in Figure 2.2. Measure IB1, IC1, IB2, and IC2. Calculate β1= IC1/IB1and β2= IC2/IB2.3. What is the overall current gain, βtot= IC2/IB1? Use the formula you derived in the prelab to calculatethe total current gain from β1and β2and compare the calculation to your
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