UNIVERSITY OF CALIFORNIA AT BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences EE105 Lab Experiments Experiment 3 Bipolar Junction Transistor Characterization 1 Objective 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 operation regions and determine its characteristic values DC current gain and Early voltage VA The transistor we will use is the 2N4401 an NPN device It is strongly suggested that you read and understand the section on BJT physics before beginning this experiment 2 Materials Component 2N4401 NPN BJT 1 M resistor 5 k resistor 100 resistor Quantity 2 1 1 1 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 RB 1 M RC 5 k and RE 100 Set VCC to 5 V 2 Increase VBB until IC 0 5 mA Measure VBE and VBC What is the region of operation of the transistor Warning Never set VBE higher than 5 V for any of the transistors we use Doing so will permanently damage the transistor 3 Now measure IB What is the value of 4 From the value found above calculate Use to calculate IE then measure IE and check if the values agree 5 Let s examine the temperature dependence of collector current Put two fingers around Q1 to heat it then measure IB and IC have your partner heat the BJT while you measure the currents if you re having trouble doing both at the same time How does IC compare to the value you measured before you heated the transistor 1 3 2 PROCEDURE RC IC IB RB VBB VCC RE IE Figure 1 BJT measurement setup for this lab 6 Explain using the equation you know for collector current how you d expect IC to vary with temperature Does this agree with your experimental results If not explain why this might be the case Hint IS depends on the intrinsic carrier concentration ni and the diffusion coefficients Dn and Dp Intuitively how would ni Dn and Dp change with temperature How would IS change with temperature as a result 7 Look at the datasheet for the 2N4401 Does called hF E in the datasheet agree with the values given in the datasheet Hint A plot of hF E versus IC is given under Typical Characteristics If the values do not agree explain why you might see discrepancies 8 Set VBB to 4 V and VCC to 2 V Measure IB IC VBE and VBC What is the region of operation of the BJT 9 Set VBB to 3 V and VCC to 5 V Measure IB IC VBE and VBC What is the region of operation of the BJT 10 Swap the emitter and the collector of the BJT in the circuit you can do this by physically turning the device to face the opposite direction Set VBB to 4 V and keep VCC at 5 V Measure IB IC VBE and VBC What is the region of operation of the BJT 3 2 Determining the Early Voltage Using the HP4155 Increasing the collector base bias widens the depletion region at the interface As a result recombination decreases because the base is more depleted in mobile holes which are the main recombination source for injected electrons 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 IC on VCE 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 to bias the emitter at 0 V and the base at 0 6 V Sweep the collector from 0 V to 5 V Measure the current through the collector terminal 2 Run the measurement and plot IC versus VC the collector voltage What two regions of operation are shown and where is the boundary 3 Use this plot to determine the Early voltage VA Hint The HP4155 Tutorial has instructions that should help you calculate the Early voltage using Excel 4 Repeat your calculation of VA for base voltages of 0 625 V 0 65 V 0 675 V and 0 7 V you can step the base voltage in ICS to get this data Does VA depend on the base voltage VB Why 3 3 PROCEDURE 3 3 The BJT as a Diode 1 Connect a diode connected BJT i e the base and collector are shorted 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 acting as the P side of the diode 2 Run the measurement and plot the base collector current IC vs VBE What semiconductor device does this I V curve look like 3 4 The Darlington Pair Super High VCC 3 V Q1 VBB 1 2 V Q2 Figure 2 Darlington configuration for measurement 1 Construct the Darlington pair with your second BJT as shown in Figure 2 2 Measure IB1 IC1 IB2 and IC2 Calculate 1 IC1 IB1 and 2 IC2 IB2 3 What is the overall current gain tot IC2 IB1 Use the formula you derived in the prelab to calculate the total current gain from 1 and 2 and compare the calculation to your measurement
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