Prelab 3: Bipolar Junction Transistor Characterization
Name:
Lab Section:
1. For the NPN device shown in Figure 1, label IC , IB , and IE next to their respective current arrows.
Figure 1: A simple NPN device
2. What is β in terms of IC and IB ? What is α in terms of IC and IE ? Express α in terms of β.
β (IC , IB ) =
α (IC , IE ) =
α (β) =
3. Given that you are a circuit designer working with an NPN device and you want the device to have a
gm of 1 mS, what VBE value would you need to properly bias the NPN? Assume IS is 5 × 10−16 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
ICC
VBB +
−
VCC +
−
Figure 2: Circuit to simulate in SPICE
• Using the .dc command, sweep VCC from 0 V to 5 V in 0.01 V increments and step VBB from
0.6 V to 0.7 V in 0.025 V increments.
• Run the simulation and check the output file for any errors.
• If there are no errors, plot ICC versus VCC and print out a copy of the plot. Note: If your ICC is
negative, use Awaves to plot the absolute value of ICC . ICC appears to be negative because SPICE
defines ICC to be going out of the BJT.
5. The configuration shown below in Figure 3 is known as the Darlington pair. Assume Q1 has a DC
current gain of β1 and Q2 has a DC current gain of β2 . Derive the overall current gain, βtot = IC2 /IB1 ,
as a function of β1 and β2 . Do not neglect any currents.
IB1
Q1
IC2
Q2
Figure 3: Darlington configuration
βtot =
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, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
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 regions of operation and also determine its characteristic values,
such as DC current gain β and Early voltage VA . 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. CAUTION: FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY
HOT!!!
Component
1 MΩ resistor
5.1 kΩ resistor
100 Ω resistor
2N4401 NPN BJT
Quantity
1
1
4
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 RB = 1 MΩ, RC = 5.1 kΩ, RE = 100 Ω, and VCC = 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 VBB until IC = 0.5 mA. Measure VBE and VBC . The transistor
is in which region of operation?
Warning: Never set VBE 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 IB . What is the value of β?
1
3
2
PROCEDURE
RC
IC
IB
RB
VBB +
−
VCC +
−
RE
IE
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 IE . Afterwards, measure IE 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 hF E in the datasheet, and there is a plot of hF E versus IC 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 Q1 to heat it up, and then, measure IB and IC (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 IC and IB compare to the values you measured before you heated the transistor (i.e. do IC
and IB increase or decrease)?
7. Explain, using the equation for the collector current, how you would 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? 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 VBB to 4 V and VCC to 2 V. Measure
IB , IC , VBE , and VBC . What is the region of operation for 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
for the BJT?
3
3
PROCEDURE
10. 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 VBB to 4 V and keep VCC at 5 V.
Measure IB , IC , VBE , and VBC . 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 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, 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 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 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 VA for base voltages of 0.600 V, 0.625 V, 0.650 V, 0.675 V, and 0.700 V
(you can step the base voltage in ICS to get this data). Does VA depend on the base voltage VB ?
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, IC versus VBE . Which other semiconductor
device has this kind of I-V curve?
3.4
The Darlington Pair (Super High β)
VCC = 3 V
100 Ω
100 Ω
VBB +
− 1.2 V
100 Ω
Q1
100 Ω
Q2
Figure 3: Darlington configuration for measurement
1. Construct the Darlington pair as shown in Figure 3.
2. Measure IB1 , IC1 , IB2 , and IC2 . Calculate β1 = IC1 /IB1 and β2 = IC2 /IB2 .
3
PROCEDURE
4
3. What is the overall current gain, βtot = IC2 /IB1 , based on your measured values of IC2 and IB1 ?
Now, use the formula you derived in the prelab to calculate the overall current gain from β1 and β2 .
Compare this calculated βtot value to your measured βtot 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, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
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
VBE
VBC
IB
IC
β
α
Forward Active
Saturation
Cutoff
N/A
N/A
N/A
N/A
Reverse Active
Table 1: Regions of operations and measurements
3.1.2 Measure VBE and VBC . What is the region of operation?
VBE =
VBC =
3.1.3 Measure IB and compute β.
IB =
β=
1
2
3.1.4 Calculate α and use the result to calculate IE . What is the measured value of IE . Do the two values
for IE agree?
α=
(Calculated) IE =
(Measured) IE =
3.1.5 Does your calculated β value agree with the value listed in the datasheet? If not, explain why there is
a discrepancy.
3.1.6 Heat up the BJT with you fingers and record the respective values of IC and IB . How do the values
of IC and IB change as you heat up the BJT?
IB =
IC =
3.1.7 Explain, using the equation for the collector current, how you would 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? Also, how would IS change with
temperature as a result? Note: You do not need to explicitly answer the questions in the hint but do
think about them.)
3
3.1.8 Set VBB to 4 V and VCC to 2 V. Measure IB , IC , VBE , and VBC . What is the region of operation?
IB =
IC =
VBE =
VBC =
3.1.9 Set VBB to −3 V and VCC to 5 V. Measure IB , IC , VBE , and VBC . What is the region of operation?
IB =
IC =
VBE =
VBC =
3.1.10 Swap the emitter and collector connections of the BJT. Set VBB to 4 V and keep VCC at 5 V. Measure
IB , IC , VBE , and VBC . What is the region of operation?
IB =
IC =
VBE =
VBC =
Remember to fill out Table 1 using all the data you have collected thus far.
3.2.2 Attach the plot of the I-V curve to this worksheet. Label the two regions of operation and draw the
boundary between them.
3.2.3 Use the I-V curve to determine VA .
VA =
3.2.4 Repeat your calculation of VA for base voltages of 0.600 V, 0.625 V, 0.650 V, 0.675 V, and 0.700 V
(you can step the base voltage in ICS to get this data). Does VA depend on VB ? Why?
4
VB
0.600 V
0.625 V
0.650 V
0.675 V
0.700 V
VA
Table 2: Early voltage calculations
3.3.2 Attach the plot of the I-V curve to this worksheet. Which other semiconductor device has this kind of
I-V curve?
3.4.2 Measure IB1 , IC1 , IB2 , and IC2 . Calculate β1 and β2 .
IB1 =
IC1 =
IB2 =
IC2 =
β1 =
β2 =
3.4.3 What is the overall current gain, βtot = IC2 /IB1 , based on your measured values of IC2 and IB1 ?
Now, use the formula you derived in the prelab to calculate the overall current gain from β1 and β2 .
Compare this calculated βtot value to your measured βtot value.
(Measured) βtot =
(Calculated) βtot =
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, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.