Lab 5: Part II: Semiconductor Transistors
Edited 9/2015 by Stephen Albright & DGH
PURPOSE OF EXPERIMENT: Study the characteristics of a semiconductor transistor.
REFERENCES: Tipler, Chapter 9 (esp. p. 340-342); Enge, Wehr and Richards, Chapter 11;
Horowtiz & Hill, p. 11, 35, 192; Eisberg & Resnick, Chapter 13.
APPARATUS: Power Transistor, Instek D.C. Power Supply, Agilent Multimeter, HP 3468A
Multimeter, 11kΩ Decade Resistance Box and 12V Indicator Lamp.
OVERVIEW: A transistor is a semiconductor device commonly used to amplify or switch
electronic signals. Bipolar junction transistors (BJT) are a very common transistors and come in
two varieties, PNP & NPN. In a simplistic sense a BJT can be thought of as two diodes connected
cathode to cathode (PNP) or anode to anode (NPN) – Note, this simplistic model would not actually
perform as a transistor. See below:
Diode
Simplistic Model of PNP
Simplistic Model of NPN
In reality, a BJT has several significant differences from the simplistic model. In a BJT the central
region called the Base is very thin and only lightly doped. One of outside sections, called the
Emitter, is heavily doped and the other outside section, called the Collector, is also heavily doped
except for a narrow region next to the Base where it is lightly doped. These differences in thickness
and doping are what allow a BJT to function.
Accurate Model of a PNP Transistor
PROCEDURE:
1. Construct the following circuit. The diode in the figure refers to any pair of contacts on the
transistor (the junction between Base and either of the other two contacts is approximately a
diode). Note, unlike the procedures we used to study diodes, in this lab the same circuit will
be used to test for forward and reverse bias and no electrometers will be used – the accuracy
is not required.
2. Set the decade resistance box to 5kΩ, the ammeter to 0.00000A & the Voltmeter to 60V.
Circuit for measuring I & V across each pair of transistor contacts
3. Identify the transistors “Base” by applying 0.5-1V (as read across the “diode”) across each
of the three pairs of contacts. Apply the voltage in both polarities. Note: You will need to
adjust the power supply voltage and some configurations will not allow 0.5-1V across the
pair. Do not exceed 6V as read on the power supply.
4. Determine if the transistor is PNP or NPN by observing the polarity of the forward and
reverse bias relative to the Base.
5. Identify the transistor’s “Emitter” and “Collector” by applying ~14V in both polarities
across the two unidentified contacts. The current will be noticeably higher when the
Collector is forward biased relative the Base than when the Emitter is forward biased.
6. Construct the following circuit. Be sure to Note: the polarity of the Collector - Base
connection is reverse bias. That is, the Base is positive (less negative) relative to the
Collector:
Circuit for measuring IB & IC
7. Set the fine dial on the Resistance Box to 1kΩ (Full CW). Set the power supply to ~11V
(not greater than 12V), the IC Ammeter to 6A and the IB Ammeter to 0.00000A. Decrease
the resistance one click at a time and record IB & IC for each step - include 1kΩ . Warning:
Do not leave IC set at maximum (0 resistance) for very long (~ 1minute), the transistor could
overheat and be destroyed.
8. Plot IC vs IB.. From the graph, estimate the saturation current (IC)s.
IC
(IC)s
(IC)= (IB)
IB
9. Discuss the relationship between IC & IB at higher currents.
10. How does saturation current (IC)s impact the performance of an amplifier?
11. A transistor’s gain () is defined as  = IC / IB. Plot  vs IB.
12. Over what part of this graph would an amplifier perform the best? Explain.
13. Leaving the base connected as shown, reverse the collector and the emitter connections on
the transistor. Repeat step 7. Can you still control IC & the lamp with IB? Discuss what you
observe.
Appendix:
Following are the schematic symbols for PNP & NPN transistors:
PNP Transistor
NPN Transistor