Download Practical 2P12 Semiconductor Devices

Practical 2P12
Semiconductor Devices
What you should learn from this practical
Science
This practical illustrates some points from the lecture courses on Semiconductor Materials and Semiconductor
Devices concerning the operation of bipolar transistors and integrated circuits.
Practical Skills
You will gain experience in constructing very simple electronic circuits and in characterising them using digital
multimeters (DMM’s) and an oscilloscope. You will gain experience in the operation of an SEM.
Overview of the practical
This practical has three elements:(1)
to investigate the electrical characteristics of a bipolar transistor,
(2)
to study some of the basic electrical characteristics of a standard type 741 integrated circuit operational
amplifier, using an oscilloscope to measure the medium frequency signals,
(3)
to use an SEM to study a similar 741 op amp using voltage contrast.
Experimental
(1)
Investigation of the electrical characteristics of a bipolar transistor
You are provided with a BU406 power transistor, a prototype board, a power supply, a 1 kΩ resistor, three
DMM’s and some lengths of wire. The BU406 is an npn transistor and its circuit symbol is shown in fig. 1. It
can be considered to be composed of a “sandwich” of three adjacent layers of differently doped semiconductor,
in this case silicon. For an npn transistor, the first layer, called the emitter, is heavily doped n-type, next to this is
a very thin layer of p-type material known as the base, and the final layer, again n doped, is the collector. In this
way the device can be thought of as two back-to-back pn junctions. Under normal operating conditions the
emitter base junction is forward biased which in this case entails making the emitter more negative than the base,
whilst the base collector junction is reverse biased which is achieved by making the base more negative than the
collector.
The prototype board is used to construct simple electronic circuits quickly without the need for making soldered
joints. Electrical connections are made by pushing wires into the holes in the surface of the board. The holes in
the board are arranged in rows with all the holes in any given row electrically connected to each other whilst
being electrically isolated from all other holes on the board. There are 94 rows each with 5 interconnected
contacts on the board you are provided with. Electrical contact between wires is achieved simply by pushing
them into holes which lie anywhere in the same row on the board. The next set of electrical contacts can then be
made by using holes in another row of the board and so on until the circuit is completed.
Circuit diagrams are used to show correctly the interconnections in a circuit, but they may not represent very
well the physical layout of the components. Wires are only interconnected if they are marked with a dot where
they cross in the circuit diagram.
Experimental Procedure
a) Measure the I-V characteristics of the emitter base junction of the transistor by constructing the circuit as
shown in figure 2. DO NOT apply more than 10V reverse bias otherwise the transistor will be destroyed.
Note: the input resistance of the DMM when configured to measure voltage is 10MΩ.
Use your data to compare the characteristics of the junction with those predicted by the the ideal diode
equation:
  qV  
I = I 0 exp  - 1
  kT  
Try to give reasons for any differences that you find.
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2P12 − Semiconductor Devices
b) Verify that the I-V characteristics of the collector base junction are similar th those you have just measured
for the emitter base junction. (No more than 20 data points are required).
c) Investigate the behaviour of the transistor by constructing the circuit shown in figure 4. In this case use the
BU406 transistor with the large heat sink attached. Verify the relationship Ie = Ib + Ic for the currents flowing
respectively in the emitter, base and collector. Note that a relatively small current in the base of the transistor
can be used to control a much larger current through the collector. For a power transistor such as the BU406
this allows relatively large power dissipation loads to be driven.
CAUTION: The transistor, resistor and load will get HOT during these measurements! Do not leave on for
periods of more than approximately one minute.
(2)
Investigation of the electrical characteristics of an op amp
The industry standard 741 integrated circuit operational amplifier chip is available from many manufacturers in
the usual plastic encapsulation, which we use in the bench test box, and in the more expensive metal can format
used, with the top removed, in the SEM. The actual silicon chip is similar in both cases and contains 20
transistors, 11 resistors and a few capacitors.
The standard circuit symbol for a op amp is the triangle, with the various connections shown in Fig.5. In the
usual top view of the plastic encapsulated version there are 8 pins numbered in an anticlockwise sequence
starting from the top left corner, which can be identified by the notch in the ‘top’ of the package and the indent
spot against pin 1. In the 741 frequency compensation is internal and pins 1 and 5 for offset null are rarely
required, making this circuit very easy to use at medium frequencies, such as audio applications. The power
connections are usually plus and minus 15 volts from a stabilised power supply, and the input and output signals
can then be up to ± 12 volts, at low (mA) current.
The two basic applications - there are hundreds of others - use negative feedback for stable and predictable
performance; see figure 6. A proportion of the output signal determined by the choice of feedback resistor Rf is
fedback to produce, as far as possible, a zero voltage difference between the two input terminals of the amplifier.
The gain of the amplifier depends on the mode of connection (whether inverting etc.) and the value of the input
(R1) and feedback (Rf) resistors.
The prewired box provided for the experiment contains three units.
(i)
A 741 operational amplifier circuit with R1 = 2000 ± 5 Ω, which can be measured with the digital
multimeter connected across the yellow terminal (A, inverting) and the adjacent blue terminal (C). The
(blue terminal) connections for Rf (C and D) are brought out to the front panel so that various values of Rf
can be selected (R f > R1); together with a switch to connect internally either the input signal to A and V =
0 to B for inverting operation, or to reverse them for the non-inverting mode.
(ii)
A square wave oscillator circuit with decade switched frequencies between 1Hz and 100kHz, and a
controllable output amplitude (< 1V to > 10V).
(iii)
A stabilised power supply (CAUTION: BOX CONTAINS MAINS VOLTAGES) for the internal
circuitry and with external connections needed to power the 741 IC in the SEM.
Experimental Procedure
NB:
The characteristics of the input and output signals can be measured with the dual trace oscilloscope. To
avoid confusion ensure that the controls are set to calibrated (CAL) positions and that the variable (VAR)
controls are switched off (CAL).
Set channel 2 on the oscilloscope to a vertical sensitivity of 0.1V/division and timebase of 0.5mS/div, and use
the test box front panel controls to set the output of the oscillator circuit to give a 0.5V square wave AC output at
1kHz. This will be V i. After setting the amplitude of the input signal the channel 2 sensitivity can be reduced to
0.5V/div and the trace positioned so that it does not overlap the output trace, e.g. at the bottom of the CRT
screen. From time to time check these values for any possible drift, especially during the first few minutes of
operation. A separate trigger input (EXT TRIG) for the oscilloscope is recommended and the signal is available
at the right hand end of the front panel.
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2P12 − Semiconductor Devices
Select at least six resistor values in the range 2.2kΩ to 50κΩ and fit each in turn across terminal C and D as the
feedback resistor R f. For accurate results measure the exact value of each medium tolerance resistor used for Rf
- and remember that your hands have finite resistance! For each value of Rf use the other oscilloscope channel
(CH1) to measure the output voltage (Vo) and thereby to determine the gain (G = Vo/V i ), in each mode of
operation.
Plot the gain (G) against the resistance ratio (Rf/R1; Rf > R1) and establish the (medium frequency) gain
relationship(s) for the inverting and non-inverting configurations. A V = 0 reference level can be obtained by
disconnecting the input to the oscilloscope or by selecting the GND input with the front panel switch. Estimate
the accuracy of your measurements and include a diagram of the interconnections used to obtain the data.
3.
Characterisation of an op amp using an SEM
Move the experimental box and oscilloscope to the SEM room and ask a demonstrator or the technician to install
the DELICATE op amp test rig into the SEM for you. Check with the technician that the SEM is booked for
you!
The spatial resolution of the SEM has made it a major technique for integrated circuit evaluation and design
validation, as well as in some quality control applications. The voltage sensitivity of the secondary electron
signal also enables voltage measurements to be made with high spatial resolution in an SEM. This technique
uses the effect known as voltage contrast.
Make the necessary interconnections and operate the op amp with the fixed R1 and Rf which have been fitted,
using a 10V square wave signal, initially at 1Hz frequency. You should then be able to identify the large
(square) output smoothing capacitor from the periodic change in intensity. Increase the SEM magnification until
the scan is contained within the area of uniform intensity variation on the capacitor.
Connect one oscilloscope channel to the output of the op amp, this will measure directly the voltage on the
output smoothing capacitor. Connect the other channel to the SEM video output to measure the voltage produced
by the SEM secondary electron signal after detection and subsequent amplification by the SEM electronics.
Using 1kHz pulses vary the INPUT level to the op amp and obtain a calibration of video output voltage Vs
circuit output voltage for the SEM video output voltage as a function of the voltage on the output smoothing
capacitor. Try three different settings of the SEM video controls and comment on their affect on the calibration
curve. Establish the gain of the circuit by using the oscilloscope to measure directly the electrical INPUT to the
op amp and a calibrated video signal to measure the output. Comment on the accuracy of the method and
suggest ways to improve it. Obtain electron micrographs of the op amp during operation which demonstrate the
voltage contast effect you have been using.
Rough timetable
Day 1:
Electrical characteristion of the bipolar transistor and write up
Day 2 & 3:
Characterisation of the op amp and write up.
The report
Aims:
Methods:
Results:
Discussion:
Sum up.
State clearly what the experiments aim to find out.
Explain in bare detail what you did.
Describe what you observed at an appropriate level of detail.
Explain your results. Make sure you have answered the questions posed in the text.
A good length would be about 1000 words. Do not write more than 1500 words.
. References
Horowitz and Hill “The art of electronics”, Cambridge Univ Press, 1980.
Goldstein et al “Advanced scanning electron microscopy”.
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2P12 − Semiconductor Devices
1 kΩ
A
PSU
Figure 1
Figure 2
Figure 3
Figure 4
V
Figure 5
Figure 6
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2P12 − Semiconductor Devices
Practical Questionnaire
Year:
Materials Second Year
Term:
Trinity 00
Practical no.
2P12 – Semiconductor Devices
1) Was the practical timed correctly relative to the lectures ?
far too early -3
-2
-1
spot on
1
2
3
far too late
2) Did you think you got out of the practical what you were meant to ?
not at all 0
1
2
3
4
5
completely
3) Did you find writing up the practical:
very difficult 0
1
2
3
4
5
no problem
4) Was the practical:
completely useless
0
1
2
3
4
5
very useful
5) Was the practical:
far too short
-3
-2
-1
spot on
1
2
6) Was the practical:
very boring
0
1
2
3
4
5
very interesting
7) Was the Junior Demonstrator
completely useless 0
1
2
3
4
5
very helpful
8) Did the practical script give you enough inforrmation to do the practical ?:
not enough info -3
-2
-1
spot on
1
2
confusing
3
3
far too long
too
much
Any other comments ?
(particularly for any problems exposed by responses to the questions above)
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2P12 − Semiconductor Devices
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