2p12semiconductordevices2009
TRANSCRIPT
-
7/31/2019 2P12semiconductordevices2009
1/11
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 (DMMs) 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 k resistor, three DMMs and some lengths of wire. The
1 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
2/11
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.
2 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
3/11
Wires are only interconnected if they are marked with a dot where they
cross in the circuit diagram.
Experimental Procedurea) 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 measurevoltage is 10M.
Use your data to compare the characteristics of the junction with those
predicted by the ideal diode equation:
= 1
kT
qVexpII 0
Try to give reasons for any differences that you find.
b) Verify that the I-V characteristics of the collector base junction are
similar to 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
3 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
4/11
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.
4 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
5/11
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
feedback 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, 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 (Rf > 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.
5 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
6/11
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 Vi. 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.
Select at least six resistor values in the range 2.2k to 50k and fit each in
turn across terminal C and D as the feedback resistor Rf. 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/Vi), 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
6 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
7/11
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
7 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
8/11
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 contrast effect you have been
using.
Rough timetable
Day 1: Electrical characterisation of the bipolar transistor and write up
Day 2 & 3: Characterisation of the op amp and write up.
The report
Aims: State clearly what the experiments aim to find out.
Methods: Explain in bare detail what you did.
Results: Describe what you observed at an appropriate level of
detail.
Discussion: Explain your results. Make sure you have answered the
questions posed in the text.
Sum up.
A good length would be about 1000 words. Do not write more than
1500 words.
8 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
9/11
References
Horowitz and Hill The art of electronics, Cambridge Univ Press, 1980.
Goldstein et al Advanced scanning electron microscopy.
9 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
10/11
Practical Questionnaire
Second Year: MSOM & MEM
Term: Michaelmas
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 3 far too long
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 information to do the practical?
not enough info -3 -2 -1 spot on 1 2 3 too much / tooconfusing
PTO
10 2P12 Semiconductor Devices
-
7/31/2019 2P12semiconductordevices2009
11/11
11 2P12 Semiconductor Devices
Any other comments? (particularly for any problems exposed by responses
to the questions above:
Please hand in at the same time as your report.