2P12semiconductordevices2009

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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

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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.



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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



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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.



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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.



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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



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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



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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.



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References

Horowitz and Hill The art of electronics, Cambridge Univ Press, 1980.

Goldstein et al Advanced scanning electron microscopy.



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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



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Any other comments? (particularly for any problems exposed by responses

to the questions above:

Please hand in at the same time as your report.

2P12semiconductordevices2009

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