Practical 2P12 Semiconductor Devices

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1 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: I = I exp qv 0 kt -1 Try to give reasons for any differences that you find. 1 2P12 Semiconductor Devices

2 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 I e = I b + I c 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 R f 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 (R 1 ) and feedback (R f ) resistors. The prewired box provided for the experiment contains three units. (i) (ii) (iii) A 741 operational amplifier circuit with R 1 = 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 R f (C and D) are brought out to the front panel so that various values of R f can be selected (R f > R 1 ); 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. A square wave oscillator circuit with decade switched frequencies between 1Hz and 100kHz, and a controllable output amplitude (< 1V to > 10V). 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. 2 2P12 Semiconductor Devices

3 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 R f - and remember that your hands have finite resistance! For each value of R f 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 (R f /R 1 ; R f > R 1 ) 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 R 1 and R f 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: The report Aims: Methods: Results: Discussion: Sum up. Characterisation of the op amp and write 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, Goldstein et al Advanced scanning electron microscopy. 3 2P12 Semiconductor Devices

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5 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 spot on far too late 2) Did you think you got out of the practical what you were meant to? not at all completely 3) Did you find writing up the practical: very difficult no problem 4) Was the practical: completely useless very useful 5) Was the practical: far too short spot on far too long 6) Was the practical: very boring very interesting 7) Was the Junior Demonstrator completely useless very helpful 8) Did the practical script give you enough inforrmation to do the practical?: not enough info spot on too much / too confusing Any other comments? (particularly for any problems exposed by responses to the questions above) 5 2P12 Semiconductor Devices