IN3170, spring 2020, mandatory labratory exercise 1: Characterizing - - PDF document

IN3170, spring 2020, mandatory labratory exercise 1: Characterizing MOSFETs (deadline 17-Feb-2020, 10:00!)

• P. H¨

afliger & Sebastian Wood Institute of Informatics University of Oslo e-mail: hafliger@ifi.uio.no January 22, 2020

Abstract This first lab is meant to give a feeling for matching theoretical device parameters to real measurements on the example of CMOS field effect transistors. It will also introduce the students to work with electronic equipment in basic analog circuit testing. It is the first in a series of three lab tasks that will be graded and will count 40% towards the final grade. The first lab, however, will only be ’pass’ or ’fail’. The students are required to pass this lab assignment. The deadline is February 17th, 10:00! It is a hard deadline! Do not miss it! Plan to submit well ahead of the deadline! We will use devilry.ifi.uio.no for submission of you lab report.

1 Lab Rules

1.1 Safety

Voltages over 40 Volt can in some cases be harmful, even though it usualy requires more than that. The lab equipment is thus not able to provide voltages higher than 36V. Do not use equipment other than that provided in the lab! If a part of the skin is covered with a conductive fluid or is pierced and exposed to such voltages, a current could bypass the “insolator” of the skin and run through the body. If this current passes through the heart it can cause fibrillations or even cardiac arrest. Even higher more extreme currents could also give rise to internal burns. If this happens to anybody or something else happens in the lab, seek medical help immediately: heart fibrillations can last and cause trouble long after the incident. Also notify the person in charge of the lab. 1

Some electronic components can explode if they are exposed to high currents. This is important to remember when working with electrolytic capacitors. How- ever, none of the capacitors provided in the lab are electrolytic capacitors. Never bring your own electronic components into the lab!

1.2 Conduct

Good routines are necessary to make the work in the lab effective and safe:

• Food and drinks are prohibited from every lab.
• In general everybody is responsible for keeping the lab tidy.
• Always turn off the power supply before you start adding and/or removing

components.

• Use an ESD protection wrist strap when handling ICs and other sensitive

components.(ESD: electrostatic discharge)

• Always clean up after using equipment and tools:

– Turn off all equipment, except for lab computer. – Throw away cutoffs and vacuum clean the desk, chair, and floor if nessesary. – Place all components you have used back to their respective places. (Do this while you work, if you have a component you don’t use anymore, put it back.)

• When you leave; the desk should be clean and ready for the next group.
• Read the information posters in the lab describing what to do in case of

fire or medical emergency.

2 Report and Group Assignments

2.1 Requirements for the Lab Report (read carefully!)

You are required to execute the tasks and answer all the questions posed below and to submit a report on your work. The report needs to be explaining clearly what you have done, how you have done it, what the results were and what you conclude from them. Make sure to answer all questions! Supply the report with drawings of the circuits (including the values of the components and parameters you used where appropriate, e.g. bias voltages/currents, component sizes etc.) and measurement setups, and show your measurements in graphs! Use labels in the schematics that you draw, such as M1, M2 (M is often used fro labelling CMOS transistors), opamp1, I1, V1 etc. You should then use those labels in your text, since it is much easier to write: ’transistor M1 in figure 1’ than ’the transistor third from the top and second from the left in the righthand side 2

circuit in figure 1’. MANDATORY: Include a photograph of your circuit into the report!

2.2 Graded Mandatory Group Assignments

Note that this is part of the courses exam and strict rules apply as described in the document http://www.mn.uio.no/ifi/english/studies/admin/mandatory-assignments/ index.html. The page explains the significance of mandatory assignments in a course and in particular group assignments. It also specifies your responsibility to not plagiarize anybody else’s work and that you are required to conduct and understand your own experiments and obtain your own results, while you are still allowed and encouraged to exchange advice and experiences also between groups. Each group must deliver a written lab report using the Devilry online sub- mission system before the hard deadline indicated in the title. Note that you can submitt multiple times and the last submission before the deadline will be graded, so it might be a good idea to plan to submit preliminary versions well before the deadline. The points given for this lab assignment will determine if the lab assignment is accepted or rejected. You will need to pass this lab assigment in order to be admitted to the exam. The next two lab assignments will be weighted as 20% of the total score of the course, i.e. your final grade. Each task is labeled with how many points it will contribute towards the score.

3 Lab Task

3.1 Introduction

The MOS transistor is the fundamental building block in microelectronics. De- pending on both usage and production the properties of the device varies sig-

• nificantly. During production the MOS transistor may be adapted to different

settings like high voltage devices, high speed devices and analog devices and so

• n. In spite of this production flexibility, the major application of MOS transis-

tors as switches in digital systems is setting the standard for devices available in

• silicon. Now, as digital systems are “moving out” interfacing directly with the

real world through sensors and actuators (buzzword: cyberphysical systems), also analog interfacing functions must be included. A significant challenge is therefore to design high performance analog circuits in crude digital technology. The goal of this exercise is to characterize off-the-shelf MOS-FET transistors that we know very little about a-priory. We are going to use the ELVIS II board as a platform that provides a remov- able bread board to build your test circuits, built-in instruments for measuring electronic circuits (refered to as ’virtual instruments (VI)), and a convenient GUI on the computer to control those instruments. The ELVIS board has some limitations in functionality when it comes to automated measurements involv- ing multiple of those built-in instruments. In particular for this assignment, 3

there is no VI for characterizing MOSFETs. The name of the ’three wire cir- cuit analyzer’ sounds very promissing, but it is made for bipolar transistors and the controlling wire is current controlled to control the base current, while for MOSFETs we would need to control the voltage of the gate. The graphic programming language LabView offers the possibility to define new VIs your- self, but it’s not a very intuitive language (at least not to your lecturer). We’ll employ another fix for this lab, using the two wire analyzer and the variable voltage source and some creative wireing.

3.2 Tools

• The NI-ELVIS bord and plug-in bread bord

You shall use a bread board to plug in cables and discrete components to compose your test circuits. The bread bord can be removed from the socket in the ELVIS bord and each group can keep one for the duration

• f the course. Thus, once you are done for the day you can leave your

components in place and lock your bord away or take it home with you until your next lab session. The ELVIS bord has some built in instruments that are displayed on the computer at each work place. You shall use those instruments to characterize your circuit. Some general advice: plan your layout of components and cables on the bread bord before you start plugging them in and try to make things compact and organized.

• The NI ELVISmx Instrument Launcher

BEFORE you launch the NI ELVISmx Instrument Launcher you should switch on the ELVIS board, such that the software (SW) recognizes the hardware (HW) correctly. Note that there are two power switches, one

• n the right hand side on the back of the board and one on top of th

board also on the right near the back. You may launch the NI ELVISmx Instrument Launcher from the icon on your Windows desktop in the lab and you will get a list of icons representing the various instruments that are built in into the ELVIS board. See figure 1. Note that the interface looks now a bit more modern than in the description of IN1080.

• The NI ELVISmx 2-wire VI Analyzer

Another NI ELVISmx instrument i the 2-wire VI analyzer and when you click it its front panel pops up and looks as depicted in figure 2. It let’s you sweep a voltage and measure a current accross a device/circuit. The connections on the bread board left hand side between which you need to place your circuit under analysis are DUT+ and DUT- . For more help, press the ’help’ button. Note that this instrument will execute a series of measurements that you could have performed by hand. This is

• ften a good thing, but has some pitfalls: you should know in some detail

4

Figure 1: The NI ELVISmx Instrument Launcher 5

Figure 2: The NI ELVISmx 2-wire VI Analyzer Virtual Instrument 6

Figure 3: The NI ELVISmx Oscilloscope Virtual Instrument how the results are obtained, because sometimes results of automated measurements can be something else than what you think they should be, because they are not exactly obtained as you think they are ...

• NI ELVISmx Variable Power Supply

Start it from the Instrument Launcher and consult the help pages or ask your lab assistant if something is unclear.

• The NI ELVISmx Oscilloscope

When you open the Oscilloscope in the instrument launcher the virtual instrument pops up like in figure 3. You’ll have two channels to observe waveforms on the oscilloscope that are conected to BNC ports labelled CH0 and CH1 on the ELVIS board, the upper left red circle in figure 4. Per default they should be connected with BNC cables to ports BNC1 and BNC2 such that you can connect them by plugging in wires into the bread board to the right of the labels BNC 1 ± and BNC 2 ± (the lower red circle). Note that you should connect the minus terminal to a GROUND. You can briefly check out its function by connecting the output of the 7

Figure 4: Connections on the ELVIS II bread board, with the oscilloscope con- nections highlighted. 8

function generator (output labelled FGEN on the breadboard and you need to launch its VI) to one of the channels and play around with the various knobs. More help for the instrument is available if you press on the help button in the VI front panel. That’s true for all instruments! An important function/button is the ’write to log’ which will export the graph in a text file that you can make readable by MATLAB with a few simple edits. Then you can create nice graphs for your report or do some more adavanced analysis of the data in MATLAB.

• NI ELVISmx Function Generator

Start it from the Instrument Launcher and consult the help pages or ask your lab assistant if something is unclear.

• CMOS transistors

The IC that contains a set of individual transistors is labeled ’MC14007UBCP’. There may be other IC’s containing the number 4007 in their name and those contain the same arrangement of transistors with the same pin- assignment and can also be used. The difference is that they are in a different CMOS process technology, so the transistor properties will be

• different. A data sheet will be available in the lab or through the course

pages that shows which pins are to be used as bulk, source, drain, and

• gate. Do not forget to connect the power pins (14 and 7)!
• MATLAB

We will be using MATLAB for some excersises and it’s the best tool for plotting all of your results as nice graphs. An important function/button on all ELVIS VIs is the ’write to log’ which will export the graph in a text file that you can make readable by MAT- LAB with a few simple edits. Then you can create nice graphs for your report or do some more adavanced analysis of the data in MATLAB. Thus, you should bring a working knowledge of MATLAB to this course. If you have none, get a crash course from a fellow student who has used it! It is a powerful mathematics tool with a command line interface. One useful function is ‘help’. ‘help <command name>’ will display an explanation

• n how to use ‘<command name>’. Another help function that helps you

find functions that you do not know the exact name of is ‘lookfor’. Type ‘help lookfor’ to learn more.

3.3 General Advice

• Draw a schematic before you start assembling components on the PCB!

Label pins on the PCB and in the schematics (!) clearly in order to keep your overview. Debuging will be much, much easier that way!

• Come to the lab with a work plan: Read the entire lab task beforehand

and make a plan how to proceed. Put yourself a goal for a lab session. 9

Read the relevant book chapters in order to understand the entire lab. Be ready with questions already before the lab if there are still things unclear.

3.4 Lab Tasks

Task 1 (4p): Provide about 10V power to the chip containing the MOS FETs. Use one output of the Variable Power Supply VI. Make sure to connect the bulks of NFETs to the lower power supply and the bulks of PFETs to the higher power supply. Connect the two wire analyzer between drain and source of an nFET on the IC. Connect the gate to another output of the variable voltage source. Set the gate to 5 different voltages and generate 5 iDS vs. vDS plots. Export the result to MATLAB to make nice plots and to be able to use the results in some computations later on. Try and see if you can choose the different voltages to cover weak inversion, moderate inversion, and strong inversion. However, weak inversion might not be doable as the current measurement by the ELVIS board is likely not accurate enough to measure sufficiently small currents. Check it out and comment it in your report! Determine the parameter λ and VA for this transistors from these

• plots. Describe precisely how you derive the parameters from the plots.

Do the same for a PFET transistor as well controlling its source drain and gate approprietly! Task 2 (4p): Disconnect the gate from the constant voltage supply and short it to the drain, thereby turning the transistor in a two terminal device, i.e. a diode connected transistor. Thus, if you now sweep the voltage in the two wire analyzer, you do get a curve that is very similar to a iDS vs. vGS plot. Show that plot for both NFET and PFET and use first a linear and then a logarithmic y-axis. (The latter to check whether the current meaurement instrument is capable of covering the weak inversion region.) Describe how these plots might differ from a standard iDS vs. vGS plot. Can you correct the curve to compensate some of these differences and get a resullt that is even closer to a ’standard’ iDS vs. vGS plot? Can you derive the parameters vtn, vtp and kn from these measurments? Task 3 (2p): Plot the plots from tasks 1 and 2 again and add into the same plots the theoretical curves with your parameters that you have derived from the measurements. Preferably, use the full EKV model: then you do not have to worry about the different regions of operation. Do the curves match? Task 4 (4p): Now that you have worked with an NFET and a PFET you can connect the two to form an inverter. From your measurements in task 1 and 2, can you predict where the switching threshold of that inverter will be? Explain! 10

Connect the input to the function generator and set the function generator to produce a triangular waveform moving from Gnd/Vss to Vdd and back

• again. Connect both the input and output to the two channels of the
• scilloscope (to get two voltage curves that are synchronized. Plot the

I/O relationship. Does the switching threshold match your prediction? 11