AIM-Lab: A SYSTEM FOR REMOTE CHARACTERIZATION OF ELECTRONIC DEVICES AND CIRCUITS OVER THE INTERNET T. A. Fjeldly1*2,M. S. Shur', H. Shen', and T. Ytterda13 'Rensselaer Polytechnic Institute, Troy, NY 'Center for Technology, Norwegian University of Science and Technology, Kjeller, Norway 3Nordic VLSI, Trondheim, Norway Tel: (47) 6484 4747, Fax: (47) 6381 8146, E-mail:
[email protected]
Abstract: We report on the development of AIM-Lab, a a CMOS test chip and a silicon carbide (Sic) diode, is used remotely operated laboratory for characterization of electronic devices and circuits over the Internet. Specifically, we apply the AIM-Lab concept in a seniorfgraduate course on semiconductor devices and circuits, which comes with a laboratory module that includes up to 9 experiments performed on a CMOS chip and a S i c diode. AIM-Lab has been used in the distance learning programs at Rensselaer Polytechnic Institute and at the Norwegian University of Science and Technology.
as a lab module in courses on semiconductor devices and circuits at the senior or first year graduate level. By analyzing the experimental characteristics obtained from individual devices, the students can extract SPICE parameters, which can be used for simulating the behavior of circuits. These simulations can be compared with experiments on simple circuits in AIM-Lab, such as CMOS inverters and ring oscillators. For the theoretical part of such a course, we use our new textbook "Introduction to Device Modeling and Circuit Simulation" [2], which includes the description of AIMINTRODUCTION Spice [3], a circuit simulator that we use as a tool for With the mass proliferation of the Internet, interesting analyzing the experimental data obtained from AIM-Lab. possibilities have emerged for extending its use into new areas, including remote education - a rapidly growing part of SYSTEM CONFIGURATION AND FEATURES the university curricula. By utilizing the Internet and the World Wide Web, the potential exists for offering courses to The implementation of AIM-Lab is based on a cliendserver students world-wide, without other technical requirements architecture, as shown in Fig. 1. The server, written in Microsoft Visual C++, includes two main components. One than a personal computer and a telephone line. Laboratory courses are a vital part of engineering of them is a TCPLIP (Transmission Control ProtocoVIntemet ducation, but so far, lab courses have been considered Protocol) server socket that receives commands sent over the impractical for remote education. On the other hand, user- Internet. The second component, the Driver Interface Layer -. nendly, computer-controlled instrumentation is revolution- (DIL),interfaces between the instrument driver and the 7ing the way measurements are being made, and is now higher levels of the server 1451. D E sends the commands to ,emitting net-based techniques to be utilized for setting up the instrument driver, which uses the HPIB EEE 488.2 emote laboratory access. standard protocol to drive the instruments. Such a remote laboratory can be used in conjunction vith courses on semiconductor devices and circuits, and be Iffered to remote students on a global scale allowing them to lbtain hands-on experience in semiconductor device charac--rization. This removes a major obstacle for establishing a 'I mndless and complete remote engineering education I I umculum. As an added benefit, this technology may offer Client I ' d e n t s the opportunity to work with sophisticated Socket I I quipment, of the kind they are more likely to find in an I tdustrial setting, and which may be too expensive for most I Genuah'm I -hook to purchase and maintain. I I Here, we present our results on the development of and Handing I S-perience with AIM-Lab (Automated Internet MeasureModule ! I -1 x t Laboratory), an Internet-based remote laboratory on HPIB InstrumentDriver I- _ _ _ _ _ - - _ - _I _ _ _ miconductor device characterization [ 11. Presently, AIMFig. 1. System configuration of AIM-Lab[6,7]. 'h, which includes 9 experiments that can be performed on
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The server is a Windows based multi-document interface (MDI) application, which drives the instrument and hence the experimental circuits via the HPIB card. The server is designed to allow multi-user and multiexperiments. No experiment failure or errors caused by the clients will lead to malfunction of the server. If any experiment takes a very long time to finish, which suggests a failure, it will be discarded and hence will not affect the other experiments [ 6 ] . Upon launching the server application, all information on the available experiments is read from a file prepared by the instructor. This information is sent to the client at login time. Java is the programming language of choice on the client side, since it offers the flexibility of a GUI (Graphics User Interface) design, convenient network programming, and platform independence. The client has a pop-up window that, on one hand, provides GUI interactions to the user, and on the other hand, communicates directly with the server.
The GUI interface is created according to the experiment infomation received from the server upon client initialization. The user can access the AIM-Lab Web site [I] using a Web browser, where heishe opens the client window, selects an experiment, and enters parameters for the experiment (such as voltage ranges, step size, etc.). The experiment is initiated by activating the "Start Experiment" menu item in the "Operation" pop-up menu. Some of the windows encountered are shown in Fig.2. The instructions from the client are then sent via the TCPAP client socket to the server, which runs the experiment and returns the experimental data to the client. The results are presented in the client Browser window as columns of numerical data and a graph, as shown in Fig. 3 for NMOS current-voltage (I-V) characteristics. The numerical data can be copied from the window for further processing by the user.
Fig. 2. AIM-Lab information window for the NMOS I-V characteristics experiment (background),the Client window with menus and instructions for running the experiment (middle right), window for selecting experiment (lower left), and window with panel for setting experimental parameters and initiating the experiment (lower right).
Fig. 3. AIM-Lab client side window with experimental results (NMOS I-V characteristics)correspondingto the instructionssubmitted by the client (see Fig.2).
AVAILABLE EXPERIMENTS Currently, seven experiments have been implemented in AIM-Lab, including one Sic diode experiment, two NMOS experiments, two PMOS experiments, and two CMOS inverter experiments (see Iower left window in Fig.1). The MOSFET/CMOS experiments are pcrlormed on the AIMSpice CMOS test chip [6] shown in Fig. 4. The experimental instrumentation is an HP4142B DC Sourcehlonitor with eight source monitor units (SMUs). The diode I- V characteristics are measured by applying a staircase sweep voltage to the S i c diode and monitoring the current. The experimental parameters to be entered in the Source Setup pop-up panel include the start, stop, and step voltages for the sweep. The user may also select a zompliance current for protection. The diode voltage is iimited to (-lOOV, 3V). This experiment can be used for :iudying various properties of the diode, such as the cut-in :oltage V,; and departure from ideal diode theory. ixperimental details from the region around V,.;are shown in z'ig. 5, revealing a relatively high value for V,, indicative of he large band-gap of Sic. The large band-gap makes this liaterial very interesting for use in high temperature .lectronics and for the fabrication of blue light-emitting :iodes (LEDs). The NMOS and PMOS ll1-Vl~ Characteristics are leasured by applying a staircase sweep voltage ( V , ) :tween drain and source of the devices and monitoring the cain current. A constant voltage is applied between gate and 3urce (VgJ during each sweep, and a group of IlrVd data
curves can be acquired by varying the gate voltage between sweeps. A bias voltage (Vb) can also be applied to the substrate. Pado
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Fig. 4. Layout of AIM-Spice CMOS test chip. NMOS and PMOS transistors of different dimensions are shown in the upper and lower sections of the chip, respectively. Ring oscillators are shown in the middle. The biases shown are those used for obtaining NMOS characteristics.
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Fig. 5. Details of SIC diode characteristics for applied voltages between 2 and 3 V. The cut-in voltage is about 2.5 V.
The source is grounded, as indicated for an NMOS in Fig. 4. The Characteristics for the NMOS device are shown in Fig. 2. As part of the students' analysis of these characteristics, they extract SPICE parameters for the devices. This can be done by analyzing the characteristics manually. For example, the linear parts of the characteristics contain information about series resistances and carrier mobility, and the carrier saturation velocity can be found from the saturation current. Likewise, the transistor transfer (l(,-Vx) characteristics are measured by applying a staircase gate-source sweep voltage, and monitoring the drain current at different values of the drain-source voltage. From the subthreshold parts of the transfer characteristics, we obtain the subthreshold ideality factor and the drain-induced barrier lowering (DIBL) parameter (see [2, 81). The CMOS inverter is composed of one NMOS and one PMOS, with their gates connected together as input, and their drains connected together as output. The transfer I-V characteristics are measured by applying a staircase sweep voltage Vi to the input, and applying a current source I,, to the output while monitoring the output voltage. The source voltage V, and the substrate voltage VI?of the PMOS are set to constant positive voltages, and the source and substrate of the NMOS are grounded. Fig. 6 shows a comparison of an experimental inverter transfer characteristic (symbols) and the same characteristic simulated in AIM-Spice (continuous curve) using the MOSFET Level 7 model. For the simulation, crude values of SPICE parameters extracted for the constituent NMOS and PMOS transistors are used. A better fit can always be obtained by a more careful extraction combined with a final optimization step for the SPICE parameters.
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Fig. 6. Comparison of experimental (symbols) and simulated transfer characteristic (continuous curve) of the AIM-Lab CMOS inverter. The simulation is performed in AIM-Spice using crude SPICE parameters values extracted from the NMOS and PMOS charac:teristics.
EXPERIENCE IN COURSE WORK We have experience from the use of AIM-Lab as a module in the first-year graduate course "Semiconductor Devices and Models I" (SDM-I) [9] at Rensselaer Polytechnic Institute (RPI) and in the senior course "Device Modeling and Circuit Simulation" at N,orwegian University of Science and Technology ( "U). The courses were parts of the distance learning program at the two institutions. SDM-I at RPI was a course for first year graduate students and qualified senicirs offered to both on-campus students and to distance leaning students from GM, IBM, Pitney Bowie, and other companies. A total of 23 students were enrolled in the pilot course. All course materials were posted on the Web [9] and the students did not have to remember any additional URLs, etc. The experiments were used in a classroom to illustrate and reinforce the basic principles of the operation of a field-effect transistor ( E T ) as well as to demonstrate .some non-ideal effects, which limit the FET performance. ,4n ability to change the voltage and current ranges and to zoom at certain features of current-voltage characteristics was especially useful. We also tried to compare the measured data with FET models. This was done in two different ways. First, the students had to comment if the results of the measurements made sense given only the geometry of the device under test. They used order-of-magnitude values of the electron mobility and saturation velocity to roughly estimate the maximum device current ancl transconductance. This led to a fairly lively discussion in c:lass. Only then, different FET models were used, from a very simple constant mobility model to the more sophisticated AIM-Spice Level 7 model, (see [2,3]) in order to try to fit the device characteristics.
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The students were also asked to provide a feedback and critique the user interface. Their comments and critique helped to improve the remote laboratory. It was a very positive and rewarding experience. Similar experiences were obtained at NTNU, where the students also were given assignments in parameter extraction and in the simulation of the measured inverter characteristics using the extracted parameters. For the comparison we used the MOSFET Level 7 model of AIMSpice. This and other models were also presented as a part of the course [2,3]. All in all, we feel that AIM-Lab did provide a new and very useful dimension to the courses. So much so that we would like to expand AIM-Lab to include several additional experiments using more advanced CMOS integrated circuits and also alternative technologies such as bipolar transistors, GaAs metal-semiconductor FETs (MESFETs) and heterostructure FETs (HFETs). We envision having several remote laboratory sites shared by several universities and, possibly, even some remote laboratory sites maintained by the semiconductor device vendors. This will make teaching semiconductor courses more realistic and practical and will add a much-needed experimental dimension to what have traditionally been purely theoretical lectures.
ALTERNATIVE SYSTEM CONFIGURATIONS Several companies and researchers offer software packages that should potentially allow us to use Internet for remote lab applications. The Internet Developers Toolkit from National Instruments, Inc., which is the add-on utility of the LabVIEW application, makes virtual instrument (VI) front panels viewable from standard Web browsers by converting the front panel into images [IO]. Both the Componentworks [ 113 from National Instruments, Inc. and HPVee [ 121 from Hewlett Packard Co. supply ActiveX controls, which can be embedded in user applications and be suitable as the front end user interface for remote instrument control. The embedded micro interface technology (EMIT) from emWare is another approach for controlling and monitoring electronic devices on the Web [13]. The skinny servedfat client configuration and the data communication through their customized serial protocol, called emNET, help reduce the cost of the system. In the Interactive Electronics Laboratory at University of Illinois at Chicago (UIC) [14], the users are granted a limited capability to set up their own circuits. This is achieved by significantly increasing the system complexity. G circuit switching mechanism, including the hardware and .he software, is incorporated into the system. The user needs '0 download software from their web site, and set up the mvironment. Further, to prevent malicious use of the ;ystem, security issue is much more severe. The speed is :is0 decreased at the current network bandwidth. At present,
the UIC system includes only two bipolar junction transistor (BJT) experiments, one for characterization of a BJT and one for measuring a BJT inverter. These approaches have their advantages and disadvantages. The ActiveX based applications rely on the Internet Explorer Web browser and Windows based PC platforms. The actual communication part is not implemented in this approach. emWare has a problem in switching between different instruments or between different applications because of the customized interface between EMIT and the instruments [15]. The UIC system suffers from increased complexity and reduced speed. In a preliminary investigation, our group has considered several approaches. Two of these are based on the functionalities of LabVIEW [16], and a third utilized a dedicated software package written in Java (clientlserver communication) and C++ (server/instrument communication) [4, 51, a predecessor to the present approach. The first approach used LabVIEW and the powerful add-on utility of the Internet Developers Toolkit. This allowed VI front panels to be viewed in and operated from common Web browser. Netscape, for example, allows these panels to be automatically updated over the Web as animated images, using client-pull or server-push update methods. However, this is not always suitable over the Internet, in part because the data communication by animated images represents a large transfer overhead. In the second approach, we utilized the primitive communication capability of LabVIEW and the Java computer language. A standalone LabVIEW VI server was created based on the TCPDP primitives in LabVIEW. The server listens and accepts the measurement request through a TCPDP socket and calls the virtual instrument drivers to perform the experiment. The results are then sent back to the client. The client side is implemented in Java and communicates directly with the server utilizing a standard Web browser. The user can set and send the measurement request and receive a response by means of the Java applet, which can be downloaded from the server side. The use of Java makes the user interface quite flexible and enables us to perform experiments more efficiently compared to using the HTTP server with the Internet toolkit. The availability of these alternative approaches gives the instructor a choice between a simple implementation in the first approach, and browser independence and system efficiency in the second and in the present approach.
CONCLUSION AIM-Lab provides real laboratory experiments via the Internet. It offers a valuable tool for remote engineering instruction that can not be replaced by simulation software packages. The purpose of AIM-lab is to provide not only an experimental component for remote education courses, but also to reveal the great potentials of remote laboratory
technologies. It is important that we provide easy access of the experiments to all visitors of the Web site. AIM-lab provides the users with pre-designed experiments. It takes the users, including those who use a low speed modem at home, less than a minute to set up and run an experiment, and receive results. On the other hand, more features to enhance the lab experience can easily be added to the current system. We have had positive experience with AIM-Lab as an integral module in a remote distance education courses, and have also had encouraging feedback on AIM-Lab from Internet users in the US and worldwide, including valuable comments and suggestions. Eventually, based on the AIMLab concept, courses and course modules within many disciplines of engineering and science may be offered to remote students any place in the world, including those who otherwise would be precluded by distance and lack of resources.
ACKNOWLEDGMENT We gratefully acknowledge the support of the Rensselaer Strategic Initiative Program and the DIGITALIS project at "Uwith funding from the Research Council of Norway. We are thankful for the equipment grant from Hewlett Packard company, and for the donation of LabVIEW from National Instruments, Inc. The AIM-Spice Test Chip was fabricated by LG Electronics and was kindly supplied by Professor K. Lee at Korean Advanced Institute of Science and Technology (MIST).
REFERENCES [ 11 AIM-Lab URL: http://nina.ecse.rpi.edu/shur/remote
[2] T. A. Fjeldly, T. Ytterdal, and M. Shur, Introduction to Device Modeling and Circuit Sirnulatioil, Wiley & Sons, New York, NY (1998).
[3] AIM-Spice URL: http://vrww.aimspice.com/ [4] V. Kristiansen, Remotely operated experiments on electric circuits over the Internet - An implementation using Java, M. Sc. thesis, Norwegian University of Science and Technology, 1997. [5] B. Dalager, Remotely operated experiments on electric circuits over the Internet - Realizing a clientherver solution, M. Sc. thesis, Norwegian University of Science and Technology, 1998. [6] H. Shen, Z. Xu, B. Dalager, V. Kristiansen, 0. Stram, M. S.Shur, T. A. Fjeldly, J. I& and T. Ytterdal, Conducting Laboratory Experiments over the Internet IEEE Trans. on Education, Vol. 42, No. 3, pp. 180-185 (1999). [7] T. A. Fjeldly, M. S. Shur, H. Shen, and T. Ytterdal, "Automated Internet Measurement Laboratory (AIM-Lab) for Engineering Education", accepted for publication in Proceedings of 1999 Frontiers of Education Conference, San Juan, Puerto Rico, Noveimber, 1999. 181 K. Lee, M. S. Shur, T. A. Fjeldly, and T. Ytterdal, Semiconductor Device Modeling for VLSI, Prentice Hall, Englewood Cliffs, NJ, 1993. 191 For detailed course description and lecture overheads see URL: http://nina.ecse.rpi.edu/shur/sdml/index.htm. [lo] LabVIEW 4.1 Internet Toolkit Reference Manual, 1997. [ 111 LabVIEW 4.1 User Function and VI Reference Manual, 1997. [ 121 HPVee URL:http://www.tmo.hp.comltmo/piaM[PVEE/ 'I
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PIATop/English/index.html [ 131 emWare URL: http://wvvw.emware.com/ [ 141 Interactive Electronics Laboratory at University of Illinois at Chicago URL: http://www.mal.uic.edu/marble [ 151 M. Howard and C.Sontag, "Managing Devices with the Web", BYTE, Vol. 45, September (1997). [16] Z. Xu, Modeling and characterization offield effect transistors based on unified charge control model, M. Sc. thesis, Rensselaer Polytechnic Institute, 1998.
