ONLINE LAB - 3

 

Name of the experiment: Bipolar Device Characterization

Objective

The objective of this experiment is to study the behavior of the BJT and its I-V characteristics from a remote place (online).

Concept of Online Education

Online Education (OE) refers to a mode of education and a system where the interacting learner and the teacher are separated by space where the interaction can be done through high speed internet. It is an alternative method of instructional process to the traditional or conventional method. It enables a large segment of the learners with necessary aptitude to learn more knowledge and professional competence. Since OE is a form if instruction, which is capable of catering for large number of students, it is impossible to deliver the instruction and teaching without the help of a PC and an internet connection. The easily available internet connectivity is helping to distribute the content and teach the same to the distance learners. This is an effective media and has been extensively used for educational purposes to spread literacy or to give formal and non-formal education all over the world. In the present decade, the online media is dominating in distance education in the developed and developing countries.

In India, continuous efforts are being made to improve the quality and quantity of distance education and several educational commissions have examined and made recommendations for bringing about the required innovations to meet the needs of the distance education system. Moreover, several research studies have indicated that the effective use of new instructional strategies through communication and information technologies, which provide individualized instructions like Learning Module, Programmed Learning Material (PLM) and Computer Based Instructional (CBI) materials. These methods together with its allied communication technologies reach a large number of learners and also help in improving the quality of teaching-learning process in distance education. Now, with the introduction of digital technologies like telecommunication, Interactive television (I-TV) and Virtual conferences (Video conferencing, teleconferencing, audio conferencing and computer conferencing), the virtual learning was established in distance education for teaching, learning and evaluation. In that regard, recently developed online experimentation is playing an important role for online education in India.

Concept of Online Labs

Advancing technology has opened many doors in education. The next step in this direction is interactivity at teaching. Student is able to, not only to see what is involved, but he or she is able to learn from hands on experience. Using computers can be a very effective way of accomplishing this. Students are more motivated and can learn more effectively if they have the opportunity to conduct experiments. Experiments allow a student to compare reality with simulations, collaborate with each other, and give them opportunity to follow their curiosity. Experiments allow a student to compare reality with simulations, collaborate with each other, and give them opportunity to follow their curiosity. Unfortunately, many engineering courses do not include lab component because of significant expense and space considerations. In response to this, I-Lab created remote web accessible laboratories are providing a new framework of science and engineering courses. Remote laboratories allow for much more efficient use of laboratory equipment and give students the opportunity to conduct experiments from the comfort of his home, with an Internet accessible browser. These online Internet accessible labs are important in several learning situations. The first of these is the distance learning scenario. In this situation, learners execute a laboratory oriented course or exercise from their homes or places of employment. Individual learners are remote from each other so that collaboration is distributed. There are currently an increasingly large number of efforts to provide the online analog of the university classroom in various parts of the world. However, there are comparatively few efforts to provide the online analog of the university laboratory, as lectures are much simpler to implement in the Internet environment. However, laboratory learning is a key part of a well designed curriculum. As the number of distance learners and distance learning programs increase, the demand for online laboratory access will also increase. This could for example, also make them available to other national community colleges or partnering Universities and colleges all around the world. So, laboratory based learning experiences that traditionally have been possible only at universities with abundant funds for research are now accessible to many. Third scenario of application is integration of reality into live lectures and seminars. In this situation, teachers present to classroom audience a live (but remote) experiment or demonstration controlled by the instructor. In this scenario, the lab is brought online to the classroom. Economic, space, and cost issues are extremely important and must be considered in setting-up any distance as well as conventional learning environment. Online Laboratories hold promise of being up to three orders of magnitude cheaper to setup than conventional laboratories, requiring less space to run the experiments and being accessible to much larger audience and utilized round the clock.

 

Typical online Internet accessible laboratory consists of:

 

v      Lab device, instrument or pilot plant equipment for tele-presence showing the lab to remote users

v      Teleconferencing equipment or at least built-in chatting capabilities for collaboration among students and instructor

v      Control software allowing users to perform experiments, program lab devices and/or run pilot plant.

 

Introduction to BJT

Bipolar Junction Transistors (BJT) was the first three terminal semiconductor devices with useful amplifying properties.The BJT has three different layers sandwiched together. If an n-type semiconductor is added to the p-side of a p-n junction diode, then it results in the formation of an n-p-n type of transistor. Adding a p-type semiconductor to the n-side forms a p-n-p type transistor. Hence its two types are n-p-n and p-n-p. However, the middle layer called the Base is very thin and lightly doped. Of the outer regions, one is called the Emitter and is very heavily doped and the other is called the Collector and is less heavily doped in comparison to the Emitter. 

The bipolar structure provides a number of natural advantages.

1.      Electrons travel from the emitter to the collector, perpendicular to the wafer surface.

2.      The entire emitter area conducts current.

3.      The input voltage directly controls the density of the carriers that provide the output current, Ic.

4.      The turn-on voltage of BJT, VBE is relatively independent of device size and process variations.

5.      The input capacitance tends to scale with the operating current to the extent that it is dominated by the diffusion capacitance.

                      

                       Fig 1:- BJT Symbols

 

Modes of operation: There are four modes of operations of BJT in four different quadrants of the x-y coordinate system. They are forward active (Q IV), reverse active (Q II), cutoff (Q III), and saturation (Q I) modes.

Forward Active mode: It is the most important mode of operation and is being used as amplifier in analog circuits. The emitter base junction is forward biased here.

Reverse Active mode: Here the emitter base junction is reverse biased and the roles of emitter and collector are reversed. This mode of operation is mainly used in digital circuits.

Cutoff mode: In this mode, both the collector-base junction and emitter-base junction are reverse biased. So, this mode is used for transistors that are in standby mode, i.e., turned OFF for a specific time.

Saturation mode: this is very important mode for the working of the transistors as they are switched ON in this mode. Here, both the collector-base junction and emitter-base junction are forward biased. Also, the charge is stored in the base of the transistor which makes it more difficult to turn OFF.

 

Common Base Configuration: A representative n-p-n planar bipolar transistor is shown in figure below. The n+ region is heavily doped to inject electrons into the p-type base region. The p+ region provides ohmic contact to the p-base region and the n+ collector region provides a low resistance contact to the n-collector region. The arrow of the emitter gives the direction of conventional current flow. Currents into the transistor will be taken as positive and currents flowing out of the transistor will be taken as negative. In the active mode, the emitter base junction is forward biased and the collector base junction is reverse biased.

                  

             Fig 2: - CB Configuration Circuit using n-p-n transistor

 

 

I-V Characteristics: The analysis of the current gain of the common base configuration begins with a schematic representation of the current components for the p-n-p transistor biased in active mode. The I-V characteristics of a p-n-p transistor for a common base configuration are shown below. The collector saturation current ICBO is the collector current when the emitter circuit is open. The presence of the emitter base junction influences ICBO because with the emitter circuit open, the hole gradient dpn/dx must be zero, is shown in figure for zero emitter current. The hole ­­­­­concentration variation in the base is also shown in figure for the emitter-base junction short circuited to give VEB = 0. As VCB is increased, ICBO begins to increase rapidly at VCBO. This rapid increase in current may be due to avalanche breakdown or the neutral base width has been reduced to zero and the collector base depletion region merges with the emitter base depletion region due to the reverse bias on the collector-base junction. Breakdown by this process is known as punch through.

   

                          
                  

Common Emitter Configuration: By arranging the transistor in the common emitter configuration, the output current IC is much larger than the input current IB. The normal bias polarities as well as normal current directions are also shown. For an n-p-n transistor, the bias polarities are opposite. All layers are taken as uniformly doped so that the p-n junctions are abrupt step junctions as for the common base configuration. The usefulness of bipolar transistor in common emitter configuration is that a small base current can control a much larger collector current.

 

I-V Characteristics: A representative plot of typical collector and base currents of the emitter base bias is shown in figure 4. At low currents, the base current IB varies due to space charge recombination current and/or surface recombination current, and then at higher currents, IB is due to recombination in the base. At high base currents, IB departs from the exp. (qVEB/KT) dependence due to ohmic resistance of the base region. This base resistance is generally due to the base region being thinner and more lightly doped than the emitter. The collector current results from the emitter-base diffusion current, which varies as exp. (qVEB/KT) except at high currents where the injected minority carrier concentration approached the majority carrier concentration in the base and the IE begins to vary as exp. (qVEB/KT). The variation of IE is related to the collector current as IC=α0IE, and therefore IC has same variation exp. (qVEB/KT) as IE. At small values of IC, β does not become relatively constant until the space charge recombination and/or surface recombination current are much less than the diffusion current. At larger values of IC, β decreases due to the high injection effects which give a diffusion current variation as exp. (qVEB/KT).The I-V characteristics for the common emitter configuration are shown in figure 4. The condition for the saturation when both the junctions are forward biased is somewhat different for the common emitter configuration, because saturation is now determined by the relative values of VEB and VCE. The condition for saturation can be obtained by representing the transistor as shown in figure 5. Let IC ≈ IE and IE >> IB. then the current flow IC through the two resistors results in the voltage drops of VEB and VCB. From Kirchhoff’s law, the sum of the voltages around a closed loop is zero.

    

 

        

 

 

When VCE < VEB, VCB goes negative and the collector-base junction is forward biased. This condition is called saturation and occurs in Si transistors for VCE < 1V.

 

Software(s) and Hardware(s) required

a.       PC with internet connectivity (preferably high-speed)

b.      Teamviewer software for remote user (free downloadable)

  

Procedure

Step 1 – Read the manual of your experiment from our Homepage link

Step 2 –  Register and Login, remember your “User ID” and “Password”. (Registration is required only for one time)

Step 3 – Read the procedure for doing the experiment and Click on the button to proceed

Step 4 -  Now, “log in” to connect the remote Hardware Setup (at Indian Institute of Technology, Kharagpur, India)

Step 5 - Click on the experiment of your choice (from left column)

Step 6 – Now answer the preliminary quizzes to be eligible for doing the experiment

Step 7 - click on “Book the experiment” to book the particular slot for your experiment (time and date)

Step 8 – Click on “Run Experiment” during your time and slot

 Step 9 – Give the “input parameters” as per requirement of the experiment and click on “enter”

 Step 10 – The output curve will appear on your screen gradually and automatically. The  numerical output data will be shown simultaneously.

 Step 11 – Take the graph and data for showing “Result” using other software

Step 12 - Analyze the data and graph.

Step 13 - To Log out, click on the    and close the window.

 Step 14 - Now type www.vit.ac.in/onlinelab to go the home page again.

Step 15 - You are ready for the next experiment.

 

Results

The following photograph shows the output I-V characteristics of an NPN BJT in CE configuration.

                          

 

Conclusion

The study of the BJT in common-base and common-emitter configurations is done. Of the two, the common emitter configuration has more gain as a small base current can control a much larger collector current. The study of the various modes of operation gives us information on whether the transistor can be used in either analog or digital circuits.

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