ONLINE LAB - 3
Name of the experiment: Bipolar Device Characterization
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
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
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
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
Typical online Internet accessible laboratory consists of:
Lab device, instrument or pilot plant equipment for
tele-presence showing the lab to remote users
Teleconferencing equipment or at least built-in
chatting capabilities for collaboration among students and instructor
Control software allowing users to perform
experiments, program lab devices and/or run pilot plant.
Introduction to BJT
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.
structure provides a number of natural advantages.
Electrons travel from the
emitter to the collector, perpendicular to the wafer surface.
The entire emitter area
The input voltage directly
controls the density of the carriers that provide the output current, Ic.
The turn-on voltage of BJT,
VBE is relatively independent of device size and process variations.
The input capacitance tends
to scale with the operating current to the extent that it is dominated by the
Fig 1:- BJT Symbols
Modes of operation:
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:
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
Fig 2: - CB Configuration Circuit using n-p-n transistor
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:
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
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.
< 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)
PC with internet
connectivity (preferably high-speed)
Teamviewer software for
remote user (free downloadable)
– 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,
Step 5 - Click on
the experiment of your choice (from left column)
Step 6 –
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)
– 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”
– The output curve will appear on your screen gradually and
automatically. The numerical output
data will be shown simultaneously.
– Take the graph and data for showing “Result” using other software
- Analyze the data and
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.
The following photograph shows the output I-V characteristics of an NPN BJT in
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.