ONLINE LAB - 7

Name of the experiment: Diode Breakdown Characteristics

Objective

The objective of this experiment is to study the breakdown characteristics of a diode 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 Breakdown Diodes

Diodes which are designed with adequate power dissipation capabilities to operate in the breakdown region may be employed as voltage reference or constant voltage devices. Such diodes are known as avalanche, breakdown or zener diodes.

                        Figure 1:- Circuit using zener diode used to regulate voltage across R_L against changes due to variations in load current and supply voltage

Two mechanisms for diode breakdown for increasing reverse voltage are recognized. In one mechanism, the thermally generated electrons and holes acquire sufficient energy from the applied potential to produce new carriers by removing valance electrons from their bonds. These new carriers in turn, produce additional carriers again in the process of disrupting bonds. This cumulative process is referred to as avalanche multiplication. It results in the flow of large reverse currents, and the diode finds itself in the region of avalanche breakdown. Even if the initially available carriers do not acquire sufficient energy to disrupt bonds, it is possible to initiate breakdown through a direct rupture of the bonds because of the existence of the strong electric field. Under these circumstances, the breakdown is referred to as Zener breakdown. This zener effect is now known to play an important role only in diodes with breakdown voltages below about 6 V.

At increasing reverse bias the width of the depletion region rises. To have a situation where both bands with majority carriers are at tunnel distance apart (of the order of 10 nm since no collisions should occur and from quantum mechanics there should be a responsible chance of tunneling to happen) we must have high doping concentrations. A diode that breaks down because of the zener effect at 4V must have doping concentrations (symmetric doping assumed) of at least 10¹⁸ cm^-3 at both sides.

Avalanche breakdown is caused by impact ionization of electron-hole pairs. When applying a high electric field, carriers gain kinetic energy and generate additional electron-hole pairs through impact ionization. The ionization rate is quantified by the ionization constants of electrons and holes. These ionization constants are defined as the change of the carrier density with position divided by the carrier density. The ionization causes a generation of additional electrons and holes

Quantum mechanical tunneling of carriers through the band gap is the dominant breakdown mechanism for highly doped p-n junctions. The analysis is identical to that of tunneling in a metal-semiconductor junction where the barrier height is replaced by the energy band gap of the material.

The tunneling current is obtained from the product of the carrier charge, velocity and carrier density. The velocity with which on average the carriers approach the barrier while the carrier density equals the density of available electrons multiplied with the tunneling probability. The tunneling current therefore depends exponentially on the band gap energy to the 3/2 power.

Characteristics

They are used characteristically in the manner shown in figure below. The source V and the resistance R are selected so that, initially, the diode operates in the breakdown region. Here, the diode voltage, which is also the voltage across the load R_L, is V_S and the diode current is I_Z. The diode will now regulate the load voltage against variations in load current and against variations in supply voltage V because, in the breakdown region, large changes in diode current produce only small changes in diode voltage. Moreover, as load current or supply voltage changes, the diode current will accommodate itself to these changes to maintain a nearly constant load voltage. The diode will continue to regulate until the circuit operation requires the diode current to fall to I_ZK, in the neighborhood of the ‘knee’ of the diode voltage-ampere curve. The upper limit on diode current is determined by the power dissipation rating of the diode. The breakdown voltage V_Z can be controlled quite accurately in the doping process.

                                  Figure 2:- The Voltage-Ampere Characteristics of a Breakdown diode.

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 figure below shows the forward and reverse characteristics of a zener diode.

 Conclusion

The two mechanisms viz. avalanche multiplication and impact ionization are studied here to understand the carrier flow in reverse bias of a diode. From the I-V characteristic graph, it is seen that the current is unlimited after I_ZK, so the diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

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