ONLINE LAB – 9

Name of the experiment: LED Characteristics

Objective

The objective of this experiment is to study the LED behavior 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 LEDs

LEDs are a typical pn-junction devices used under a forward bias. The basic operating mechanisms are based on the electrical and optical properties of pn-junctions and of semiconductor materials. Depending on the semiconductor material used in the light emitting layer (active layer), the wavelength of the emitted light can be anywhere within the range of visible to infrared. Most commercially available LEDs are made from III-V compound semiconductors. Some II-VI compound semiconductors such as ZnS and ZnSe are used in few LEDs emitting visible light, though these materials are not frequently used because of thee difficulty of the pn-junction formation. LEDs composed of a combination of InGaAsP and InP cover the wavelengths from 1300 to 1550 nm band are the ones usually used in optical fiber communication systems.

                                            Figure 1:- LED block schematic and energy band diagram

Device Characteristics

The output light of the LEDs originates from the spontaneous emission in the active layer, which in turn is a product of the radiative recombination of the injected electrons and holes. When a certain density of electrons is injected into an active layer having a thickness less than the carrier diffusion length, the same density of holes is also injected because of the requirement of charge neutrality. Those injected carriers distribute uniformly in the active layer because its thickness is less than the carrier diffusion length (a few micron-meters). The thickness of the active layer in LEDs is ordinarily between 0.5 and 2 µm.

The electrons and holes in the active layer radiatively recombine at a finite rate, so the injected carrier density changes with time. The variation of the density can be expressed by a behavior of the injected minority carrier.

The injected carrier density is more than 10¹⁷ cm^-3 under the usual operating conditions in LEDs. The active layer of normal LEDs is intentionally undoped   or lightly doped in order to increase light output power, since the influences of Auger recombination, intra-band adsorption, and non radiative recombination become stronger with increased impurity doping.

The light power emitted in the active layer is given by the product of the no. of photons emitted and the photon energy hv. The no. of photons created by the spontaneous recombination process in a unit volume of the active layer is given by the product of the injected carrier density, J/qd, and the internal quantum efficiency.

Current Voltage Characteristics

As shown in basic characteristics, the current increases rapidly when the applied voltage exceeds the band gap of the active layer, which is about 0.8V for the LED.

                              Figure 2: - I-V characteristics, in low bias range, for an InGaAsP/InP double heterostructure diode

  As the injected current increases, the differential resistance gradually decreases and approaches the series resistance, R_s. The series resistance for the device treated is estimated to be 1.8Ω. Here, non-radiative recombination current is dominant in the low bias range. The non-radiative recombination current, with the 2K_BT component is larger than the radiative diffusion current. Under a forward bias, some of the injected electrons and holes are confined as minority carriers at the surface depleted region (usually 2-5nm deep), where in a pn-heterojunction only the majority carrier of the wider band gap semiconductor is injected. Each minority carrier diffuses along the surface in the confined layer until it nonradiatively recombines with a majority carrier.

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

Conclusion

The study of LED I-V characteristics gives us information on the various parameters like diffusion current, recombination current, light output power, differential resistance and output efficiency.

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