ONLINE LAB - 8

Name of the experiment: Characterization of P-i-N Photo Detector

Objective

The objective of this experiment is to study the PIN diode 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 PIN diode:

A PIN diode consists of an intrinsic layer sandwiched between the opposite types of a p-n junction. The intrinsic layer has very low concentration of either n-type or p-type in the order of 10¹³ cm^-3 and a resistivity in the order of kΩ-cm. The intrinsic layer thickness (X_I) ranges from 10µm to 200µm. The outside p- and n-type layers are very heavily doped. Silicon is the main substrate material for the pin diode though GaAs was also studied.

                                                   Figure 1:- PIN Diode

The special feature of a PIN diode is a wide intrinsic layer that provides unique properties such as low capacitance, high breakdown voltage with reverse bias and carrier storage for microwave applications with forward bias. Near zero or at low reverse bias, the lightly doped intrinsic layer starts to be fully depleted. Once fully depleted, its capacitance is independent of reverse bias. Since there is little net charge within the intrinsic layer, the electric field is constant.

For silicon, the breakdown field ԑ_BD is approximately 2x10⁵ V/cm. These two equations show that the parameter X_I controls the tradeoff between frequency response (from capacitance) and power (from max. voltage). When the pin diode is under forward bias, both types of carriers are injected into the intrinsic layer and the carrier profiles are shown in figure. It is usually assumed that within the intrinsic layer, the electron and hole concentrations are the same and that they are uniform within the intrinsic layer. The current conduction is through recombination.

Characteristics

                                      Figure 2:- DC I-V Characteristics of a PIN diode

                                      Figure 3:- typical RF resistance as a function of DC forward current

A PIN diode obeys the standard diode equation for low frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. There is a lot of stored charge in the intrinsic region. At low frequencies, the charge can be removed and the diode turns off. At higher frequencies, there is not enough time to remove the charge, so the diode never turns off. The PIN diode has a poor reverse recovery time. The high-frequency resistance is inversely proportional to the DC bias current through the diode. A PIN diode, suitably biased, therefore acts as a variable resistor. This high-frequency resistance may vary over a wide range (from 0.1 ohm to 10 kΩ in some cases the useful range is smaller, though). The wide intrinsic region also means the diode will have a low capacitance when reverse biased. In a PIN diode, the depletion region exists almost completely within the intrinsic region. This depletion region is much larger than in a PN diode, and almost constant-size, independent of the reverse bias applied to the diode. This increases the volume where electron-hole pairs can be generated by an incident photon.

The diode design has some design tradeoffs. Increasing the dimensions of the intrinsic region (and its stored charge) allows the diode to look like a resistor at lower frequencies. It adversely affects the time needed to turn off the diode and its shunt capacitance. PIN diodes will be tailored for a particular use. It should be noted that since pin diode is similar to p-n junction diode, a diffusion current component should also be considered. In practice, this component is small since both regions surrounding the intrinsic layers are heavily doped.

The most interesting phenomenon for a pin diode, however, is for small signals at high frequencies, (>1/2πτ) at which the stored carriers within the intrinsic layer are not completely swept by the RF signal or by recombination. At these frequencies, there is no rectification or distortion and the pin diode behaves like a pure resistor whose value is solely determined by the superimposed DC bias or current.

For modulation and switching applications, even the mean bias point can vary with time. The upper limit of this modulation frequency is determined by the reverse recovery characteristics. When a pin diode is switched from forward bias to reverse bias abruptly, the stored charges continue to contribute to a large reverse current until they are fully drained away.

                                  Figure 4:- Carrier generation in a reverse bias PIN Diode

For practical applications, photo excitation is provided through either an etched opening in the top contact or an etched hole in the substrate. The latter reduces the active area of the diode to the size of the incident light beam. The key to achieving a high performance pin diode is to illuminate the diode through the substrate, ensure total depletion of the i-layer and operate the device at a low reverse bias. This is important for digital operation and low noise performance. 

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

Result

The figure below shows the ideal response of  resistance of a PIN diode vs. DC current.

Conclusion

Due to the presence of the intrinsic region, the PIN diode is a versatile device having applications in both forward and breakdown regions. When it is reverse biased, it is used as RF and microwave switches and as photodetectors. In forward bias, it is used as RF and microwave variable attenuators and limiters.

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