ONLINE LAB - 2

Name of the experiment: Characterization of MOSFET

Objective

The objective of this experiment is to study the MOSFET behavior and its I_D-V_DS 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 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 MOSFET

The basic structure of a MOSFET is formed by adding two heavily doped n+ regions to the MOS capacitor on p-type silicon. The MOSFET is also called as insulated gate FET (IGFET). One n+ region is designated the source, while the n+ region where the carriers flow out of the MOSFET is the drain. Application of a positive gate bias produces the conductive path (by electrons) between the source and drain. The gate length is represented by L, while the gate width is represented by W. The bulk semiconductor is called the substrate or MOSFET body. The potential applied to the gate controls the flow of carriers from the source to the drain.  

                                                             Fig 1: - MOSFET structure

When the electrons are the mobile charge carriers for the inversion, the MOSFET is termed as n-channel MOSFET. Similarly for n-type substrate with holes as mobile carriers for inversion the MOSFET is termed as p-channel MOSFET. When the MOSFET is off (i.e., gate to source voltage-V_GS = 0), it is termed as enhancement mode MOSFET. When MOSFET is ON for V_GS = 0, it is termed as depletion mode MOSFET.

Basically, there are three regions of operation for the MOSFET. They are described below. Here we consider a device made up of a p-substrate, an insulator, a metal gate, and an n+ drain and source. We assume that the source and the drain are connected to ground.

Cutoff Region (V_G ≤ V_T): For gate voltage less than or equal to threshold voltage, no inversion layer is formed and the drain current is zero. This segment of the axis which coincides with V_D is known as cutoff. In cutoff, with no inversion layer, the structure between the n+ drain and source consists of two n₊ p-junction diodes back to back, so that the current for any voltage V_D applied between drain and source is, for all practical purposes, zero.

Linear Region (V_D = V_D1 and V_G > V_T): For the linear and saturation regions, the voltage applied to the gate, V_G is greater than the threshold voltage, VT and an inversion layer of electrons (n-layer) is formed in the SiO interface by capacitor action between the metal gate and the semiconductor.  With a fixed V_G, a small increase in V_D causes the channel to be biased positively with respect to the substrate, which is at ground potential. This reverse bias widens the layer.

Linear Region (V_D = V_D2 > V_D1): As V_D is increased, beyond the linear segment of the characteristics, the depletion layer becomes wider and the inversion layer charge density, decreases with largest decrease occurring at the drain end , where the voltage across the oxide (V_G-V_D), has its smallest value. The drain voltage negates some of the effects of the gate voltage. If we increase V_D2 above V_D1, the current also increases to I_D2 resulting in a smaller slope as shown in figure. This process continues as V_D is increased, causing an increase of I_D but with a smaller slope dI_D/dV_D, until the point is reached where the increase of V_D culminates in pinch-off at the drain end where the electron density in the channel has become zero. The effect of the gate voltage that exceeds V_T has been negated by the increase in V_D resulting in the termination of the linear region.

Saturation Region (V_D3 ≥ V_SAT and I_D = I_SAT): The inversion charge density at the drain end of the channel is determined by the voltage across the oxide, (V_G - V_D). When that becomes equal to or less than V_T, charge at the drain becomes zero and V_D3 = V_G - V_T. The drain current reaches a constant value at V_D ≥ V_D3. The region where the current is constant for V_D ≥ (V_SAT = V_D3) is labeled the saturation region. Here, V_D1, V_D2, V_D3 are the drain voltages at various gate voltages and locus of pinch-off points.

                                                                        Fig 2: The three regions of operations of an n-channel MOSFET.

I_D – V_DS Characteristics: A source and drain are added to the MOS Capacitor to form a MOSFET. A heavily doped n⁺ region is shown adjacent to the MOS capacitor gate. When the gate voltage VGS exceeds the threshold voltage V_T, then an inversion layer is formed near the SiO2 - Si interface. The n⁺ source region can supply electrons to the inversion region without depending on the thermal generation rate as required for the MOS capacitor. Next, an n+ drain region has been added so that electrons can flow from source to the drain through the inversion layer when a positive drain voltage V_DS is applied to the drain. This electron flow constitutes the drain current I_D. Current into the drain is taken as a positive current. The positive gate voltage controls the number of electrons in the inversion layer and hence controls the drain current. In depletion mode, device is ON at zero gate bias. To turn the device OFF, a negative gate bias is required as shown in figure.

The analysis of the MOSFET requires the derivation of source-to-drain current (I_D) as a function of drain-to-source voltage (V_DS) for a given gate-to-source voltage (V_GS) when V_GS exceeds the threshold voltage V_T. I_D is obtained for small V_DS to illustrate the analysis procedure.

                                                 Fig 4: - n-channel depletion-mode characteristics

The curve named V_DS=V_DS(SAT) is the locus of the pinch-off points in figures 3 and 4. A more complex model can be assumed by taking into account the effect of reverse biased n-junction space charge at the drain. These ID vs. VDS characteristics are derived from p-type substrate where the minority carriers for inversion are electrons.

Transfer characteristics:

                                                                           Fig 5: The transfer characteristics of an n-channel MOSFET

The transfer characteristics of an n-channel MOSFET is given in Fig. 3.The values of I_D are measured at two values of V_GS. These values are termed as the threshold voltages, V_T. If V_T < 0, it is termed as depletion mode and V_T > 0, it is enhancement mode.

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 graphs show the Transfer characteristics and I-V characteristics of a MOSFET. The experiment can be done at a remote place using the actual hardware lab facilities to get the following output of MOSFET.

Conclusion

The variation in the gate biasing turns the device ON and OFF. A clear picture of the modes of operation of MOSFET is shown in the I_D-V_DS and transfer characteristics.

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