Session – 1
Session Name: MOSFET Transistor
Author Name: Pragya Kushwaha
Department: Electronics and Communication
Subject/Course: VLSI Design
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Session Objectives At the end of this session, the learner will be able to:
Explain MOSFET.
Describe the physical structure of MOSFET and discuss its types.
Dramatize the working of a MOSFET and discuss its current-voltage
characteristics.
Teaching Learning Material
Board.
Presentation Slides.
LCD Projector.
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Session Plan
Time (min)
Content Learning Aid and
Methodology Faculty
Approach
Typical Student Activity
Learning Outcomes (Blooms + Gardeners)
10 Introduction to
MOSFET Transistor Brainstorming Chalk and Talk
Conducts Facilitates
Defines
Participates Recalls Answers
Remembering Understanding Intrapersonal Interpersonal
Linguistic
15 Physical Structure and
Types of MOSFET
Slide Presentation
Chalk and Talk Look and Tell
Quiz
Explains Conducts
Listens Observes
Participates Answers
Comprehends
Remembering Understanding Intrapersonal Interpersonal
Visual Linguistic
20 Operation of MOSFET
Role play Chalk and Talk
Slide Presentation
Conducts Facilitates Explains
Participates Observes
Comprehends
Remembering Understanding
Applying Intrapersonal Interpersonal Kinesthetic
Visual Logical
15 Conclusion Pictorial Quiz Conducts Facilitates Questions
Discusses Recalls Answers
Remembering Understanding Intrapersonal Interpersonal
Visual Linguistic
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Session Inputs
Introduction to MOSFET Transistor
We can begin the session on MOSFET Transistor by asking few
questions to the learners to introduce them to the basic terms
associated with the topic.
Suggested Activity: Brainstorming
We can ask the following simple questions to the learners.
1. What is full form of MOSFET?
2. What ‘M’ stands for in MOSFET?
3. What ‘O’ stands for in MOSFET?
4. What ‘S’ stands for in MOSFET?
5. What ‘FET’ stands for in MOSFET?
6. How many layers it consist of?
Learners may give various answers. We can write these on the board, and
discuss and analyze the answers with the help of suitable examples. After
removing the irrelevant answers, we can conclude the following:
1. Metal Oxide Semiconductor Field Effect Transistor
2. M - Metal
3. O - Oxide
4. S - Semiconductor
5. FET - Field Effect Transistor
6. It means MOSFET has 3 layers during its formation: metal, oxide and
semiconductor.
Metal (or poly-silicon doped heavily to act like a metal)
Oxide (SiO2, Acts as an insulator.)
Semiconductor (One can selectively change the carrier type
to n-type or p-type.)
Field Effect (Device is controlled by an electric field as opposed to
current.)
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Figure1: NMOSFET(left) and PMOSFET (right)
MOSFET Transistor can be defined as:
The metal–oxide–semiconductor field-effect transistor (MOSFET,
MOS-FET, or MOS FET) is a transistor used for amplifying or
switching electronic signals.
An insulated-gate field-effect transistor or IGFET is a related term
almost synonymous with MOSFET. The term may be more
inclusive, since many "MOSFETs" use a gate that is not metal, and
a gate insulator that is not oxide.
Physical Structure and Types of MOSFET
We can explain the topic physical structure of MOSFET Transistor by
showing some pictures on presentation slides to the learners to
introduce them with the physical structure of the transistor.
The following figure shows the physical structure of the MOSFET
transistor.
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Suggested Activity: Look and Tell Quiz
We can ask the following simple questions to the learners:
1. How many terminals are there in figure 2 (symbol)?
2. Lists the terminal names shown in figure 2.
3. Identify the types of substrate shown in figure 1?
The following are the expected answers from the learners:
1. MOSFET is a four terminal device (body or substrate terminal is generally
connected with source terminal hence not shown in figure).
2. There are two types of substrate n-type and p-type.
3. Source Terminal, Drain terminal, Gate terminal, Substrate terminal.
MOSFET is a four terminal device.
PMOSFET- MOSFET with n type substrate hence majority carriers
are HOLES in this type of MOSFET.
NMOSFET- MOSFET with p type substrate hence majority carriers
are ELECTRONS in this type of MOSFET.
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Types of MOSFET Transistor:
The channel can be of n-type or p-type (see figure 1), and is
accordingly called an NMOSFET or a PMOSFET (also commonly
NMOS, PMOS).
NMOS has high switching speed in comparison to PMOS,
because NMOS has electrons as majority carriers while PMOS has
holes as majority carriers.
Operation of MOSFET
With the help of the following role-play, the working of MOSFET can
be discussed.
Suggested Activity: Role Play
To begin with this activity, let us call upon 16 learners from the class. These
learners can be informed in prior about their roles. They can be asked to
come prepared with the dialogues assigned to them. We can appoint 2
learners to maintain the decorum of the class during role play. One learner
can explain the task from the backend while the others are enacting their
roles.
2 learners (standing) from the group of 16 acts as drain and source terminal
potential of MOSFET. Chair is the symbol of Gate terminal.
The value of gate to source voltage (Vgs) can be given by the observing
learners; so as to make everyone involved. This can also alert the attention of
everyone.
The play begins as follows:
Step 1: Gate voltage of MOSFET is less than Threshold voltage Vtn=1v.
The sitting learners are having Vgs value less than Vtn i.e. 0.6volt, 0.7volt.
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Cut off region: No channel formation
Step 2: Now, next values are applied to the gate terminal. The sitting learners
are showing nearly equal and higher values of Vgs (i.e. 1v, 1.5v, 2v….), which
is higher than Vtn= 1v.
MOSFET Transistor is ‘ON’
Step 3: Learners are showing the stage when MOSFET gets Gate voltage
Vgs>= Vtn (see figure 4). Electrons are going from source terminal to drain
terminal. Here sitting learners are playing role of moving electrons.
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N-Channel is created
Step 4: After channel formation, drain current starts flowing from drain
terminal to source terminal. Here two learners are sitting between source and
drain terminal showing current flow in channel. The learner sitting in left hand
side is showing symbol of linear drain current Id when Vds<Vgs-Vt (see figure
5b), while the learner sitting on right hand side is showing the saturation
current Idsat when Vds>Vgs-Vt (see figure 5c).
MOSFET: A Voltage Controlled Current Source:
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Figure3: NMOSFET ‘ON’ state
In MOSFETs, a voltage on the oxide-insulated gate electrode can
induce a conducting channel between the two other contacts
called source and drain. Hence it is also called voltage
controlled current device.
Threshold Voltage (Vt) - The value of VGS where the drain current
just begins to flow (see figure 4).
PMOSFET- Vgs is negative to switch ON the MOS transistor
(Vgs<Vt).
NMOSFET- Vgs is positive to switch ON the MOS transistor
(Vgs>Vt).
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Figure4: Drain current increased after Threshold voltage (Vgs>Vt)
Current-Voltage characterstics of NMOSFET is shown in figure 4.
After getting the the condition of Vgs>Vt, A channel is created
by electrons and in results Drain current Id starts flowing from
drain to source.
Then after Vgs>Vt when current starts flowing, current increases
further with respect to the drain to source potential Vds.
We get linear region of MOSFET when Vds< Vgs-Vt and
Saturation region of MOSFET when Vds> Vgs-Vt as shown in figure
5.
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Figure5: I-V Characteristic of MOSFET
Conclusion
Finally, we can conclude the session by revising the key points
discussed so far, through the following Pictorial Quiz activity.
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Suggested Activity: Pictorial Quiz
To begin with this activity, the learners can be divided into two groups
(columns wise). We can select one representative from each group. The
group members can collectively help their team representative to identify the
keyword written on the power point slide by giving clues in the form of its
definition, function, etc. If he/she identifies the keyword correctly, one point
can be given to the group. The group with highest point can be declared as
winners.
The following are some sample questions based on pictures:
1. Identify which type of MOSFET are these?
The possible answer:
Left hand side MOSFET is NMOS and right hand side MOSFET is PMOS
2. Fill in the blank: MOSFET has 4 terminals: DRAIN, (…….), SOURCE, and
SUBSTRATE.
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The possible answer:
Gate
3. NMOS transistor will ON when Vgs<Vt. Statement is True/ False.
The possible answer:
False
Click the below link to view the PPT for some more sample questions.
http://www.mission10x.com/mission-
10x/Documents/Pictorial%20Quiz_Pragya_KIT.ppt
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Summary In this session, we learnt to:
Explain MOSFET.
Describe the physical structure of MOSFET and discuss its types.
NMOS
PMOS
Dramatize the working of a MOSFET and discuss its current-voltage
characteristics.
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Assignment
Find some applications of MOSFET.
MOSFET works as a switch: prove this fact.
Relate MOSFET with Pass transistor and Transmission gate.
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References
http://www.youtube.com/watch?v=QO5FgM7MLGg
www.cmosvlsi.com/lect1.ppt
www.acsel-lab.com/Books/Digital_System.../DigSysClkCh8_v1.ppt
www.eng.buffalo.edu/courses/ee240/studentprojects/.../group1.ppt
userwww.sfsu.edu/~larryk/engr301/Mosfet%20Basics.ppt
en.wikipedia.org/wiki/MOSFET
www.doe.carleton.ca/~tjs/21-mosfetop.pdf
www.ece.sunysb.edu/~adoboli/355/class_7.ppt