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