Slide 1 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Microelectronic Circuits

Introduction to MOSFET MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Slide 2 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Construction

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Slide 3 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Slide 4 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Source Drain

SiO2 Insulator (Glass)

Gate

holes

electrons

5 volts

electrons to be

transmitted

Step 1: Apply Gate Voltage

Step 2: Excess electrons surface in

channel, holes are repelled.

Step 3: Channel becomes saturated

with electrons. Electrons in source

are able to flow across channel to

Drain.

P

N N

MOSFET Operation

Slide 5 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET

Slide 6 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

MOSFET is operating as a linear resistance device whose value is controlled by vgs infinite resistance at vgs ≤ vt

The induced channel will be enhanced after gate voltage becomes more than the threshold voltage , hence the name enhanced type MOSFET

Slide 7 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

When VDS is increased by keeping VGS constant > Vt

VDS appears as a voltage drop across the length of the channel. i.e when we traverse from the Source to Drain , the voltage drop increases from 0 to vDS

Slide 8 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

Thus the voltage between gate and points along the channel decreases from vGS at the source and vGS-vDS at the drain end.

Due to this channel depth is no longer uniform, being deepest at the source and shallowest at the drain end

Slide 9 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

As the vDS is increased , the channel becomes more tapered and its resistance increases correspondingly. Thus iD---vDS curve is not a straight line

Slide 10 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

When the vDS is increased to a value such that , the voltage between the Gate and channel is reduced to Vt (vDS = vGS - vt )

At this moment , the channel width at the drain end almost zero and the channel is said to be pinched off

Increasing vDS beyond this value has no effect on the channel shape and current through the channel remains constant. (at vDS = vGS - vt )

Slide 11 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET Operation

Drain current saturates and MOSFET is said to be in the saturation region .

vDS at which saturation occurs is denoted by vDSsat=vGS-vt

The Device operates in a saturation region if vDS > vDSsat

Slide 12 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

iD and vDS relationship

regionSaturationVvL

Wki

regiontriodevvVvL

Wki

tGSnD

DSDStGSnD

2'

2'

2

1

2

1

Where W= channel width, L = Channel length =Process transconductance parameter (Determines the value of MOSFET transconductance.)

'

nk

Slide 13 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

iD vs vDS characteristics

Slide 14 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

NMOS Symbol

Slide 15 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Equivalent Circuit

Slide 16 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET as an Amplifier

Slide 17 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

MOSFET as an Amplifier

Slide 18 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Biasing of MOSFET

Two Supplies Method

Slide 19 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Biasing of MOSFET

Voltage Divider Bias Method

22

1tGSoxnD VV

L

WCI

SDGSG RIVV

Slide 20 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Numerical based on Biasing

It is required to design a circuit to establish a dc drain current ID = 0.5 mA. The MOSFET is specified to have Vt = 1V and . Use VDD = 15. calculate the percentage change in the value of ID obtained with the MOSFET threshold voltage Vt = 1.5 V

21 VmALWkn //'

Slide 21 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Biasing Using Drain to Gate Feedback

Biasing the MOSFET using a large drain-to-gate feedback resistance, RG.

DDGSDD

DDDDDSGS

RIVV

IRVVV