MOSFET

introduction

In vented in year 1960 by Dawon Kahng and Martin Atalla mosfet is by far the most common fet in both digital and analog circuits.

A three terminal voltage controlled device as the operation of mosfet is controlled by the electric field established by aplying voltage between gate &body of mosfet

Mosfet is a unipolar device. 

Construction of mosfet

Operation of a mosfet Inversion layer is

formed due to the applied voltage

No gate current required to maintain the inversion layer

The applied voltage controls the current between gate & source

Mosfet operation can be separated under foll. three modes:

1. Cut-off region

2. Linear region

3.Saturation region

2. Triode or linear region: When S > Vtn and V

Classification of mosfet On the basis of operation of

mosfet,it is basically classified as:- Enhancement mosfet Depletion mosfet

The enhancement mosfet A mosfet is said to be enmos the drain

current is enhanced by the application of gate voltage

p p

n(substrate)

Induced p -channel

s g d

V-I characteristics of emos(p-channel)

Drain characteristics Transfer curve

-5

-6

-2-4

-10

Vgs(-)

-15

--

-Vds

Id

Vgst

Idss

-3-2

-1

+4 -4 -8 Vgs

Id

The depletion mosfet A mosfet is said to be a demos when a

channel induced between the source and the drain is of the same type of impurity as used for the source & drain.

- n nnn

p

s g d

V-I characteristics of dmos(n-channel)

Drain characteristics Transfer curve

-3 -2 -1 1 2 31

2

3

Vt

depletion

enhancement

Vgs

Id

+2+10

-1-2

Vgs

Id

246

5 10

15 Vds

cmos cmos is a combination of n-channel and p-

channel mosfet fabricated on single chip. In cmos the mosfets r complemented in a way

to keep one mosfet conducting on giving an input keeping the other non-conducting.

N+ N+ P+ P+P-well

N-substrate

s2 g2 D2 D1

g1

s2

Applications of mosfet Mosfet is extensively used as a resistor by

connecting the gate to the drain. the resistance so obtained is non linear in nature.

Mosfet is also used as a capacitor as the layer of SiO2 forms a parallel plate capacitor .

mosfets are extensively used in switching applications .

Advantages of mosfet MOSFETs are extensively usedfor digital

switching is that the oxide layer between the gate and the channel prevents any DC current from flowing through the gate, reducing power consumption.

Mos digital circuits r typically preferred over bipolar circuits as they r more economical and smaller in size.

Mosfets can be scaled down in size,in modern ICs the channel lengths r scaled down to a tenth of a micrometer has resulted in great improvement in mosfet operation.

Cmos devices have a high packing density allowing more number of ICs on a single chip.Cmos devices consume less power & do not produce as much heat as compared to other devices.

Bibliography The referred books and sites are: http://ece-www.colorado.edu/~bart/

book/mosfet.htm en.wikipedia.org/wiki/Transistor http://www.webopedia.com/TEKM/M/

MOSFET.html Micorelectronics(Millman &Grabel) Intregated electronics(Millman&Halkias)