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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

C H A P T E R 5

MOS Field-Effect Transistors

(MOSFETs)

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Device Structure

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Two back-to-back diodes, High Resistance (Giga Ohms), No Current Flow

Operation with Zero Gate Voltage

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Creating a Channel for Current Flow

-vGS (gate to source voltage)

-Vt: threshold voltage (vGS which channel

starts conducting)

-Current flowing when vDS applied

-Effective voltage (or overdrive voltage):

-Charge in the channel:

-Oxide capacitance:

-Gate to source capacitance:

effOVtGS vvVv ≡≡−

OVox vWLCQ )(=

ox

oxox

tC

ε=

WLCC ox=

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Applying a small vDS

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

))()((

])()([(

]))([(

tGSoxnDS

DStGSoxnD

DSOVoxnD

VvL

WCg

vVvL

WCi

vvL

WCi

−=

−=

=

µ

µ

µ

Conductance gDS of the channel:

Applying a small vDS

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Applying a small vDS

))()((

1

1

tGSoxn

DS

DS

DS

VvL

WC

r

gr

−

=

=

µ

)(L

WCk oxnn µ=

oxnn Ck µ='

Process transconductance

parameter

MOSFET transconductance

parameter

MOSFET behaves as a

linear resistance

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Operation as vDS is increased

Channel becomes

more tapered and its

resistance increases

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Operation for vDS>>VOV

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

P-Channel

MOSFET

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

P-Channel MOSFET

tpGS Vv ≤

tpGS Vv ≥

oxnn Ck µ='

)(L

WCk oxnn µ=

P-Channel transistor transconductance parameter

P-Channel transistor process transconductance parameter

Use absolute value

Threshold voltage

“Formulae are the same,

switch sign of voltages”

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.10 Cross-section of a CMOS integrated circuit. Note that the PMOS

transistor is formed in a separate n-type region, known as an n well. Another

arrangement is also possible in which an n-type body is used and the n device

is formed in a p well. Not shown are the connections made to the p-type body and to the n well; the latter functions as the body terminal for the p-channel

device.

Complementary MOS or CMOS

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.11 (a) Circuit symbol for the n-channel enhancement-type MOSFET.

(b) Modified circuit symbol with an arrowhead on the source terminal to

distinguish it from the drain and to indicate device polarity (i.e., n channel). (c)

Simplified circuit symbol to be used when the source is connected to the body

or when the effect of the body on device operation is unimportant.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Table 5.1 Regions of Operation of the

Enhancement NMOS Transistor

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.12 The relative levels of the terminal voltages of the enhancement

NMOS transistor for operation in the triode region and in the saturation region.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.15 Large-signal equivalent-circuit model of an n-

channel MOSFET operating in the saturation

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.19 (a) Circuit symbol for the p-channel enhancement-type

MOSFET. (b) Modified symbol with an arrowhead on the source

lead. (c) Simplified circuit symbol for the case where the source is

connected to the body.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Table 5.2 Regions of Operation of the Enhancement PMOS Transistor

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 5.20 The relative levels of the terminal voltages of the

enhancement-type PMOS transistor for operation in the triode region

and in the saturation region.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure E5.7

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 5.3.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 5.4

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 5.6..

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 5.7.