Chapter 5 MOSFET 1 - University of Saskatchewanengr 5 MOSFET_1.… · Title: Microsoft PowerPoint -...

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)


Device Structure


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

Operation with Zero Gate Voltage


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=


Applying a small vDS


))()((

])()([(

]))([(

tGSoxnDS

DStGSoxnD

DSOVoxnD

VvL

WCg

vVvL

WCi

vvL

WCi

−=

−=

=

µ

µ

µ

Conductance gDS of the channel:




))()((

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


Operation as vDS is increased

Channel becomes

more tapered and its

resistance increases


Operation for vDS>>VOV


P-Channel

MOSFET


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”


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


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.


Table 5.1 Regions of Operation of the

Enhancement NMOS Transistor


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.


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

channel MOSFET operating in the saturation


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.


Table 5.2 Regions of Operation of the Enhancement PMOS Transistor


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.


Figure E5.7


Example 5.3.


Example 5.4


Example 5.6..


Example 5.7.

Chapter 5 MOSFET 1 - University of Saskatchewanengr 5 MOSFET_1.… · Title: Microsoft PowerPoint -...

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