Outline

• Channel Length Modulation

PMOS

Role of Substrate

Temperature Effects

2/12/2015

Outline

Channel Length Modulation

1

Recap

• NMOS Operation in

Cut off ,Linear and Saturation mode

Related Equations

2/12/2015

Recap

Cut off ,Linear and Saturation mode

2

MOSFETMOSFET

When VGS=0 V and V=0 V and VDS =1 V

When VGS=0 V and V=0 V and VDS =3 V

When VGS=2 V and V=2 V and VDS =0 V

When VGS=2 V and V=2 V and VDS =1 V

When VGS=5 V and V=5 V and VDS =0 V

When VGS=5 V and V=5 V and VDS =1 V

When VGS=10 V and V=10 V and VDS =10 V

Effect of Channel Length Modulation

Effect of Channel Length Modulation

Effect of Channel Length Modulation

Effect of Channel Length Modulation

Linear/Triode Region

2/12/2015

•MOS transistor can be modelledrDS whose value is controlled

Linear/Triode Region

13

modelled as linear resistorcontrolled by vDS

Large Signal equivalent Model (saturation)

•Since the drain current independentsaturated mosfet behaves as idealis controlled by vGS

Large Signal equivalent Model (saturation)

independent of drain voltage, ,theideal current source whose value

and

• The effect of channel-length modulation is less for a longchannel MOSFET than for a short

and L

length modulation is less for a long-channel MOSFET than for a short-channel MOSFET.

iD - vDS Characterstics (Channel Length Modulation)Characterstics (Channel Length Modulation)

Effect of L on channel length Modulation

2/12/2015

Effect of L on channel length Modulation

17

iD - vDS Characterstics (Channel Length Modulation)Characterstics (Channel Length Modulation)

Chapter 4 MOS field-Effect Transistors (MOSFETs)

Figre 4.17Modelling Channel Length ModulationEffect Transistors (MOSFETs)

Figre 4.17Channel Length Modulation

NMOS TransistorNMOS Transistor

Enhancement - mode

•Devices are cutoff with zero gate bias •Apply +ve voltage to gate to make channel

Depletion - mode

•Devices conduct with zero gate bias voltage•Channel exists. Apply -ve voltage to turn off

Conduction characteristics for transistor

zero gate bias voltageApply +ve voltage to gate to make channel

zero gate bias voltageve voltage to turn off

Conduction characteristics for nMOStransistor

n

D

S

G

Enhancement - mode

•Devices are cutoff with zero gate bias voltage•Apply +ve voltage to gate to make channel

Conduction characteristics for transistor

(assumingfixed Vds)

n

Depletion - mode

•Devices conduct with zero gate bias voltage•channel exists. Apply -ve voltage to turn off

Conduction characteristics for nMOStransistor

D

S

G

PMOS TransistorTransistor

Chapter 4 MOS field-Effect Transistors (MOSFETs)

Figure 4.18PMOS Transistor - Symbolic RepresentationEffect Transistors (MOSFETs)

Figure 4.18Symbolic Representation

Conduction characteristics for pMOS transistor

(assuming fixed Vds)

Conduction characteristics for transistor

(assuming fixed Vds)

HomeWork

2/12/2015

• Voltage and Current Equations for PMOS in

Linear and Saturation mode

HomeWork

26

Voltage and Current Equations for PMOS in

Linear and Saturation mode

Role of substrate

• In many applications the source terminal is connected to the substrate terminal B which results in the between the substrate and the induced channel having zero (cutoff) bias

2/12/2015

Role of substrate

In many applications the source terminal is connected to which results in the PN junction

between the substrate and the induced channel having

27

Role of substrate• The reverse bias voltage will

.This in turn reduces the channel depth

• To return to its former state

VGS

2/12/2015

Role of substrateThe reverse bias voltage will widen the depletion region

.This in turn reduces the channel depth

To return to its former state -------has to be increased

28

Role of substrate

• The effect of VSB on the channel can be most convinently represented as a change in the threshold voltage Vt

Vto - threshold voltage for VSB = 0

- Fabrication process

2/12/2015

Role of substrate

on the channel can be most convinently represented as a change in the

threshold voltage for VSB = 0

Fabrication process

29

Role of substrate

• If VSB changes then thereVt , which in turn results ineven though vGS is constant

• The body (substrate ) controls ias another gate for the MOSFET , a phenomenon known as Body Effect

2/12/2015

Role of substrate

there is incremental change inin incremental change in iD

constant

The body (substrate ) controls iD thus the body acts for the MOSFET , a phenomenon

30

Temperature Effects

2/12/2015 EEE C424/ECE C313

Temperature Effects

EEE C424/ECE C313 31

Temperature Effects

2/12/2015 EEE C424/ECE C313

Temperature Effects

EEE C424/ECE C313 32

Breakdown and input Protection

2/12/2015 EEE C424/ECE C313

Breakdown and input Protection

EEE C424/ECE C313 33

Chapter 4 MOS field-Effect Transistors (MOSFETs)

Figure 4.9CMOS TransistorEffect Transistors (MOSFETs)

Figure 4.9Transistor