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|V. |. GS. A Switch!. An MOS Transistor. What is a Transistor?. The MOS Transistor. Polysilicon. Aluminum. MOS Transistors - Types and Symbols. D. D. G. G. S. S. Depletion. NMOS. Enhancement. NMOS. D. D. G. G. B. S. S. NMOS with. PMOS. Enhancement. Bulk Contact.

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what is a transistor

|V

|

GS

A Switch!

An MOS Transistor

What is a Transistor?
the mos transistor
The MOS Transistor

Polysilicon

Aluminum

mos transistors types and symbols
MOS Transistors -Types and Symbols

D

D

G

G

S

S

Depletion

NMOS

Enhancement

NMOS

D

D

G

G

B

S

S

NMOS with

PMOS

Enhancement

Bulk Contact

current voltage relations a good ol transistor

-4

x 10

6

VGS= 2.5 V

5

Resistive

Saturation

4

VGS= 2.0 V

Quadratic

Relationship

(A)

3

VDS = VGS - VT

D

I

2

VGS= 1.5 V

1

VGS= 1.0 V

0

0

0.5

1

1.5

2

2.5

V

(V)

DS

Current-Voltage RelationsA good ol’ transistor
current voltage relations the deep submicron era

-4

x 10

2.5

VGS= 2.5 V

Early Saturation

2

VGS= 2.0 V

1.5

Linear

Relationship

(A)

D

I

VGS= 1.5 V

1

VGS= 1.0 V

0.5

0

0

0.5

1

1.5

2

2.5

V

(V)

DS

Current-Voltage RelationsThe Deep-Submicron Era
velocity saturation

5

u

= 10

sat

)

s

/

m

(

n

u

x

= 1.5

x

(V/µm)

c

Velocity Saturation

Constant velocity

Constant mobility (slope = µ)

perspective
Perspective

I

D

Long-channel device

V

= V

GS

DD

Short-channel device

V

V

- V

V

DSAT

GS

T

DS

i d versus v gs

-4

x 10

-4

x 10

6

2.5

5

2

4

1.5

(A)

3

(A)

D

D

I

I

1

2

0.5

1

0

0

0

0.5

1

1.5

2

2.5

0

0.5

1

1.5

2

2.5

V

(V)

V

(V)

GS

GS

ID versus VGS

linear

quadratic

quadratic

Long Channel

Short Channel

i d versus v ds

-4

-4

x 10

x 10

2.5

6

VGS= 2.5 V

VGS= 2.5 V

5

2

Resistive

Saturation

VGS= 2.0 V

4

VGS= 2.0 V

1.5

(A)

(A)

3

D

D

VDS = VGS - VT

I

I

VGS= 1.5 V

1

2

VGS= 1.5 V

VGS= 1.0 V

0.5

1

VGS= 1.0 V

0

0

0

0.5

1

1.5

2

2.5

0

0.5

1

1.5

2

2.5

V

(V)

V

(V)

DS

DS

ID versus VDS

Long Channel

Short Channel

simple model versus spice

-4

x 10

2.5

VDS=VDSAT

2

VelocitySaturated

1.5

Linear

1

VDSAT=VGT

0.5

VDS=VGT

Saturated

0

0

0.5

1

1.5

2

2.5

Simple Model versus SPICE

(A)

D

I

V

(V)

DS

a pmos transistor

-4

x 10

0

-0.2

-0.4

(A)

D

I

-0.6

-0.8

-1

-2.5

-2

-1.5

-1

-0.5

0

V

(V)

DS

A PMOS Transistor

VGS = -1.0V

VGS = -1.5V

VGS = -2.0V

Assume all variables

negative!

VGS = -2.5V

the gate capacitance

Polysilicongate

Source

Drain

W

x

x

+

+

n

n

d

d

Gate-bulk

L

d

overlap

Top view

Gate oxide

t

ox

+

+

n

n

L

Cross section

The Gate Capacitance
gate capacitance
Gate Capacitance

Cut-off

Resistive

Saturation

Most important regions in digital design: saturation and cut-off

gate capacitance1
Gate Capacitance

Capacitance as a function of the degree of saturation

Capacitance as a function of VGS

(with VDS = 0)

diffusion capacitance
Diffusion Capacitance

Channel-stop implant

N

1

A

Side wall

Source

W

N

D

Bottom

x

Side wall

j

Channel

L

Substrate

N

S

A

linearizing the junction capacitance
Linearizing the Junction Capacitance

Replace non-linear capacitance by

large-signal equivalent linear capacitance

which displaces equal charge

over voltage swing of interest

the sub micron mos transistor
The Sub-Micron MOS Transistor
  • Threshold Variations
  • Subthreshold Conduction
  • Parasitic Resistances
threshold variations

V

V

T

T

Threshold Variations

Low

V

threshold

Long-channel threshold

DS

VDS

L

Threshold as a function of

Drain-induced barrier lowering

the length (for low

V

)

(for low

L

)

DS

sub threshold conduction

-2

10

Linear

-4

10

-6

Quadratic

10

(A)

D

I

-8

10

Exponential

-10

10

VT

-12

10

0

0.5

1

1.5

2

2.5

V

(V)

GS

Sub-Threshold Conduction

The Slope Factor

S is DVGS for ID2/ID1 =10

Typical values for S:

60 .. 100 mV/decade

sub threshold i d vs v gs
Sub-Threshold ID vs VGS

VDS from 0 to 0.5V

sub threshold i d vs v ds
Sub-Threshold ID vs VDS

VGS from 0 to 0.3V

summary of mosfet operating regions
Summary of MOSFET Operating Regions
  • Strong Inversion VGS >VT
    • Linear (Resistive) VDS <VDSAT
    • Saturated (Constant Current) VDS VDSAT
  • Weak Inversion (Sub-Threshold) VGS VT
    • Exponential in VGS with linear VDS dependence
future perspectives
Future Perspectives

25 nm FINFET MOS transistor