Lecture #38 OUTLINE The MOSFET: • Bulk-charge theory • Body effect parameter • Channel length modulation parameter • PMOSFET I-V • Small-signal model Reading: Finish Chapter 17, 18.3.4 Spring.
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Transcript Lecture #38 OUTLINE The MOSFET: • Bulk-charge theory • Body effect parameter • Channel length modulation parameter • PMOSFET I-V • Small-signal model Reading: Finish Chapter 17, 18.3.4 Spring.
Lecture #38
OUTLINE
The MOSFET:
• Bulk-charge theory
• Body effect parameter
• Channel length modulation parameter
• PMOSFET I-V
• Small-signal model
Reading: Finish Chapter 17, 18.3.4
Spring 2007
EE130 Lecture 38, Slide 1
Problem with the “Square Law Theory”
Qinv Coxe VG VT VS VC
• Ignores variation in depletion width with distance y
Spring 2007
EE130 Lecture 38, Slide 2
Modified (Bulk-Charge) Model
VG VT
• linear region: VD VDsat
m
W
m
I Dlin Coxe eff (VG VT VDS )VDS
L
2
• saturation region: VD VDsat
I Dsat
where m 1
Spring 2007
VG VT
m
W
Coxe eff (VG VT ) 2
2mL
Cdm
3T
1 oxe
Coxe
WT
EE130 Lecture 38, Slide 3
since Si 3 Si O2
MOSFET Threshold Voltage, VT
The expression that was previously derived for VT is the
gate voltage referenced to the body voltage that is
required reach the threshold condition:
2qNA Si (2F VSB )
VT VFB VSB 2F
Cox
Usually, the terminal voltages for a MOSFET are all
referenced to the source voltage. In this case,
2qNA Si (2F VSB )
VT VFB 2F
Cox
and the equations for IDS are
W
m
Coxe eff (VGS VT VDS )VDS
L
2
VDS VDSsat VGS VT / m
I Dlin
Spring 2007
EE130 Lecture 38, Slide 4
I Dsat
W
Coxe eff (VGS VT ) 2
2mL
VDS VDSsat VGS VT / m
The Body Effect
Note that VT is a function of VSB:
2qN A Si (2F VSB )
VT VFB 2F
Cox
2qN A Si (2F )
2qN A Si (2F )
2qN A Si (2F VSB )
VFB 2F
Cox
Cox
Cox
2qN A Si
VT 0
2F VSB 2F VT 0 g
Cox
where g is the body effect parameter
2F VSB 2F
When the source-body pn junction is reverse-biased, |VT|
is increased. Usually, we want to minimize g so that IDsat
will be the same for all transistors in a circuit
Spring 2007
EE130 Lecture 38, Slide 5
MOSFET VT Measurement
• VT can be determined by plotting IDS vs. VGS,
using a low value of VDS
IDS
VGS
Spring 2007
EE130 Lecture 38, Slide 6
Channel Length Modulation Parameter, l
• Recall that as VDS is increased above VDsat, the width DL of
the depletion region between the pinch-off point and the
drain increases, i.e. the inversion layer length decreases.
I Dsat
1
1 DL
1
L DL L
L
DL VDS VDSsat
DL
l VDS VDSsat
L
I Dsat
Spring 2007
W
Coxe eff (VGS VT ) 2 1 l VDS VDSsat
2mL
EE130 Lecture 38, Slide 7
P-Channel MOSFET
• The PMOSFET turns on when VGS < VTp
– Holes flow from SOURCE to DRAIN
DRAIN is biased at a lower potential than the SOURCE
VG
VS
• VDS < 0
VD
GATE
P+
IDS
P+
N
VB
• IDS < 0
• |IDS| increases with
• |VGS - VTp|
• |VDS| (linear region)
• In CMOS technology, the threshold voltages
are usually symmetric: VTp = -VTn
Spring 2007
EE130 Lecture 38, Slide 8
PMOSFET I-V
• Linear region: 0 VDS
I DS
VGS VTp
m
W
m
Coxe p ,eff (VGS VTp VDS )VDS
L
2
• Saturation region: VDS
I DS I Dsat
VGS VTp
m
W
Coxe p ,eff (VGS VTp ) 2
2mL
m = 1 + (3Toxe/WT) is the bulk-charge factor
Spring 2007
EE130 Lecture 38, Slide 9
Small Signal Model
id gd vd gmvg
• Conductance parameters:
gd
I D
VD V
lI Dsat 0
G const
I D
gm
VG
Spring 2007
EE130 Lecture 38, Slide 10
VD const
Weff Coxe
mL
(VGS VT )
Inclusion of Additional Parasitics
Spring 2007
EE130 Lecture 38, Slide 11
Cutoff Frequency
• fmax is the frequency where the MOSFET is
no longer amplifying the input signal
– Obtained by considering the small-signal model
with the output terminals short-circuited, and
finding the frequency where |iout / iin| = 1
Weff
gm
1
f max
(VGS VT )
2Coxe 2m L
L
Increased MOSFET operating frequencies are
achieved by decreasing the channel length
Spring 2007
EE130 Lecture 38, Slide 12