Download MOS Transistors

ECE340
ELECTRONICS I
MOSFET TRANSISTORS AND
AMPLIFIERS
MOSFET
• METAL-OXIDE-SEMICONDUCTOR FIELD EFFECT
TRANSISTOR
• VOLTAGE - CONTROLLED DEVICE
• LOW POWER DISSIPATION
MOSFET
METAL
OXIDE
OXIDE
SOURCE
CHANNEL
L
OXIDE
DRAIN
NMOSFET ENHANCEMENT MODE
DEVICE
-VS
+VD
+VG
METAL
OXIDE
OXIDE
OXIDE
N TYPE SOURCE
N TYPE DRAIN
DEPLETION LAYER
DEPLETION LAYER
P TYPE SUBSTRATE
-VB
MOSFET “ON” CONDITION
+VD
VG > VTN
ID
METAL
OXIDE
OXIDE
n+
p
electrons
OXIDE
n+
MOSFET PARAMETERS
• iD – DRAIN CURRENT
• VTP,VTN – THRESHOLD VOLTAGE (VTH)
• vDS – DRAIN TO SOURCE VOLTAGE
• vGS – GATE TO SOURCE VOLTAGE
• vB – BULK VOLTAGE
THRESHOLD VOLTAGE
• VOLTAGE REQUIRED TO CREATE AN INVERSION
LAYER OF CHARGE UNDER THE GATE OXIDE
• POSITIVE FOR n-CHANNEL DEVICES
• NEGATIVE FOR p-CHANNEL DEVICES
BULK VOLTAGE
• LOWEST VOLTAGE AVAILABLE FOR NMOS (NCHANNEL) DEVICES
• HIGHEST VOLTAGE AVAILABLE FOR PMOS (PCHANNEL) DEVICES
• REVERSE-BIASES PN JUNCTIONS
MOSFET CAPACITANCE
• POSITIVE OR NEGATIVE VOLTAGE AT GATE
TERMINAL INDUCES CHARGE ON GATE METAL
• CHARGE OF OPPOSITE TYPE ACCUMULATES IN
CHANNEL
• FORMS MOSFET CAPACITOR
OXIDE CAPACITANCE
Cox 
 ox
tox
 ox  3.9 o
 o  8.85 10
14
F / cm
PARAMETER DEFINITIONS
• n,p - ELECTRON OR HOLE MOBILITY
• ox – PERMITTIVITY OF OXIDE
• tox – OXIDE THICKNESS
• (W/L) – ASPECT RATIO
MOSFET OPERATION
• SOURCE TERMINAL IS GROUNDED
• GATE AND DRAIN VOLTAGES REFERENCED TO
SOURCE VOLTAGE
• VOLTAGE IS APPLIED TO GATE TERMINAL TO
INDUCE CHARGE IN THE CHANNEL
CHARGE FLOW
• CHARGE IS PULLED INTO CHANNEL FROM DRAIN
AND SOURCE REGIONS
• CHARGE FLOWS FROM SOURCE TO DRAIN AS
DRAIN VOLTAGE IS INCREASED
DEVELOPMENT OF MOSFET
EQUATIONS
dq  C ox Wdx v GS  vx   VTH 
dvx 
v DS   Ex   
dx
dx
velocity of channel charge 
dt
DEVELOPMENT OF MOSFET
EQUATIONS
dx
velocity of channel charge 
dt
dq
i
dt
 dq  dx 
 i    
 dx  dt 
dvx 
i D  i  i D  μ n C ox Wv GS  vx   VTH 
dx
DEVELOPMENT OF MOSFET
EQUATIONS
dvx 
iD  i  iD   nCoxW vGS  vx   VTH 
dx
iD dx   nCoxW vGS  vx   VTH dvx 
L
i
0
D
dx 
v DS
  C W v
n
0
ox
GS
 vx   VTH dvx 
DEVELOPMENT OF MOSFET
EQUATIONS
W
iD   nCox 
L
1 2 

 vGS  VTH vDS  vDS 
2


k n'   nCox
W
iD  k 
L
'
n
1 2 

 vGS  VTH vDS  vDS 
2


N-CHANNEL MOSFET EQUATIONS
vGS  VTH
 iD  0
vDS  vGS  VTH 
W
 iD  k 
L
vDS  vGS  VTH 
1 ' W 
2
 iD  k n  vGS  VTH 
2 L
'
n
1 2 

 vGS  VTH vDS  vDS 
2


MOSFET CHARACTERISTICS
ID
1.5mA
vGS3
1.0mA
vGS2
0.5mA
vGS1
0mA
0V
2V
4V
6V
vDS
8V
10V
12V
TRANSCONDUCTANCE
PARAMETER COMPONENTS
• MOBILITY
• ELECTRIC PERMITTIVITY
• OXIDE THICKNESS
• ASPECT RATIO
TRANSCONDUCTANCE
PARAMETER PHYSICS
W 
K  k' 
L
  ox 
  k '  Cox
k '  
 tox 
Cox 
 ox
tox
n-CHANNEL MOSFET OPERATION IN
CUTOFF REGION
vGS  VTH
iD  0
n-CHANNEL MOSFET OPERATION IN
LINEAR REGION
vDS  vGS  VTH
1 2

iD  K n vGS  VTH vDS  vDS 
2


n-CHANNEL MOSFET OPERATION IN
SATURATION REGION
vDS  vGS  VTH
1
2
iD  K n vGS  VTH 
2
p-CHANNEL MOSFET OPERATION IN
CUTOFF REGION
vSG  VTH
iD  0
p-CHANNEL MOSFET OPERATION IN
LINEAR REGION
vSD  vSG  VTH
1 2

iD  K p vSG  VTH vSD  vSD 
2


p-CHANNEL MOSFET OPERATION IN
SATURATION REGION
vSD  vSG  VTH
1
2
iD  K p vSG  VTH 
2
NMOS INCREMENTAL RESISTANCE
IN THE LINEAR REGION
1

2
iD  K n vGS  VTH vDS  vDS 
2


vDS  1  iD  K n vGS  VTH vDS
rDS
 iD
 
 vDS



1 v DS small
 rDS  K n VGS  VTH 
1
vGS VGS
PMOS INCREMENTAL RESISTANCE
IN THE LINEAR REGION
1 2

iD  K p vSG  VTH vSD  vSD 
2


vDS  1  iD  K p vSG  VTH vSD
 iD 

rSD  
 vSD 
1 vSD small
 rDS  K n VSD  VTH 
1
vSG VSG
MODULATED CHANNEL IN
SATURATION REGION
VD>>VG
+VD
VG > VTN
ID
METAL
OXIDE
OXIDE
n+
OXIDE
n+
TAPERED CHANNEL
p
NMOS INCREMENTAL RESISTANCE
IN SATURATION REGION
1
2
iD  K n vGS  VTH  1  vDS 
2
 iD
rO  
 vDS
rO  I D 



1
1
vGS VGS
  Kn 
2
 rO   
VGS  VTH  
  2 

1
PMOS INCREMENTAL RESISTANCE
IN SATURATION REGION
1
2
iD  K p vSG  VTH  1  vSD 
2
 iD 

rO  
 vSD 
rO  I D 
1
1
vSG VSG
  Kp 
2
VSG  VTH  
 rO   
  2 

1
DEPENDENCE ON DRAIN VOLTAGE
1
rO 
λI D
λ  channel length modulation parameter
1 ' W
2
VGS  VTH 
ID  k n
2
L
PSPICE MOSFET SYMBOLS
p-channel enhancement
n-channel enhancement
NMOS LARGE SIGNAL MODEL
G
+
VGS
S
D
G
+
1 ' W
2
k n  VGS  VTH 
2 L
rO
VDS
S
DEVELOPMENT OF MOSFET SMALLSIGNAL MODEL
vGS  VGS  vgs
iD  I D  id
TOTAL CURRENT AND VOLTAGE

1 'W
VGS  vgs   VTN
iD  k n
2 L
iD 


2
1 'W
VGS  VTH 2  2VGS  VTH vgs  vgs2
kn
2 L


1 'W
VGS  VTH   vgs
 iD  k n
2 L

1 'W
VGS  VTH 2  2VGS  VTH vgs
v  1  iD  k n
2 L
2
gs


2
COMPONENTS OF TOTAL CURRENT
iD  I D  id
1 'W
2
' W
iD  k n VGS  VTH   k n VGS  VTH v gs
2 L
L
W
VGS  VTH vgs
id  k
L
'
n
MOSFET TRANSCONDUCTANCE
W
VGS  VTH vgs
id  k
L
'
n
id
gm 
v gs
W
VGS  VTH 
gm  k
L
'
n
ALTERNATIVE
TRANSCONDUCTANCE EQUATION
1 'W
2


I D  kn
VGS  VTH
2 L
 VGS  VTH
2I D

' W
kn
L
W
' W
VGS  VTH   g m  2kn I D
gm  k
L
L
'
n
SMALL-SIGNAL MODEL
g
VCC
+
vgs
s
id
d
+
gmvgs
rO
vds
s