EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS
(N-Channel Metal Oxide Semiconductor)
Transistor
gate
source
metal
n-type
h
e e e
h
drain
metal
oxide insulator
h
metal
n-type
h
e e e
p-type
h
metal
h
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS Transistor in Equilibrium
gate
source
metal
n-type
+ + +
_
_
metal
oxide insulator
_
h
h
drain
metal
_
n-type
+ + +
_
_
p-type
h
h
metal
When the transistor is left alone, some electrons from the
n-type wells diffuse into the p-type material to fill holes.
This creates negative ions in the p-type material and
positive ions are left behind in the n-type material.
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS Transistor in Cutoff
VGS > 0
gate
- +
source
metal
n-type
+ + +
_
_
_
metal
oxide insulator
_
_ _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
metal
When a small, positive VGS is applied, holes “move away”
from the gate.
Electrons from complete atoms elsewhere in the p-type
material move to fill holes near the gate instead.
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS Transistor Channel
VGS > VTH(N)
gate
- +
source
metal
n-type
+ + +
_
_
metal
oxide insulator
ee_ e _e_ e _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
h
h
h
h
h
metal
When VGS is larger than a threshold voltage VTH(N), the
attraction to the gate is so great that free electrons collect
there.
Thus the applied VGS creates an induced n-type channel
under the gate (an area with free electrons).
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS Transistor Drain Current
- +
VGS > VTH(N)
VDS > 0
gate
- +
source
metal
n-type
+ + +
_
_
metal
oxide insulator
ee_ e _e_ e _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
h
h
h
h
h
metal
When a positive VDS is applied, the free electrons flow from the
source to the drain. (Positive current flows from drain to source).
The amount of current depends on VDS, as well as the number of
electrons in the channel, channel dimensions, and material.
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS Transistor
VDS
- +
VGS
gate
- +
source
metal
n-type
ID
IG
drain
metal
oxide insulator
metal
n-type
p-type
metal
G
IG
ID
S
-
VDS +
D
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS I-V CHARACTERISTIC
G
IG
ID
S
-
VDS +
D
• Since the transistor is a 3-terminal device, there is no
single I-V characteristic.
• Note that because of the insulator, IG = 0 A.
• We typically define the MOS I-V characteristic as
ID vs. VDS
for a fixed VGS.
EECS 40 Spring 2003 Lecture 16
S. Ross
MODES OF OPERATION
For small values of VGS, VGS ≤ VTH(N), the n-type channel is not
formed. No current flows. This is cutoff mode.
When VGS > VTH(N), current ID may flow from drain to source, and
the following modes of current flow are possible.
The mode of current flow depends on the propelling voltage, VDS,
and the channel-inducing voltage, VGS – VTH(N).
When VDS < VGS – VTH(N), current is starting to flow. ID increases
rapidly with increased VDS. This is triode mode.
When VDS ≥ VGS – VTH(N), current is reaching its maximum value.
ID does not increase much with increased VDS. This is called
saturation mode.
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS I-V CHARACTERISTIC
Cutoff Mode
• Occurs when VGS ≤ VTH(N)
ID = 0
Triode Mode
• Occurs when VGS > VTH(N) and VDS < VGS - VTH(N)
W
ID 
μnCOX VGS  VTH(N)  VDS /2 VDS
L
Saturation Mode
• Occurs when VGS > VTH(N) and VDS ≥ VGS - VTH(N)
W
1
2
ID 
μnCOX VGS  VTH(N)  1 λn VDS 
L
2
EECS 40 Spring 2003 Lecture 16
S. Ross
NMOS I-V CHARACTERISTICS
ID triode mode
saturation mode
VGS = 3 V
VDS = VGS - VTH(N)
VGS = 2 V
VGS = 1 V
VDS
cutoff mode
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS
(P-Channel Metal Oxide Semiconductor)
Transistor
gate
source
metal
p-type
metal
oxide insulator
drain
metal
p-type
n-type
metal
Same as NMOS, only p-type and n-type switched
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS Transistor Channel
VGS < VTH(P) < 0
gate
- +
source
metal
p-type
_ _ _
metal
oxide insulator
h +h+h + h + h +
+ + +
e
e
e
e
e
metal
p-type
_ _ _
+ + +
n-type
e
drain
e
e
e
e
metal
When VGS is more negative than a threshold voltage VTH(P), the
gate attracts many positive ions and holes (repels electrons)
Thus the applied VGS creates an induced p-type channel
under the gate (an area with positive ions).
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS Transistor Drain Current
- +
VGS < VTH(P) < 0
VDS < 0
gate
- +
source
metal
p-type
_ _ _
metal
oxide insulator
h +h+h + h + h +
+ + +
e
e
e
e
e
metal
p-type
_ _ _
+ + +
n-type
e
drain
e
e
e
e
metal
When a negative VDS is applied, the positive ions flow from the
source to the drain. (Positive current flows from source to drain).
The amount of current depends on VDS, as well as the number of
ions in the channel, channel dimensions, and material.
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS TRANSISTOR
G
IG
ID
S
-
VDS +
D
Symbol has “dot” at gate. NMOS does not.
ID, VGS, VDS, and VTH(P) are all negative.
These values are positive for NMOS.
Channel formed when VGS < VTH(P). Opposite for NMOS.
Saturation occurs when VDS ≤ VGS – VTH(P). Opposite for NMOS.
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS I-V CHARACTERISTIC
Cutoff Mode
• Occurs when VGS ≥ VTH(P)
ID = 0
Triode Mode
• Occurs when VGS < VTH(P) and VDS > VGS - VTH(P)
W
ID   μpCOX VGS  VTH(P)  VDS /2 VDS
L
Saturation Mode
• Occurs when VGS < VTH(P) and VDS ≤ VGS - VTH(P)
W
1
2
ID   μpCOX VGS  VTH(P)  1 λp VDS 
L
2
EECS 40 Spring 2003 Lecture 16
S. Ross
PMOS I-V CHARACTERISTICS
VDS
cutoff mode
VGS = -1 V
VGS = -2 V
VDS = VGS - VTH(P)
VGS = -3 V
ID
saturation mode
triode mode