The MOSFET
Metal Oxide Semiconductor Field Effect Transistor
• A practical reality in 1960’s
• Simpler manufacturing process
• For digital circuits the benefits are
–
–
–
–
MOSFET (also known as IGFET)
Enhancement mode N-channel MOSFET (NMOSFET)
L
Low power
High density
Easier fabrication
Low cost
W
G
• Digital and analog circuits can be easily
implemented
• Preferred technology for mixed analog/digital
systems is CMOS (Complementary MOS)
MOSFET Symbols
N-channel
FETs
P-channel
FETs
M1
S
D
N+
N+
reverse
biased
Cox
P-type
Substrate
(Body)
B
MOSFET Operation Simplified (Fluid Dynamics Analog)
VT = turn-on Voltage (threshold voltage)
1) Cut off: VGS < VT, VDS = 0
G
S
D
N+
N+
Source
Tank
VGS
Gate
VT
M1
2) Strong Inversion: VGS > VT, VDS = 0
G
M1
S
D
N+
N+
M1
If bulk is connected to proper power supply:
M1
B at lowest
chip potential
M1
B at highest
chip potential
inverted polarity (substrate p-type,
surface n-type => inversion
VGS
VT
Drain
Tank
MOSFET Current Voltage Relationship
3) Linear or Triode: VGS > VT , 0 < VDS < VGS - VT
IDS
VGS VT
0
Cutoff
K'=uCox
IDS
K'
2
º
VDS
Wª
V
V
V
T
DS
« GS
» VGS t VT , VDS VGS - VT
2 ¼
L ¬
Linear, triode, non-saturation
4) Saturation: VGS > VT , VDS > VGS - VT
IDS
K' W
VGS VT 2
2 L
VGS t VT , VDS t VGS - VT
Saturation
W, L are device geometry parameters
Flow becomes independent of VDS
N-channel device I-V Characteristics
Model Parameter Names in SPICE
VGS3
VGS-VT
Parameter
SPICE Param
Units
K’
KP
A/V2
VTO
VTO
V
ID
M1
+
VGS
-
+
VDS
ID
Nonsaturation
Saturation
-
VGS2
VGS1
VDS = -1/O
VDS
MOSFET Operation (VGS < VT)
P-channel device I-V Characteristics
IDS
VDS
VDS = 0, VGS > 0 but < VT
VGS=-1.5V
VGS > 0 Ÿ + charge on gate
- charge in substrate
S
VGS +
G
VGS=-2.5
Saturation
VGS=-3.5
Nonsaturation
|VGS|- |VT|
M1
D
Electric field repels holes
leaving ionized acceptor
atoms Ÿ depletion region
IDS
Drain Current for VDS > 0 but Small
MOSFET Operation (VGS | VT)
The gate voltage required to produce the inversion layer is called the
threshold voltage VT
VGS !! VT
VDS = 0, VGS increasing and | VT
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Depletion region becomes
deeper; almost as deep as
that under the N+ regions
Ÿ Surface is as attractive for
electrons as N+ regions
Ÿ Electrons can enter from
the N+ regions
The induced n-region under the gate is of n-type
Ÿ Surface is inverted and the electron layer is called the
inversion layer
The inversion layer forms a channel for current flow when VDS > 0
VDS ! 0 | 0
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N+
ID
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R channel
ID
N+
1
L
nCox (VGS VT ) W
VDS
R channel
nCox
W
(VGS VT )VDS
L
k'
W
(VGS VT )VDS
L
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