mos fetch 7 intro
TRANSCRIPT
Introduction toMetal-Oxide-
SemiconductorField Effect Transistors
(MOSFETs)
Chapter 7, Anderson and Anderson
MOSFET• History• Structure• Future• Review• Threshold Voltage• I-V Characteristics• Modifications to I-V:
– Depletion layer correction (Sup. 3)– Mobility, Vsat– Short Channel Effects– Channel Length Modulation– Channel Quantum Effects
• MOSFET Scaling and Current Topics (Literature + Sup. 3)• Subthreshold Behavior• Damage and Temperature (Sup. 3)• Spice (Sup. 3)• HFET, MESFET, JFET, DRAM, CCD (Some in Sup. 3)
MOSFET History (Very Short!)• First Patents:
– 1935• Variable Capacitor Proposed:
– 1959• Silicon MOS:
– 1960• Clean PMOS, NMOS:
– Late 1960s, big growth!• CCDs:
– 1970s, Bell Labs• Switch to CMOS:
– 1980s
Structure: n-channel MOSFET(NMOS)
pn+n+
metal
LW
sourceS
gate: metal or heavily doped poly-Si G drain
Dbody
B
oxide
IG=0
ID=ISIS
x
y
(bulk or substrate)
MOSFET Future (One Part of)
• International Technology Roadmap for Semiconductors, 2008 update.
• Look at size, manufacturing technique.
From Intel
Structure: n-channel MOSFET(NMOS)
pn+n+
metal
LW
sourceS
gate: metal or heavily doped poly-Si G drain
Dbody
B
oxide
IG=0
ID=ISIS
x
y
(bulk or substrate)
MOSFET ScalingECE G201
Fin (30nm)
Gate
BOX
prevents “top” gate
Circuit Symbol (NMOS)enhancement-type: no channel at zero gate voltage
G
D
S
B (IB=0, should be reverse biased)
ID= IS
IS
IG= 0
G-GateD-DrainS-SourceB-Substrate or Body
Structure: n-channel MOSFET(NMOS)
pn+n+
metal
LW
sourceS
gate: metal or heavily doped poly-Si G drain
Dbody
B
oxide
IG=0
ID=ISIS
x
y
(bulk or substrate)
Energy bands
(“flat band” condition; not equilibrium) (equilibrium)
Flatbands! For this choice of materials, VGS<0 n+pn+ structure ID ~ 0
pn+n+
n++
LW
sourceS
gateG drain
Dbody
B
oxide
+-
VD=Vs
Flatbands < VGS < VT (Includes VGS=0 here). n+-depletion-n+ structure ID ~ 0
pn+n+
n++
LW
sourceS
gateG drain
Dbody
B
oxide
+-
+++
VD=Vs
VGS > VT n+-n-n+ structure inversion
pn+n+
n++
LW
sourceS
gateG drain
Dbody
B
oxide
+-
+++++++++
- - - - -
VD=Vs
VGS>VT
Channel Charge (Qch)
Qch
Depletion regioncharge (QB) is dueto uncovered acceptor ions
pn+n+
n++
L W
Ec(y) with VDS=0
(x)
Increasing VGS decreases EB
EB
y0 L
EF ~ EC
Triode Region A voltage-controlled resistor @small VDS
G
pn+n+
metal
S DB
oxide
+-
++++++
- - - -
VGS1>Vt
pn+n+
metal
S DB
oxide
+-
++++++
+++
- - - - - -
VGS2>VGS1
pn+n+
metal
S DB
oxide
+-
++++++
+++
- - - - - - - - -
VGS3>VGS2+++
ID
VDS
0.1 v
increasingVGS
Increasing VGS puts more charge in the channel, allowing more drain current to flow
cut-off
Saturation Regionoccurs at large VDS
pn+n+
metal
sourceS
gateG drain
Dbody
B
oxide
+-
+++++++++
VDS large
As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off
Saturation Regionoccurs at large VDS
pn+n+
metal
sourceS
gateG drain
Dbody
B
oxide
+-
+++++++++
VDS large
The saturation region is when the MOSFET experiences pinch-off.
Pinch-off occurs when VG - VD is less than VT.
Saturation Regionoccurs at large VDS
pn+n+
metal
sourceS
gateG drain
Dbody
B
oxide
+-
+++++++++
VD>>Vs
VGS - VDS < VT or VGD <
VDS > VGS - VT
VT
Saturation Regiononce pinch-off occurs, there is no further increase in
drain currentID
VDS
0.1 v
increasingVGS
triode
saturation
VDS>VGS-VT
VDS<VGS-VT
Band diagram of triode and saturation
Simplified MOSFET I-V EquationsCut-off: VGS< VT
ID = IS = 0Triode: VGS>VT and VDS < VGS-VT
ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS
2]Saturation: VGS>VT and VDS > VGS-VT
ID = 1/2kn’(W/L)(VGS-VT)2
where kn’= (electron mobility)x(gate capacitance)
= n(ox/tox) …electron velocity = nE
and VT depends on the doping concentration and gate material used (…more details later)
Energy bands
(“flat band” condition; not equilibrium) (equilibrium)
VGS>VT
Channel Charge (Qch)
Qch
Depletion regioncharge (QB) is dueto uncovered acceptor ions
Threshold Voltage Definition
VGS = VT when the carrier concentration in the channel is equal to the carrier concentration in the bulk silicon.
Mathematically, this occurs when s=2f ,
where s is called the surface potential