Transcript Metal-Oxide-Semiconductor Fields Effect Transistors (MOSFETs)

Introduction to
Metal-Oxide-Semiconductor
Field Effect Transistors
(MOSFETs)
Chapter 7, Anderson and Anderson
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)
body
(bulk or
B
substrate)
source
S
y
gate: metal or heavily doped poly-Si
G
drain
IG=0
D
ID=IS
IS
metal
oxide
n+
n+
p
x
L
W
MOSFET Future (One Part of)
• International Technology Roadmap for
Semiconductors, 2006 update.
• Look at size, manufacturing technique.
MOSFET Scaling
ECE G201
Gate
prevents “top” gate
Fin (30nm)
BOX
Circuit Symbol (NMOS)
enhancement-type: no channel at zero gate voltage
D
ID= IS
G
B
(IB=0, should be reverse biased)
IG= 0
IS
S
G-Gate
D-Drain
S-Source
B-Substrate or Body
Energy bands
(“flat band” condition; not equilibrium)
(equilibrium)
Flatbands! For this choice of materials, VGS<0
n+pn+ structure  ID ~ 0
body
B
source
S
gate
G
- +
drain
D
VD=Vs
n++
oxide
n+
n+
p
L
W
Flatbands < VGS < VT (Includes VGS=0 here).
n+-depletion-n+ structure  ID ~ 0
body
B
source
S
gate
G
- +
drain
D
VD=Vs
+++
n++
oxide
n+
n+
p
L
W
VGS > VT
n+-n-n+ structure  inversion
body
B
source
S
n+
gate
G
- +
+++
+++
+++
n++
oxide
----p
L
drain
D
VD=Vs
n+
W
Channel Charge (Qch)
VGS>VT
Depletion region
charge (QB) is due
to uncovered acceptor ions
Qch
n++
n+
p
L
n+
W
(x)
Ec(y) with VDS=0
Increasing VGS decreases EB
EB
EF ~ EC
0
L
y
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 fs=2ff ,
where fs is called the
surface potential
Triode Region
A voltage-controlled resistor @small VDS
B
S
D
- +
+++
+++
metal
- oxide
- - -
n+
VGS1>Vt
ID
increasing
VGS
n+
p
B
S
D
-+
+++
+++
+++
metal
- -oxide
- - --
n+
VGS2>VGS1
G
n+
p
cut-off
B
S
n+
D
-+
+++
+++
+++ +++
metal
- - -oxide
-----p
VDS
0.1 v
VGS3>VGS2
n+
Increasing VGS puts more
charge in the channel, allowing
more drain current to flow
Saturation Region
occurs at large VDS
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
body
B
source
S
gate
G
-
+
drain
D
VD>>Vs
+++
+++
+++
metal
oxide
n+
n+
p
Saturation Region
occurs at large VDS
The saturation region is when the MOSFET
experiences pinch-off.
Pinch-off occurs when VG - VD is less than VT.
body
B
source
S
gate
G
-
+
drain
D
VD>>Vs
+++
+++
+++
metal
oxide
n+
n+
p
Saturation Region
occurs at large VDS
VGS - VDS < VT
VDS > VGS - VT
body
B
source
S
gate
G
-
+
drain
D
VD>>Vs
+++
+++
+++
metal
oxide
n+
n+
p
Saturation Region
once pinch-off occurs, there is no further increase in
drain current
saturation
ID
triode
increasing
VGS
VDS>VGS-VT
VDS<VGS-VT
VDS
0.1 v
Band diagram of triode and saturation
Simplified MOSFET I-V Equations
Cut-off: VGS< VT
ID = I S = 0
Triode: VGS>VT and VDS < VGS-VT
ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS2]
Saturation: VGS>VT and VDS > VGS-VT
ID = 1/2kn’(W/L)(VGS-VT)2
where kn’= (electron mobility)x(gate capacitance)
= mn(eox/tox) …electron velocity = mnE
and VT depends on the doping concentration and
gate material used (…more details later)