ECE 440: Lecture 34
MOS Field-Effect Transistor (FET)
The MOSFET is just the MOS capacitor with Source/Drain
terminals added in.
GATE LENGTH, Lg
OXIDE THICKNESS, Tox
Gate
Source
Drain
Substrate
JUNCTION DEPTH, Xj
How does it work?
 Gate electrode voltage (VGS) controls mobile charge
sheet under ________________
 Voltage between Source and Drain (VDS) sweeps the
mobile charges away, creating ______________ (ID)
Desired characteristics (remember water faucet analogy):
 “On” current ___________
 “Off” current ___________
Illinois ECE440
Prof. Eric Pop
1
Typical 2-D cross-section view of NMOSFET:
Note direction of carrier flow, and of current flow.
Gate voltage (VG) controls Source-to-Drain current (ID).
“Source” terminal refers to source of ______________
NMOS
PMOS
N+ poly-Si
P+ poly-Si
N+
P+
N+
P+
n-type Si
P-type Si
For current flow:
Illinois ECE440
Prof. Eric Pop
2
Simplest CMOS circuit is the inverter:
CIRCUIT SYMBOLS
N-channel
MOSFET
P-channel
MOSFET
CMOS INVERTER CIRCUIT
VDD
VOUT
S
INVERTER
LOGIC SYMBOL
VDD
D
VIN
D
VOUT
VIN
S
GND
0
VDD
Note the PMOS and NMOS gates are tied together to VIN.
So are the drains, to VOUT.
When VIN = 0 then NMOS is _____ and PMOS is _____
When VIN = VDD then NMOS is _____ and PMOS is _____
Illinois ECE440
Prof. Eric Pop
3
Theory of MOSFET:
 When VGS < VT the channel is ______________
depletion layer
 When VGS > VT the channel is ______________
 If small drain voltage (VD > 0) is applied _______________
Will charge sheet move by drift or diffusion?
Current ≈ width × charge sheet × velocity
What is the inversion charge: |Qinv| ≈
What is the velocity: v ≈
Illinois ECE440
Prof. Eric Pop
4
At low VDS inversion layer essentially acts like a resistor!
I DS  WQinvv  WCox VG  VT eff 
 VDS 


V


G
T
eff 
L


 WC V
ox
What about higher drain voltages VDS?
Must take into account variation of potential along channel,
0 < Vx < VDS. So inversion layer charge at any point is
|Qinv(x)| = Cox(VGS – VT – Vx)
And the current is:
IDS,lin =
VDS 
W

  eff Cox VGS  VT 
VDS
L
2 

Still linear in voltage! This is the linear region.
 When VDS = VGS – VT the channel becomes _____________
Illinois ECE440
Prof. Eric Pop
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 When VDS > VGS - VT the un-inverted (drain depletion)
region increases, as does the ____________________
Any increase in VDS:
 Reduces the amount of inversion charge, but…
 Increases the lateral field (charge velocity)
The two effects cancel each other out, so at high VDS the
drain current is no longer a function of VDS! The current
saturates to a value only dependent on VGS (i.e. charge).
Putting in VDS = VGS – VT (the pinch-off, i.e. saturation
condition) in the previous equation:
I DS , sat
Illinois ECE440
1W

eff Cox (VGS  VT ) 2
2 L
Prof. Eric Pop
6
Plot and label an example NMOSFET:
What about IDS vs. gate voltage (VGS)?
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Prof. Eric Pop
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What is the “effective mobility” μeff in the channel?
Can we go look it up in the bulk-silicon charts?
(Vgs + V t + 0.2)/6Toxe (MV/cm)
(NFET)
(PFET)
–(Vgs + 1.5V t – 0.25)/6Tox e (MV/cm)
Scattering mechanisms affecting mobility in channel:
 Charged impurity (Coulomb) scattering
 Lattice vibration (phonon) scattering
 Surface roughness scattering
Illinois ECE440
Prof. Eric Pop
8