Lecture : Frequency Response

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Lecture Frequency Response

• Adapted from Prof. Brodersens slides

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MOS Intrinsic Capacitances

• Structure capacitances
• Channel capacitances

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MOS Structure Capacitances
lateral diffusion
Poly Gate
Source n
Drain n
xd
xd
Top view
W
Ldrawn
tox
n
Leff
4
MOS Channel Capacitances

• The gate-to-channel capacitance depends upon the
  operating region and the terminal voltages

CGS CGCS CGSO
CGD CGCD CGDO
G
D
S
n
CGB CGCB
p substrate
B
5
Average Distribution of Channel Capacitance

• Channel capacitance components are nonlinear and
  vary with operating voltage
• Most important regions are cutoff and saturation
  since that is where the device spends most of its
  time

6
MOS Diffusion Capacitances

• The junction (or diffusion) capacitance is from
  the reverse-biased source-body and drain-body
  pn-junctions.

G
D
S
n
p substrate
CSB CSdiff
CDB CDdiff
B
7
Source Junction View
channel-stop implant (NA)
W
source bottom plate (ND)
xj
junction depth
channel
substrate (NA)
side walls
LS
Cdiff Cbp Csw Cj AREA Cjsw PERIMETER
Cj LS W Cjsw (2LS W)
8
MOS Capacitance Model
G
CGS CGCS CGSO
CGD CGCD CGDO
CGS
CGD
S
D
CSB
CGB
CDB
B
CSB CSdiff
CDB CDdiff
CGB CGCB
9
Layout of Two Chained Inverters
0.125
0.5
10
Components of CL (0.25 ?m)
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