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<TITLE> 3.2 Transistor Parasitic Capacitance</TITLE></HEAD><!--#include file="top.html"--><!--#include file="header.html"-->
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<H1 CLASS="Heading1">
<A NAME="pgfId=160199">
</A>
3.2 <A NAME="29091">
</A>
Transistor Parasitic Capacitance</H1>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=160218">
</A>
Logic-cell delay results from transistor resistance, transistor (intrinsic) parasitic capacitance, and load (extrinsic) capacitance. When one logic cell drives another, the parasitic input capacitance of the driven cell becomes the load capacitance of the driving cell and this will determine the delay of the driving cell.</P>
<P CLASS="Body">
<A NAME="pgfId=187894">
</A>
Figure <A HREF="#17045" CLASS="XRef">
3.4</A>
shows the components of transistor parasitic capacitance. SPICE prints all of the MOS parameter values for each transistor at the DC operating point. The following values were printed by PSpice (v5.4) for the simulation of Figure <A HREF="CH03.1.htm#19386" CLASS="XRef">
3.3</A>
:</P>
<TABLE>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFigure">
<A NAME="pgfId=160227">
</A>
<IMG SRC="CH03-9.gif" ALIGN="BASELINE">
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFigureTitle">
<A NAME="pgfId=184980">
</A>
FIGURE 3.4 <A NAME="17045">
</A>
Transistor parasitic capacitance. (a) An <SPAN CLASS="EmphasisPrefix">
n</SPAN>
-channel MOS transistor with (drawn) gate length L and width W. (b) The gate capacitance is split into: the constant overlap capacitances <SPAN CLASS="EquationNumber">
C</SPAN>
<SUB CLASS="Subscript">
GSOV</SUB>
, <SPAN CLASS="EquationNumber">
C</SPAN>
<SUB CLASS="Subscript">
GDOV</SUB>
, and <SPAN CLASS="EquationNumber">
C</SPAN>
<SUB CLASS="Subscript">
GBOV </SUB>
and the variable capacitances <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
GS</SUB>
, <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
GB</SUB>
, and <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
GD</SUB>
, which depend on the operating region. (c) A view showing how the different capacitances are approximated by planar components (<SPAN CLASS="EquationNumber">
T</SPAN>
<SUB CLASS="Subscript">
FOX</SUB>
is the field-oxide thickness). (d) <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BS</SUB>
and <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BD</SUB>
are the sum of the area (<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BSJ</SUB>
, <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDJ</SUB>
), sidewall (<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BSSW</SUB>
, <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDSW</SUB>
), and channel edge (<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BSJ</SUB>
<SUB CLASS="Subscript">
GATE</SUB>
, <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDJ</SUB>
<SUB CLASS="Subscript">
GATE</SUB>
) capacitances. (e)–(f) The dimensions of the gate, overlap, and sidewall capacitances (L<SUB CLASS="Subscript">
D</SUB>
is the lateral diffusion).</P>
</TD>
</TR>
</TABLE>
<P CLASS="ComputerFirst">
<A NAME="pgfId=184354">
</A>
NAME m1 m2 <BR>
MODEL CMOSN CMOSP <BR>
ID 7.49E-11 -7.49E-11<BR>
VGS 0.00E+00 -3.00E+00<BR>
VDS 3.00E+00 -4.40E-08<BR>
VBS 0.00E+00 0.00E+00<BR>
VTH 4.14E-01 -8.96E-01<BR>
VDSAT 3.51E-02 -1.78E+00<BR>
GM 1.75E-09 2.52E-11<BR>
GDS 1.24E-10 1.72E-03<BR>
GMB 6.02E-10 7.02E-12<BR>
CBD 2.06E-15 1.71E-14<BR>
CBS 4.45E-15 1.71E-14<BR>
CGSOV 1.80E-15 2.88E-15<BR>
CGDOV 1.80E-15 2.88E-15<BR>
CGBOV 2.00E-16 2.01E-16<BR>
CGS 0.00E+00 1.10E-14<BR>
CGD 0.00E+00 1.10E-14<BR>
CGB 3.88E-15 0.00E+00</P>
<P CLASS="Body">
<A NAME="pgfId=170326">
</A>
The parameters <SPAN CLASS="BodyComputer">
ID</SPAN>
(<SPAN CLASS="EquationVariables">
I</SPAN>
<SUB CLASS="SubscriptVariable">
DS</SUB>
), <SPAN CLASS="BodyComputer">
VGS</SPAN>
, <SPAN CLASS="BodyComputer">
VDS</SPAN>
, <SPAN CLASS="BodyComputer">
VBS</SPAN>
, <SPAN CLASS="BodyComputer">
VTH</SPAN>
<A NAME="marker=171090">
</A>
(V<SUB CLASS="Subscript">
t</SUB>
), and <SPAN CLASS="BodyComputer">
VDSAT</SPAN>
<A NAME="marker=171089">
</A>
(V<SUB CLASS="SubscriptVariable">
DS</SUB>
<SUB CLASS="Subscript">
(sat)</SUB>
) are DC parameters. The parameters <SPAN CLASS="BodyComputer">
GM</SPAN>
<A NAME="marker=171086">
</A>
, <SPAN CLASS="BodyComputer">
GDS</SPAN>
<A NAME="marker=171087">
</A>
, and <SPAN CLASS="BodyComputer">
GMB</SPAN>
<A NAME="marker=171088">
</A>
are small-signal conductances (corresponding to ∂<SPAN CLASS="EquationVariables">
I</SPAN>
<SUB CLASS="SubscriptVariable">
DS</SUB>
/∂<SPAN CLASS="EquationVariables">
V</SPAN>
<SUB CLASS="SubscriptVariable">
GS</SUB>
, ∂<SPAN CLASS="EquationVariables">
I</SPAN>
<SUB CLASS="SubscriptVariable">
DS</SUB>
/∂<SPAN CLASS="EquationVariables">
V</SPAN>
<SUB CLASS="SubscriptVariable">
DS</SUB>
, and ∂<SPAN CLASS="EquationVariables">
I</SPAN>
<SUB CLASS="SubscriptVariable">
DS</SUB>
/∂<SPAN CLASS="EquationVariables">
V</SPAN>
<SUB CLASS="SubscriptVariable">
BS</SUB>
, respectively). The remaining parameters are the parasitic capacitances. Table <A HREF="#38143" CLASS="XRef">
3.1</A>
shows the calculation of these capacitance values for the <SPAN CLASS="EmphasisPrefix">
n</SPAN>
-channel transistor <SPAN CLASS="BodyComputer">
m1</SPAN>
(with W<SPAN CLASS="Symbol">
</SPAN>
=<SPAN CLASS="Symbol">
</SPAN>
6<SPAN CLASS="Symbol">
m</SPAN>
m and L<SPAN CLASS="Symbol">
</SPAN>
=<SPAN CLASS="Symbol">
</SPAN>
0.6<SPAN CLASS="Symbol">
m</SPAN>
m) in Figure <A HREF="CH03.1.htm#19386" CLASS="XRef">
3.3</A>
(a). </P>
<TABLE>
<TR>
<TD ROWSPAN="1" COLSPAN="3">
<P CLASS="TableTitle">
<A NAME="pgfId=186243">
</A>
TABLE 3.1 <A NAME="38143">
</A>
Calculations of parasitic capacitances for an n-channel MOS transistor.</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186249">
</A>
<SPAN CLASS="TableHeads">
PSpice</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186251">
</A>
<SPAN CLASS="TableHeads">
Equation</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186253">
</A>
<SPAN CLASS="TableHeads">
Values</SPAN>
<SUP CLASS="Superscript">
1</SUP>
<SPAN CLASS="TableHeads">
for VGS</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
=</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
0V, VDS</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
=</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
3V, VSB</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
=</SPAN>
<SPAN CLASS="Symbol">
</SPAN>
<SPAN CLASS="TableHeads">
0V</SPAN>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186255">
</A>
<SPAN CLASS="BodyComputer">
CBD</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=225741">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BD</SUB>
= <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDJ</SUB>
+ <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDSW</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226768">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BD</SUB>
= 1.855 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–13</SUP>
+ 2.04 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–16</SUP>
= 2.06 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–13</SUP>
F</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186267">
</A>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226767">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDJ</SUB>
+ A<SUB CLASS="Subscript">
D</SUB>
C<SUB CLASS="Subscript">
J</SUB>
( 1 + <SPAN CLASS="EquationVariables">
V</SPAN>
<SUB CLASS="SubscriptVariable">
DB</SUB>
/<SPAN CLASS="Symbol">
f</SPAN>
<SUB CLASS="Subscript">
B</SUB>
)<SUP CLASS="Superscript">
–mJ</SUP>
(<SPAN CLASS="Symbol">
f</SPAN>
<SUB CLASS="Subscript">
B</SUB>
= <SPAN CLASS="BodyComputer">
PB</SPAN>
)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226849">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDJ</SUB>
= (4.032 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–15</SUP>
)(1 + (3/1))<SUP CLASS="Superscript">
–0.56</SUP>
= 1.86 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–15</SUP>
F</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186279">
</A>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226940">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDSW</SUB>
= P<SUB CLASS="Subscript">
D</SUB>
C<SUB CLASS="Subscript">
JSW</SUB>
(1 + <SPAN CLASS="EquationVariables">
V</SPAN>
<SUB CLASS="SubscriptVariable">
DB</SUB>
/<SPAN CLASS="Symbol">
f</SPAN>
<SUB CLASS="Subscript">
B</SUB>
)<SUP CLASS="Superscript">
–mJSW</SUP>
</P>
<P CLASS="TableEqn">
<A NAME="pgfId=317243">
</A>
(P<SUB CLASS="Subscript">
D</SUB>
may or may not include channel edge)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226852">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BDSW</SUB>
= (4.2 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–16</SUP>
)(1 + (3/1))<SUP CLASS="Superscript">
–0.5</SUP>
= 2.04 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–16</SUP>
F</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=186291">
</A>
<SPAN CLASS="BodyComputer">
CBS</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=226968">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BS</SUB>
= <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BSJ</SUB>
+ <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BSSW</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqn">
<A NAME="pgfId=227065">
</A>
<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
BS</SUB>
= 4.032 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
–15</SUP>
+ 4.2 <SPAN CLASS="Symbol">
¥</SPAN>
10<SUP CLASS="Superscript">
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