1. CAPACITANCE ESTIMATION
Area capacitances of layers
Standard unit of capacitance □Cg
Some area capacitance calculations

2. Area capacitances of layers
There are many layers and thus forms parallel plat capacitance effect
Dielectric thickness , we can calculate area capacitance as follows:
C=εo ε ins A farads D
D= thickness of the dioxide in cm
A = area of the plate in cm2
εo = permittivity of free space-8.854x10-14f/cm
ε ins = relative permitivity of sio2=4.0

3. Typical values of area capacitance are set out in table for 5µm technology
Value in pF*10-4/µm2(relative values in brackets)
Gate to channel
4 (1.0)
Diffusion
1 (.25)
Poly to sub
0.4 (.1)
M1 to sub
0.3 (0.075)
M2 to sub
0.2 (0.05)
M2 to M1
0.4 (0.1)
M2 to poly
0.3 (0.075)
Relative value = specified value / gate to channel value for that technology

4. Standard unit of capacitance □Cg
The unit □Cg is defined as gate-to-channel capacitance of a MOS transistor having W=L=feature size
□Cg was evaluated for any MOS, for example 5µm MOS circuits:
Area/standard square = 5µm X 5µm = ?
Capacitance value (from table) = 4 X 10-4 pF/µm2
Thus , standard value □Cg = 25µm2 X 4 X 10-4 pF/µm2 = ?

5. Some area capacitance calculations
W=3λ
area capacitance calculated by relative to that of a standard gate
Consider the area in metal 1
Capacitance to substrate = relative area X relative C value
=1.125 □Cg
Consider the same area in polysilicon = ?
Consider the same area in n-type diffusion = ?

6. Total capacitance CT=Cm+Cp+Cg =7.20 □Cg
3 λ 4λ
Metal 1λ
2 λ
Diffusion
2 λ
Polysilicon
Total capacitance CT=Cm+Cp+Cg =7.20 □Cg

7. Total capacitance CT=Cm+Cp+Cg = Total capacitance CT=Cm+Cp+Cg = 7 □Cg
Metal 1
|< λ > I
<- 4 λ -> I
< λ > I
3 λ 4λ 1λ
2 λ
Diffusion
2 λ
Polysilicon
Total capacitance CT=Cm+Cp+Cg =
Total capacitance CT=Cm+Cp+Cg = 7 □Cg