capacitor using CMOS transistors ?

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sarathdhulipalla

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Is it possible to fabricate a component having the functionality of a capacitor using CMOS transistors ?
 

Normally, if a process is a dual poly process one goes in for
dual poly caps. But most processes are single poly processes.

It is a common practise to use the MOS cap a capacitor
| G
--------
D----| | |----S
|------|-------| VSS

Hope the illustration makes sense......it is common to choose a NMOS device over PMOS for such capacitors.

Regards,
 

Hey, I guess the illustration is all screwed up....well one terminal is the gate....and connect drain & source to bulk connected to VSS forms the other terminal (ofcourse - u now have a problem, a capacitance is formed only between a node a ground)

Regards,
 

the gate oxide of the cmos is a cap, remember C=e/tox

for nmos, tie the gate to vdd and the D/S to ground
for pmos, tie the gate to vss and the D/S to vdd
 
If you have control over the layout, you can get a better capacitor by placing the nmos transistor inside an NWell so that the bottom plate is all N type beneath the poly. Similarly for dual well processes put the pmos transistor inside a PWell
 
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