# Measurement of Cmos Parasitic capacitors

i've had a question which asks if we assume that capacitance of capacitors with w/l of 1u/0.5 are then what are capacitance of capacitors (with the actual w/l)

now i know cgs in saturation for example is cgs0+2/3Cox(w)(L) but wouldn't Cgs0 change with new size ? if so how much ? and what about csb ? isnt it zero ? also in other questions i've seen asking capacitance of Cgb or Cgd too what about measuring them ?

• are you using discrete components, or is this for an IC design? In the first case, components have datasheets that list capacitance over voltage (example), in the second case, this is something that you typically measure using a test wafer containing the cells you plan to use. – Marcus Müller Jan 5 at 10:33
• From a theoretical point of view, you're right, the capacitance changes, as a function of the distance between carriers / conductors. But: involving gate, this is something that depends on the geometry of your transistor, so it becomes impossible to calculate manually, typically. – Marcus Müller Jan 5 at 10:34
• @MarcusMüller this is when we are assuming an IC design but im preparing for exam and this is all that i have : capacitance of capacitors with certain w & L and then their actual capacitance with their real size (W) & (L) – Pooya Estakhri Jan 5 at 12:31
• then that's a "depends on the geometry, can't be generally answered", or it depends on some assumptions that we don't know. – Marcus Müller Jan 5 at 12:37

## 1 Answer

Some capacitances are (roughly) proportional to the gate width, some to the gate length, and some to the gate area. The question is asking you to figure out how $$\C_{gs}\$$ and $$\C_{sb}\$$ will change as the physical dimensions change.

Basically, the question is asking whether you know what these capacitances really mean and where they exist in the physical device.