MOSFET cutoff frequency question

mosfet cutoff frequency

For a 0.18um and 0.8um technology MOSFET, which has a higher cutoff frequency?

Thx

cutoff frequency mosfet

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IanP

mos cutoff frequency

0.18um has higher cutoff frequency for sure.

The narrower or shorter the channel gets, the lower the parasitic capacitance and channel resistance. Not just cutoff frequency, the maximum bandwidth ft of MOS is limited by these two factors. Another factor is gm but that is determined by other two parameters namely the W and L.

If this sounds too complicated, think about 8um is a long channel device, which you don't get velocity saturation. But 0.18um is a narrow channel device, much shorter than a short channel device between 0.5 to 0.25um, it already hits velocity saturation even before the electrons get to leave from the source to the channel!
What does velocity saturation tells you? That's the maximum velocity an electron can get!