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Transit frequency of a MOSFET

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sharas

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transit frequency

Hi,
In Razavi, Problem 2.13 (page 40):
" The transit frequency of a MOSFET is defined as the frequency at which the small-signal current gain of the device drops to unity while the source and drain terminals are held at ground".
How can there be current in the transistor when Vsource=Vdrain=0V?
As far as I know, this is defined as triode region with 0 current. How can there be current gain?

Thanks in advance for your help.
 

transit frequency mosfet

And another question: What's the importance of this frequency? How should it be taken into account when designing a circuit?
 

transit frequency

razavi meant that the drain and source are at signal ground. (something like connecting huge caps from drain to ground). It is assumed that the transistor has been DC biased.

thats why he says "small-signal current gain".
 
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transition frequency mosfet

THis is the frequency where the transistor can provide current gain. And it's also related to how fast the transistor can be operated.
 
transition frequency of mosfet

Thanks for the answers!
 

the transit frequency gives the intrinsic (maximum) speed of the mosfet;
it also means that beyond that frequency the device cant be used as a voltage amplifier;;
 
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