why is the transition frequency defined the way it is?

[recent_circuits_grad]

transition frequency in circuits textbooks is defined as the frequency at which the short circuit current gain is unity.

how does this definition relate to its physical meaning, lets say for a MOSFET, the frequency at which the inversion channel charge in the MOSFET is not able to respond to it.

any ideas?

thanks!

 

[alperuslu]

Its physical meaning is that you can not use active device above the transition frequency . Above transition frequency active devivice behaves like passive.