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The exact definition of transistion frequency is frequency at which the current gain is equal to 1. So you put a current source between base and emittor, and a shortcut between collector and emittor.
1. ft cannot be measured (frequencies are too high to be able to define well controlled current source). It is defined through interpolation.
2. most of the time the transistor operating frequency is a fraction of the ft. (like 1/5)
3. this is just a characteristic of the die. Bond wires introduce parasitics that lower the actual ft.
So ft is just a so called "figure of merit" to compare different transistors. It does not have any direct use, because at the frequency ft, parasitics dominate transistor performance! When designing chips, ft has a clearer meaning and can be simulated!
hi,
the answer is yes,one problem in switching transistor are parasitic capacitance,higher ft => lower parasitic capacitance.I think you know what happen with pulse when go through RC circuit.
The transition frequency in MOS is defined completely the same way. It is correct that de DC input resistance is very high. But at higher frequencies, the input capacitance becomes dominant. So, the input voltage get smaller as frequency rises with a constant current source on the input. So yes, the same definition goes for MOSFETS!
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