Hi there,
Every transistor has a rise time (Tr) and a fall time (Tf). You cannot use a signal with a frequency that has a period (frequency = 1/period) smaller than the sum of these times. The datasheet usually give directly the frequency(Hz) the transistor can operate on.
To answer your question 20ns period = 50Mhz frequency.
your question is not clear.
1)switching is dependent to f and power is dependent to f ,maybe your transistor can't do it because switching has losses thus power is an important factor because transistor has a max power can bear.
2)the datasheet give a transfer frequency(Ft) and means that this frequncy is the max frequncy that transistor can act as amplifier but in practical circuit this is true for Ft/3
Actually I want to modulate the data on laser and for that I need a fast switching transistor. So I wanna know how many number of bits I can transfer by the switching of transistor if it has total time period of 20ns...I know the frequency I get is 50MHz...but is the bit rate 50Mbits/s