what is cut off freq and break down freq in a BJT

Alpha cutoff is when alpha drops by 0.707

F sub t is the projection of the gain-frequency graph plot to the frequency where the gain is 1 (0dB).

For a discussion of cutoff frequency, look at:

http://nina.ecse.rpi.edu/shur/SDM2/N...nal/sld020.htm

For a discussion of breakdown, look at:

http://nina.ecse.rpi.edu/shur/SDM2/N...reak/Index.htm