On the definition of the cutoff frequency fT

The commonly used definition of cutoff frequency f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> of bipola transistors is predicated on the existence of a frequency domain in which the magnitude of the current gain β varies inversely with frequency. In the absence of such a domain, the definition is vague. The definition 1/f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> = d/df[lm (1/β)] for f →0 is proposed.