Key points are not available for this paper at this time.
The commonly used definition of cutoff frequency f T 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 T = d/dflm (1/β) for f →0 is proposed.
H.K. Gummel (Wed,) studied this question.