The relationship of Tf ( forward transit time) and ft in a bipolar transistor model

Someone recently asked how the ft of a bipolar is related to tf, the forward transit time of the bipolar. The tf is a model parameter while ft is not. Yet we always talk about the ft of the transistor. The answer to this question can be found in the spg website ( www.signalpro.biz) under engineering pages>engineer's corner for interested parties.