As analog engineers focus on parameters such as the transition frequency of a bipolar device to estimate frequency performance of a transistor ( which is more of a small signal parameter). We sometimes get confused if we have to use a bipolar as a switching device. Especially in a saturated mode of switching. In this case the base of the bipolar is driven hard enough ( lots of base current) to drive the transistor when it is ON to store charge in its base region. The output voltage at the collector drops to its Vsat ( a few hundred millivolts usually) and when the transistor has to leave this state it has to contend with a storage time. So when we want to estimate the switching speed of a bipolar we need to use the datasheet parameters of the device that apply to the switching mode. These parameters are: td = delay time , tr = rise time, tf = fall time and ts = storage time. Then the maximum switching frequency of the bipolar is given by ( estimate), fmax=1/( td+tr+tf+ts). Note that ft, the transition frequency does not enter into this equation at all! For more technical info and services please visit our website.