15nS switching with transistors is a bit of a dream, esp for turn off, you will need complex base drive to achieve this - how about a high spec comparator or op-amp....?
Can you post a sketched schematic of your ideal intended circuit?
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Also the device you list is only 15V open base, you will need a good gnd plane layout and a ckt that allows B-E resistors to give you a Vcex higher than 15V, you want a 3v3 signal from a uP to generate a 25V signal to drive 100mA at 15nS rise times? (~ 16MHz) are you an RF engineer specialising in BJT circuits? the load will influence the rise time - esp if there is cabling that is not impedance matched, (~250 ohm).
An hysteretic circuit might get you there, but there will be some delay to the o/p....also I doubt if the uP o/p pin rise fall times are that quick?
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Also how often are you driving this? 8MHz? there will be a power consumption in a driver going this fast....