[sdiy] Transistor timing specs

[Seb Francis]

Hi,

I'm looking for a fast-ish transistor to act as a 5V logic inverter in a 
high speed circuit.  I'm a bit confused about what the various timing 
specs actually refer to.  Can anyone enlighten me about what the 
following things actually refer to...

e.g. For a 2N3904

td Delay Time VCC = 3.0 V, VBE = 0.5 V, 35 ns
tr Rise Time IC = 10 mA, IB1 = 1.0 mA 35 ns
ts Storage Time VCC = 3.0 V, IC = 10mA 200 ns
tf Fall Time IB1 = IB2 = 1.0 mA 50 ns

There are also graphs of:
Turn-On Time vs Collector Current
Rise Time vs Collector Current
(both show times get pretty slow down at 1mA collector current)

As a bonus question: any recommendations for a suitable fast NPN 
transistor.  Ideally it should have a turn on time of <80ns at *low* 
current.  Or would I be better off using a FET?

TIA,
Seb