Please help: My experiment with a transistor switch.....

I have been studying about using a transistor as a switch at:
http://www.kpsec.freeuk.com/trancirc.htm

I built a npn transistor switch on breadboard using a MPSA06
(HFE=100min, FT = 100MHz).
I have a 4.7K resistor connected to the base which I connect to a
0-10Mhz function generator that produces +5V square wave above 0V
(positive going only).
I have a 470 Ohm resistor as the load and I connect a oscilloscope to
the collector - emmiter.
The switch is powered by a 8V power supply.

I calculated the resistor values from the web site above.

I figured that as the transistor has a current gain bandwidth product
of 100MHz I would have no problem with a 10MHz signal.

Well when I get upto around 4Khz, the mark to space ratio (duty cycle)
starts to lessen and is no longer 1:1 (50% duty cycle).
When I get to 44Khz it is 1:2
Until finally at around 140-150Khz the spike is too narrow to see on
the scope.

If this is a 100Mhz transistor why is this happening?
I want to eventually build a switch running on a 25V power supply that
will track upto 10Mhz or 20Mhz using a 2SC2078 driven by the 5V square
wave signal.
The load will be high resistance( >10K ) with a 200 Ohm 5W power
resistor in series with the output in case the output is short
circuited.

.

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