Oscilloscope bandwidth and PWM pulse width
Your scope should be able to reproduce rise and fall times down to around 3.5ns.
There is a relationship between the rise time of an edge and the signal bandwidth. The bandwidth of a square wave does not depend on the square wave frequency. It will always be higher than the frequency. The exact bandwidth in the signal will depend on the rise and fall time of the square wave.
What this means for an oscilloscope is that fast square waves may not be reproduced correctly. If the scope bandwidth is too low, the apparent rise and fall times on the scope will longer (slower) than the actual rise and fall times.
Here is the approximate relationship:
BW = 0.35 / Tr
Where BW is the bandwidth and Tr is the rise time from 10 to 90 percent. Solving for Tr you get:
Tr = 0.35 / BW
So for you, Tr = 0.35/100MHz = 3.5ns.
The scope probe can also limit rise times, but in your case you are using a 150 MHz probe, so it should be OK.
The last thing to double-check is the probe compensation. An under-compensated probe will cause the rise times to appear slower than they actually are.
If the probe compensation is OK, then most likely, the signal you are seeing on the scope is the real signal. Some RC time constant in the actual circuit may be limiting the rise and fall times.