I'm simulating this circuit in LTSpice:
The input is a 1 Mhz square wave with 10 ns rise and fall times. Plotted are the input, output as well as transistor base voltage.
Now here is what happens if we replace the Schottky by a 1N4148 silicon diode model:
It is substantially nicer! The rise time is much more steep, leading to a nice flat top. Furthermore, the off voltage is closer to zero. This niceness is also more robust against component changes.
If the simulation were realistic, why would we ever use Schottky clamps, so it can't be right.
What is wrong in the simulation?
Is it that this type of Schottky is not appropriate for the circuit (being a large-ish rectifier diode?) Or something else?
Update: Indeed, the issue with the rise time is simply that the Schottky model is a heavyweight. Scrolling through LTSpice's list of available Schottky diode models, I found one BAT48WJ. The results with this diode look a lot more reasonable in terms of rise times:
Still, the question remains is the result with the 1N4148 realistic?