Well I tried to get to get 500KHz out of a PUTJ 2N6027 and only got up to 211KHz and that was with 3 10pf caps in series. Scope shot attached,
I also said that if you charge a cap with a constant current you would get a flat ramp instead of the rc time constant ramp, not so sure now, I know I have in the past but I couldn't do it today. I put a resistor in the circuit and measured the amps and took a picture of it, then replaced the resistor with a 3 terminal current regulator LM334 and adjusted it to the same current and took another picture, put the on the same picture, attached.
Kinarfi