Meaning of "pulse duration" in transistor datasheets?

Huh, it never occurred to me that anyone would interpret it as (a).

I believe it is (b). 300us tells you the on-time, 1% implies the period (how frequent you can do it).

This is because it says pulse DURATION, and a pulse exists only while it is on.

It means that it's on for \$300\mu \mathrm{s}\$, because anything more would fry the transistor. Then off for a Good Long While to let the transistor cool down.

[example of Finite Element Modeling of thermal flows, at end of answer, for 10 micron mesh]

The thermal timeconstant of 1 micron cube of silicon is 11.4 nanoseconds.

The thermal timeconstant of 10 micron cube of silicon is 100X slower, at 1.14 microSeconds.

The thermal timeconstant of 100 micron cube is another 100X slower, at 114 microseconds.

Thus a 300uS pulse should be adequate to cause substantial heating, hopefully not to failure, of power silicon devices that are back-grinded to be 100 micron thick.

By the way, the thermal timeconstant of a cubic meter of silicon is 11,400 seconds.

^{simulate this circuit – Schematic created using CircuitLab}