Bipolar transistor. Unknown graphs
The thermal timeconstant for silicon of size 1 micron (about the junction depth for fast low-voltage bipolars) is 11.4 nanoseconds.

The thermal timeconstant for silicon of size 10 microns (about the junction depth for high-voltage bipolars) is 1.14 microSeconds.

Pulses longer than 1.14 microSeconds will cause serious heating problems, because the thermal mass will not absorb the energy, and the heat generated in the collector will flow a few microns into the emitter-base region and cause thermal runaway, a positive-feedback behavior.

The default silicon wafer thickness is 300 microns; these wafers are often thinned (the back side is ground down) to 100 microns to fit into very thin packages. The thermal time constant of 100 microns is yet another 100X slower, to 114 microSeconds.

The metal plate (flag) under the silicon have about the same thermal properties as silicon, and that gets you to about 1 millisecond of survival time.