The first instant of junction heating takes place in one dimension, where it is just the junction itself heating and heat is not spreading yet. Then the heat spreads in 2-3 dimensions depending on the transistor layout. The dimensionality determines the rate of temperature rise at a given time point. The more dimensions heat is spreading in, the slower the temperature rises. The number of dimensions starts out small and then rises. Finally we encounter convection, which adds an even more gradual rise in temperature since thermal resistance is decreasing as temperature is rising.
So the first corner in the heating waveform may correspond to one of those thermal boundaries, junction to substrate, die to case, case to ambient, etc.
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