Amid all the promotion of solid-state superlatives ranging from data rate to feature size to LED light output, one characteristic is never touted: junction temperature. That’s because junction temperature (TJ) is an undesired but unavoidable side-effect of high currents and/or switching speeds. A p-n junction, whether it is one of millions on a CPU chip or the only one within a power LED, generates heat. In the past two decades the industry has seen heat dissipation increase by orders of magnitude. Faster is better, but faster is also hotter. This trend is not without consequences. A 10° increase in TJ can cause a 50% reduction in a semiconductor device’s life expectancy. Differing thermal expansion responses where dissimilar packaging materials meet can degrade and ultimately destroy devices.

In LEDs, both brightness and color can suffer as TJ increases. And of course the twin issues of safety and cooling can impact the design of an entire system, not just the semiconductor device producing the heat. All these facts point toward the need for a thorough grasp of thermal behaviors at the chip level, and beyond that to thermal interface materials (TIM) and even heat sinks. True understanding comes with physical measurements performed on actual devices.