Why does a transistor must be derated when it is used at higher temperature (>room temperature)?

Question

show an example of derating calculation

Answer 1

Semiconductor junctions have an absolute maximum temperature at which they can be operated (or stored). At (or above) that temperature the junction fails (for a number of reasons).

When a PN junction is forward biased (or in zener reverse bias), it generates heat. For a transistor this generally means the base-to-emitter junction. This heat must be transferred to the case, then to the ambient.

Transistors have thermal characteristics as well as electrical characteristics. They have thermal resistance from junction to case, from case to ambient, or it can be lumped as junction to ambient. Thermal resistance is usually denoted by the Greek lower-case Theta. "Power" transistors have low thermal resistances, while small plastic case transistors have much higher Thetas.

The rate at which heat is eliminated from the junction is dependent on the ambient temperature and the Theta(junction to ambient). If the ambient temperature is high, then the operating temperature (the amount of power in the junction) must be reduced (derated).

Example:
Theta-j-a = 100 degreeC / Watt
Ambient temp = 85 degreeC
Max. Junction temp = 125 degreeC

Calculate the maximum operating power in the transistor
125 - 85 = 40 degrees differential

40 / Theta = 0.4 Watts max power.

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