The power consumed by semiconductor devices is converted into heat.
This generated heat causes a rise in temperature of semiconductor devices.
Semiconductor devices will operate normally as long as the temperature does not exceed an upper limit (specified as the ambient temperature and the temperature of the junctions inside the semiconductor).
When this upper limit is exceeded, the semiconductor stops operating normally and becomes damaged.
Therefore, it is necessary to successfully dissipate the generated heat so as to keep the temperature within specified level.
Thermal design is indispensable when using the semiconductor for high power application or using it under the high operating temperature.
The concept of thermal resistance is used when considering heat dissipation.
This approach is based on the similarity of the conduction of heat and the conduction of electricity.
While the fine details are omitted here, by substituting heat (=power) for current, temperature for voltage, and thermal resistance for resistance, the calculation of electricity can be applied.
In other words, we have:
Potential difference = Resistance x current Temperature difference = Thermal resistance x heat (power) Thermal resistance = Temperature difference / heat (power)
Hence, the unit of thermal resistance is °C/W.
Moreover, θ is used as a symbol. The thermal resistance between 2 points is indicated using subscript (θj-a: Thermal resistance between a junction and ambient air).
In the case of heat, the difference between a good conductor and an isolator (insulator) is not so large as for electricity. Therefore, accurate calculations using thermal resistance are not very meaningful.