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Junction Temperature vs. Ambient Temperature: Transistor Temperature Operability

Junction box working in an electronic system


Temperature can cause mood swings, thoughts of relocation, and even joy. This, of course, depends on your comfort level or ambient temperature preference. Also, being a resident of a state that affords me outdoor activity four out of four seasons a year, promotes a partiality to a warmer climate.

However, there are still times when I can easily mistake Florida, for say, Illinois during some winters. As you can imagine, this indeed fosters mood swings and thoughts of relocation.

Also, we (humans) are not the only entities affected by the ambient temperature of the environment in which we reside. Furthermore, temperature changes can even halt functionality all together in some devices. Moreover, you can count yourself lucky if you have never experienced an engine overheating while driving down I-75 in Florida, during mid-summer. Also, this brings me to another temperature classification called operating temperature or junction temperature. As previously discussed, exceeding the recommended temperature can have detrimental effects on functionality.

What is Ambient Temperature?

Temperature considerations are an essential concern in every aspect of our personal lives, professional lives, and even the devices we use to accommodate our day-to-day existence. Furthermore, in the fields of science, electronics, and computer technology as a whole, designs are built around these same temperature concerns. More specifically, two temperature classifications warrant special attention when designing electronic devices and using electronic components. The two temperature parameters I am referring to are ambient temperature and junction temperature or operating temperature.

Firstly, ambient temperature refers to the temperature of the surrounding air or environment in which the device resides, while the device is on. Also, the definition of ambient temperature is the measurement of the components or equipment itself as well as its environment. As you might imagine, this measurement is very critical to device operation, performance, and life-cycle.

Whether it is a resistor, processor, or transistor, the accuracy of the measurement and the measurement itself is crucial to maintaining performance and functionality. Moreover, every component has a recommendation or guideline set that outlines the optimum ambient temperature environment. The most important reason for obtaining the ambient temperature is the fact that it directly affects the junction temperature or operating temperature.

For example, with PCs, the ambient temperature is a measure of the air temperature of the surrounding computer equipment. Also, the most critical component in any PC is its microprocessor chip. Furthermore, this is why it (microprocessor) usually has its own cooling system in place in addition to the other fans within a PC case. Although this measurement is critical for device functionality and lifecycle, it is even more so for the microprocessor.

Finally, the optimal ambient temperature for PCs or its components ranges from 600 to 750 Fahrenheit. Furthermore, operationally speaking, the PC should be on the lower end of this spectrum if operating the PC for more extended periods. Although I have never experienced a microprocessor failure due to heat, I have seen systems that have. In summary, the ambient temperature can and will dictate a device’s overall temperature, and thus constant consideration is required.

 A handful of capacitors against a white background

Between finding the proper capacitor for your design and balancing voltage needs around it, they can be tricky to implement. 


What is Junction Temperature?

In the field of electronics, the junction temperature and the operating temperature can be the same; for example, the semiconductor in use in the device. However, the junction temperature is directly affected by the ambient temperature as well, and for integrated circuits, the following equation depicts this relationship between the two measurement parameters.

TJ = Ta + (PD x Rja)

Whereas in this equation:

TJ represents the Junction Temperature in Celsius

Ta represents the Ambient Temperature in Celsius

PD represents the power dissipation of the integrated circuit in Watts

Rja represents the Junction to Ambient thermal resistance in Celsius/Watts.


Furthermore, the junction temperature, which is short for transistor junction temperature (semiconductor), is the maximum operating temperature of the actual semiconductor in use in the electronic device. During operation, it is higher than the temperature of the part's exterior and the case temperature. Also, the difference is equivalent to the amount of heat transferring from the junction to the case multiplied by the junction-to-case thermal resistance.

Moreover, numerous physical properties can affect the temperature of semiconductor materials. This includes the thermal production of charge carriers, the diffusion rate of dopant elements, and carrier mobilities.

Glossary of Terms for Thermal Production in Semiconductor Materials

Charge carriers is the term in Physics for particle or quasiparticle that are free to move that also carry an electric charge. This is particularly true of particles that carry an electric charge in conductors (electrical). Furthermore, two examples of these particles are ions and electrons.

Dopant elements or doping agents are the traces of an impurity element that is inserted into a chemical material to modify its initial electrical or optical properties.

Carrier mobilities is a characteristic parameter (in Physics) that depicts how swiftly an electron can move through a semiconductor or metal, as an electric field pulls it.

The Importance of Maintaining Proper Junction Temperature?

Since the junction temperature is also the safe operating temperature, it is, of course, relevant to overall design considerations as well as functionality and performance considerations. Therefore, it is necessary to be able to calculate your maximum junction temperature.

Furthermore, the guidelines on the maximum junction temperature are in the datasheet that accompanies the component. Also, it is in use for the necessary case-to-ambient thermal resistance calculations for the specified power dissipation. Moreover, the results of this calculation aids in the selection process of the appropriate heat sink for the design, when applicable.

Also, with today’s current processors (PC), most manufacturers provide built-in network sensors to monitor the core temperature of the processor chip. Thus, whenever the sensor registers temperatures near the eclipse point of the TJ, it will initiate thermal throttling. This measure includes clock speed reduction, clock gating, and clock stretching, all of which reduce core temperature. However, if these measures are not sufficient, the sensor will initiate a shutdown to prevent damage due to excessive TJ. So, to obtain an approximation of the (chip) junction temperature, we use the previously discussed equation: TJ = Ta + (PD x Rja).

Transistors on a circuit

Microprocessors and transistors have very sensitive needs for operating temperatures.


In electronics, all devices require power to operate. This power is introduced through both current and voltage from a designated source or sources. However, the consumption of energy by a device creates heat and thus results in junction temperatures increasing. Overall, ambient temperature (Ta) will dictate the minimum temperature in which a device operates. Regardless of any measures in use to control heat, the device in use will always possess a higher junction temperature than its surrounding environment. Therefore, the junction temperature is dependent on ambient temperature, and thus, it affects the design and overall device’s applicational uses.

Temperature control within an electronic circuit can be a difficult process for any designer, but with Cadence’s suite of design and analysis tools, you can rest easy. Allegro PCB Designer works together with you and your analysis teams to ensure proper temperature regulation and thermal dissipation through any of your electronic designs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.