What is Thermal Runaway in BJT?

 

The leakage current in I_CBO increases in BJT with increase in temperature.  The leakage current is also called reverse leakage current. Leakage current increases in BJT for every 10degC rise in temperature. The relation between collector current and leakage current is:

I_c = βI_B + (1+β)I_CBO

From the above relationship, we see that I_c and I_CO are related. As the Collector current increases, CB junction (collector-base) temperature rises increasing the I_CO as well. So, this will also increase the temperature of the BJT leading to a condition called Thermal Runaway. We need to understand that this heat produced in the BJT is at the Collector Base (CB) junction. With Thermal Runaway, the component gets damaged. We  have to use heat sink to help component overcome thermal runaway.

Note that we are talking about CE configuration above as this is the most commonly used configuration.

In what scenarios should you be worried about Thermal Runaway?

If the transistor is used for low power applications like example as a simple switch to control a circuit, in these cases heat dissipation is not a problem as the component can handle low heat produced in that scenario. If we are using power transistors, where lot of heat is produced, thermal runaway can damage your component if heat sink is not used. Below is the example of a power transistor with metal cap. This package is called a metal can package.

Another scenario where the transistor is provided with a metal tab to help power dissipation.

In all the above scenarios, collector terminal is connected to the TAB which helps to dissipate power.