During forward biased, ð½2 is reverse biased so a leakage forward current always associated with SCR. Now as we know the leakage current is temperature dependent, so if we increase the temperature the leakage current will also increase and heat dissipation of junction ð½2 occurs. When this heat reaches a sufficient value ð½2 will break and conduction starts.
Disadvantages:
This type of triggering causes local hot spot and may cause thermal run away of the device.
This triggering cannot be controlled easily.
It is very costly as protection is costly.