SCR / Thyristor is a four layer three junction device. It mainly consists of interleaved layers of P and N layers. It can block Voltage of either polarity but conducts current only in one direction that is from Anode to Cathode under normal working of forward biasing.
Basically to forward bias an SCR we need to make Anode more positive than Cathode. So we can say that SCR is said to be forward biased. Even if it is forward biased without a suitable gate pulse the device does not turn on. We need a positive gate pulse to turn on SCR. On forward biased the junctions J1 and J3 are forward biased while J3 is reverse biased. To initiate the conduction at forward biased junction J2 we need a positive gate pulse to introduce charge carriers into junction J2.
Under reverse biased condition the junction J1 and J3 are reverse biased and J2 is forward biased. The first N layer from Anode is actually an N layer with less concentration of charge carriers, so it will block about 95% of the voltage applied across SCR. Since N layer supports large depletion layer width, normally at this stage the SCR is said to be on reverse blocking state and will conduct only a minor leakage current which is very less when compared to its operating current in Kilo Amperes range.
But when we give a positive gate pulse (positive with respect to Cathode) to a reverse biased SCR it will push charge carriers to the junction near it here it is reverse biased J3. This increased concentration will forward bias the junction and increase leakage current flow. Increased leakage current through a reverse biased SCR contribute more power loss at the junctions which in turn causes increase in reverse leakage current. Cumulatively SCR will undergo Thermal Runway and the device may get damaged.