As the gate current begins to flow from gate to cathode with the application of gate
signal. Gate current has a non uniform distribution of current density over the cathode
surface. Distribution of current density is much higher near the gate. The density decrease
as the distance from the gate increases. So anode current flows in a narrow region near
gate where gate current densities are highest. From the beginning of rise time the anode
current starts spreading itself. The anode current spread at a rate of 0.1mm/sec. The
spreading anode current requires some time if the rise time is not sufficient then the anode
current cannot spread over the entire region of cathode. Now a large anode current is
applied and also a large anode current flowing through the SCR. As a result turn on losses
is high. As these losses occur over a small conducting region so local hot spots may form
and it may damage the device.