Delay time ( td)
Initially a thyristor remains in forward blocking stat when anode to cathode forward voltage is applied and gate to cathode circuit is opened. As the gate to cathode voltage is applied a small forward leakage current flows. Delay time is the time during which anode current rises form forward leakage current to 10% of Ia when gate current is reached 90% of Ig where Ia and Ig are the final value of anode and gate current respectively. As the anode voltage falls from Va to 90% of Va where Va is the initial value of anode voltage.
Since the gate current has non uniform distribution of current density over the cathode surface Ig is much higher near the gate and its value decreases rapidly as the distance from the increases.