Delay time ( t_d)

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 I_a  when gate  current is  reached 90% of I_g where I_a  and I_g  are the final  value  of anode  and gate  current  respectively. As the anode  voltage falls  from  V_a to 90%  of Va where V_a 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.