Rise  Time (t_r)

During  rise time voltage falls from 90% V_a to 10%  where  V_a where V_a is the initial  anode  voltage. In terms  of current  time taken  by anode  current  to rise form  10% I_a 90 % I a is  the final value  of anode  current. The turn  on time  of a thyristor mainly depends  upon the  rise time. If t_r is high means ton will be high.  To reduce the time t_r should  be kept  low in limit. The rise  time t_r can  be reduced by applying  high and  current  pulses to the gate because is inversely proportional to the magnitude of Is  and its build up rate. During t_r I_a  starts spreading from the narrow  conducting  region near  the gate.