When a thyristor is forward biased i.e anode is connected to positive and cathode is connected to negative terminal of a battery, then the first pn junction (J1) and the last pn junction (J3) are forward biased due to the breakdown of the depletion layer where at these two junctions whereas the pn junction (J2) remains in reverse biased as no voltage is applied across the gate. The thyristor in this case is said to be in Forward Blocking Mode or OFF Mode as shown in the vi-characteristics curve.
When the positive voltage is applied across the gate then the depletion layer across pn junction (J2) breaks and current starts flowing through the thyristor and flowing current rises with the increase in the gate voltage until saturation occurs. After the saturation occurs, the thyristor is said to be in Forward Conduction Mode ON Mode.
A thyristor can only be on ON mode or OFF Mode which means it cannot be between ON or OFF state similar to the transistors making the thyristors suitable for the switching device