Thus, a thyristor behaves like a normal semiconductor diode after it is turned on or 'fired'. Thyristors are switched ON by a gate signal, but even after the gate signal is de-asserted (removed), the thyristor remains in the ON-state until a turn-off condition occurs (which can be the application of a reverse voltage to the terminals, or a decrease of the forward current below a certain threshold value known as the 'holding current'). Thyristors can only be turned ON using the gate lead, but cannot be turned OFF using the gate lead. Normal thyristors ( silicon-controlled rectifiers) are not fully controllable switches (a 'fully controllable switch' can be turned on and off at will).
