An SCR (Silicon Controlled Rectifier) is a four-layer (P-N-P-N), three-terminal latching device. Its behaviour is best understood through the two-transistor analogy, in which the P-N-P-N structure is modelled as an interconnected PNP and NPN transistor pair whose collectors feed each other's bases. A gate pulse injects base current that starts this regenerative feedback loop.

Sequence of operation:

• In the forward blocking state, the device is forward-biased but off, blocking current until triggered.
• A sufficient positive gate current pulse initiates regeneration. Each transistor drives the other harder until the loop gain reaches unity and the device latches fully ON.
• Once latched, the internal feedback sustains conduction on its own. The gate has done its job and loses control — the gate can no longer turn the device off.

Therefore, removing the gate signal immediately after conduction begins has no effect: the SCR continues to conduct until the anode current falls below the holding current I_H, at which point regeneration collapses and the device reverts to blocking. Turn-off is achieved by reducing anode current below I_H — via natural (line) commutation when the AC supply reverses, or forced commutation in DC circuits.

The claim that the SCR turns off immediately when the gate is removed misunderstands the latching action — that would be true of a transistor, not a thyristor. A precise 50% drop in anode current has no physical basis; anode current is set by the external load circuit, not the gate. Oscillation between ON and OFF is not a stable SCR behaviour once latched, since there is no mechanism to spontaneously interrupt the anode current.