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A single silicon-controlled rectifier can only switch on during the positive half of an AC mains wave, leaving the negative half completely dead. To control full AC power without wiring two bulky thyristors in parallel, engineers merged them into a single component: the triac.
A triac (triode for alternating current) is a three-terminal, five-layer semiconductor switch that conducts current in either direction when triggered by a gate pulse. Its three terminals are Main Terminal 1 (MT1, the reference point), Main Terminal 2 (MT2), and the Gate (G).
How can one single gate pin trigger conduction whether current flows forward or backward through the main terminals?

The Four Operating Quadrants

Because both the main terminal voltage (VMT2−MT1V_{MT2-MT1}) and the gate trigger voltage (VG−MT1V_{G-MT1}) can be either positive (++) or negative (−-) relative to MT1, the triac operates in four distinct quadrants.
Why is Quadrant IV so much harder to trigger than the other three?

Conduction Mechanisms by Quadrant

In Quadrant I (I+I^+), MT2 is positive and the Gate is positive relative to MT1. Electrons from MT1 (N2N_2) inject into P2P_2, triggering standard regenerative transistor action across the P1−N1−P2−N2P_1-N_1-P_2-N_2 path with the lowest required gate trigger current (IGTI_{GT}).
In Quadrant III (III−III^-), both MT2 and the Gate are negative relative to MT1. Gate junction P2−N3P_2-N_3 forward-biases, injecting electrons into P2P_2 that initiate main conduction across the reverse path P2−N1−P1−N4P_2-N_1-P_1-N_4 with equally high sensitivity.
In Quadrant IV (III+III^+), MT2 is negative but the Gate is pulled positive. Conduction relies on indirect junction gate triggering where carriers must diffuse across a wide depletion zone, demanding up to three times higher IGTI_{GT}.