TRIAC is the word derived by combining the capital letters from the words TRIode and a.c. As the TRIAC can conduct in both the directions, the terms anode and cathode are not applicable to TRIAC. Its three terminals are usually designated as main terminals, $M T_{1}, M T_{2}$ and gate $G,$ as in a thyristor.

The terminal $M T_{1}$ is the reference point for measurement of voltages and currents at the gate terminal and at the terminal $M T_{2}$ . The gate is near to terminal $M T_{1}$ .

The $V-I$ characteristic of a TRIAC is shown in Fig. c. This characteristics of the TRIAC are based on the terminal $M T_{1}$ as the reference point. The first quadrant is the region wherein $M T_{2}$ is positive with respect to $M T_{1}$ and vice-versa for the third- quadrant. The peak voltage applied across the device in either direction must be less than the breakover voltage in order to retain control by the gate. A gate current of specified amplitude of either polarity will trigger the TRIAC into conduction in either quadrant, assuming that the device is in a blocking condition initially before the gate signal is applied.

The characteristics of a TRIAC are similar to those of an SCR, both in blocking and conducting states, except for the fact that SCR conducts only in the forward direction, whereas the TRIAC conducts in both the directions. Depending upon the polarity of a gate pulse and biasing conditions, the main four-layer structure that turns ON by a regenerative process could be one of $P_{1} N_{1} P_{2} N_{2}, P_{1} N_{1} P_{2} N_{3},$ or $P_{2} N_{1} P_{1} N_{4}$