Explanation: A TRIAC (TRIode for Alternating Current) is a semiconductor device specifically designed to control power in AC circuits. Its internal structure is precisely described as two complementary Silicon Controlled Rectifiers (SCRs) connected in inverse-parallel, with their gate terminals tied to a common gate. Each SCR within the TRIAC handles one half-cycle of the AC waveform. When a gate pulse is applied, it triggers conduction for both positive and negative half-cycles, allowing the TRIAC to switch current in both directions (bilateral conduction), unlike a single SCR which is unidirectional. A Unijunction Transistor (UJT) is a different device used primarily for triggering and oscillations. A Field Effect Transistor (FET) is a voltage-controlled device operating on different principles. "Bilateral SCR" is not a standard device name for this specific configuration; the TRIAC is the device that achieves bilateral switching using SCR-like characteristics.