Understanding the Bi-Directional Device

The question asks to identify a specific electronic device based on its unique characteristics: being a 5-layer bi-directional device, capable of being triggered into conduction by both positive and negative voltage at its anodes, and with both positive and negative triggering pulses at its gate. Let's carefully analyze each option to determine which device perfectly matches this description.

TRIAC Device Characteristics

The term TRIAC stands for "Triode for Alternating Current". It is a semiconductor device that acts as a bi-directional switch, meaning it can conduct current in both directions. This makes it ideal for controlling AC power. Here's why the TRIAC fits all the criteria:

• 5-layer Bi-directional Device: A TRIAC can be conceptualized as two Silicon Controlled Rectifiers (SCRs) connected in inverse parallel, but integrated into a single crystal structure. While a single SCR is a 4-layer (p-n-p-n) device, the TRIAC effectively has multiple p-n junctions, giving it a 5-layer structure when considering its bi-directional operation paths (e.g., p1-n1-p2-n2-p1). This layered structure allows current flow in either direction once triggered.
• Triggering by Both Positive and Negative Voltage at Anodes: Unlike an SCR which conducts only when its anode is positive with respect to the cathode, a TRIAC is designed to conduct when either of its main terminals (MT1 or MT2, which act as anodes/cathodes depending on the voltage polarity) is positive or negative relative to the other. This inherent bi-directional capability allows it to control alternating current.
• Triggering by Both Positive and Negative Triggering Pulses at its Gate: A TRIAC has a single gate terminal. It can be triggered into conduction by applying either a positive or a negative pulse to its gate with respect to MT1, regardless of the polarity of the voltage across its main terminals (MT1 and MT2). This is one of its most distinguishing features, offering flexible control in AC circuits.

Comparison with Other Devices

Let's briefly look at why the other options do not match the description:

• SCR (Silicon Controlled Rectifier):
  • An SCR is a unidirectional device. It conducts current in only one direction (from anode to cathode) when forward biased and triggered.
  • It is a 4-layer (p-n-p-n) device.
  • It is typically triggered by a positive pulse at its gate (with respect to the cathode) when its anode is positive relative to the cathode. It cannot be triggered by negative gate pulses and does not conduct with negative anode voltage.
• UJT (Unijunction Transistor):
  • A UJT is primarily a three-terminal, single-junction semiconductor device used mainly in timing circuits, oscillators, and trigger circuits for SCRs and TRIACs.
  • It is not a bi-directional switching device for AC power control in the way a TRIAC is, nor does it fit the "5-layer" description.
• DIAC (Diode for Alternating Current):
  • A DIAC is a bi-directional device, similar to a TRIAC in that it conducts in both directions.
  • However, a DIAC is a 3-layer (n-p-n or p-n-p) device, essentially two diodes connected in inverse parallel.
  • Crucially, a DIAC has no gate terminal. It triggers into conduction when the voltage across its terminals exceeds a certain breakover voltage in either direction. It cannot be triggered by external gate pulses.

The following table summarizes the key features for clarity:

Feature	TRIAC	SCR	UJT	DIAC
Layers	5 (effective)	4	3	3
Directionality	Bi-directional	Unidirectional	Unidirectional (triggering)	Bi-directional
Gate Terminal	Yes	Yes	Yes	No
Triggering by Gate Pulses (Positive/Negative)	Both positive and negative	Positive only (with positive anode)	N/A (oscillator/trigger device)	N/A (breakover voltage)

Based on this detailed analysis, the TRIAC is the only device that perfectly aligns with all the characteristics described in the question: a 5-layer bi-directional device which can be triggered into conduction by both, positive and negative voltage at its anodes and with both, positive and negative triggering pulses at its gate. The final answer is $\boxed{\text{TRIAC}}$.