Devices with four-layer (PNPN) structures in power electronics Among the listed semiconductor devices (UJT, SCR, TRIAC), which device(s) has a four-layer PNPN structure?

Introduction / Context:
Recognizing internal layer structures helps in predicting triggering behavior and conduction characteristics of power devices. In particular, four-layer PNPN structures exhibit regenerative action central to thyristor operation.

Given Data / Assumptions:

• Options include UJT (unijunction transistor), SCR (silicon controlled rectifier), and TRIAC.
• We are asked which have four layers (PNPN).

Concept / Approach:
The SCR is the canonical four-layer device, consisting of PNPN with three junctions and three terminals (anode, cathode, gate). A TRIAC can be thought of functionally as two inverse-parallel SCRs in a single package; its physical cross-section is typically five-layer, not strictly four-layer. A UJT is a single-junction, two-layer device.

Step-by-Step Solution:
Identify SCR → classical PNPN (four-layer) structure.Identify TRIAC → effectively two SCRs; physical structure is generally five-layer, not a simple PNPN.Identify UJT → single junction, not PNPN.Therefore, the correct answer is SCR only.

Verification / Alternative check:
Device symbols and triggering models reaffirm that SCRs rely on regenerative PNPN action; UJTs and TRIACs do not match a single four-layer PNPN stack.

Why Other Options Are Wrong:
UJT: Single junction device; not PNPN.
TRIAC: Multi-layer (often five-layer) bidirectional device, not a single PNPN stack.
SCR and TRIAC: Partly incorrect because TRIAC is not strictly four-layer.

Common Pitfalls:
Assuming “two SCRs back-to-back” implies four layers; the integrated TRIAC structure differs.

Final Answer:
SCR