Shockley diode structure How many semiconductor layers does a Shockley diode (two-terminal thyristor) contain?
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A2
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B3
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C4
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D2 or 3
Answer
Correct Answer: 4
Explanation
Introduction / Context:The Shockley diode is the simplest thyristor structure—a two-terminal, four-layer device that exhibits regenerative switching. It is foundational to understanding SCRs (which add a gate terminal for controlled turn-on).
Given Data / Assumptions:
- A Shockley diode is a PNPN device with two terminals (anode and cathode).
- Exhibits breakover behavior and negative resistance region.
Concept / Approach:
Its internal structure is equivalent to two interconnected bipolar transistors (pnp and npn). Regeneration occurs when combined current gains approach unity, causing rapid turn-on at the breakover voltage. The four alternating layers (PNPN) form three junctions J1, J2, and J3.
Step-by-Step Solution:
Identify alternating layering: P–N–P–N → 4 layers.Count junctions: J1, J2, J3 → 3 junctions.Two terminals (no gate) distinguish it from an SCR (which is a gated PNPN).Verification / Alternative check:
Device symbols and equivalent transistor models confirm the four-layer architecture with two terminals.
Why Other Options Are Wrong:
- 2 or 3 layers cannot produce the characteristic thyristor breakover behavior.
Common Pitfalls:
Assuming “diode” implies two layers; the Shockley diode is a historical misnomer for a four-layer switch.
Final Answer:
4