PUT identification — The programmable unijunction transistor (PUT) is actually a member of which device family, despite its name suggesting similarity to the UJT?

Introduction:
The programmable unijunction transistor (PUT) is a classic device used in relaxation oscillators, timing circuits, and triggering applications. Its name can be misleading: although it performs functions reminiscent of a unijunction transistor, its internal structure and behavior align more closely with the thyristor family.

Given Data / Assumptions:

• Device named “programmable unijunction transistor”.
• Used for threshold triggering with a programmable firing point.
• We seek the actual device class, not just functional similarity.

Concept / Approach:

A PUT is essentially a four-layer PNPN thyristor configured and biased to emulate the UJT’s negative-resistance characteristic. The key difference is that a PUT’s firing (trigger) point is set by external resistors feeding its “gate,” allowing programmable threshold behavior. Structurally and operationally (breakover, latching), it belongs to the SCR/thyristor family rather than the FET or BJT families.

Step-by-Step Solution:

Verification / Alternative check:

Application notes and datasheets categorize PUTs alongside thyristors, often modeling them as gated PNPN devices that latch when triggered—behavior distinct from UJTs (which are three-terminal, two-junction devices) and entirely different from FETs or BJTs.

Why Other Options Are Wrong:

• UJT thyristor: UJT is not a four-layer thyristor; it has a different structure and curve.
• FET device: Field-effect operation is unrelated to PUT physics.
• TRIAC: A bidirectional thyristor; PUT is unidirectional small-signal.
• BJT: Three-layer current-controlled device, not PNPN with latching.

Common Pitfalls:

• Equating functional similarity (sawtooth generators) with structural identity.
• Forgetting that “programmable” refers to setting the trigger level via external bias.

Final Answer:

SCR