Lead configuration of a unijunction transistor (UJT): which set correctly describes its terminals?

Introduction / Context:
The UJT is a three-terminal device used for triggering and relaxation oscillators. Correctly identifying its terminal names and functions is essential for wiring timing circuits and firing thyristors.

Given Data / Assumptions:

• Device: UJT (unijunction transistor).
• Terminals: E (emitter), B1, and B2.
• Applications: sawtooth generators, SCR gate drivers.

Concept / Approach:
A UJT has a lightly doped bar of semiconductor with two base contacts (B1, B2) forming a resistive path. A single emitter junction injects carriers into the bar, exhibiting negative resistance over part of its characteristic, enabling relaxation oscillation and triggering functions.

Step-by-Step Solution:
Identify terminals: E, B1, and B2 (three leads total).Understand structure: emitter junction + resistive base channel between B1 and B2.Recognize usage: charging a capacitor until the UJT fires and discharges through B1.Therefore, the correct description is “one emitter lead and two base leads.”

Verification / Alternative check:
Datasheets and pinouts (e.g., 2N2646) show E, B1, B2 pins consistently, confirming the lead configuration.

Why Other Options Are Wrong:

• a) and b): Each lists only part of the terminals, not the full set.
• c) Two emitters are not present in a UJT.
• e) Collectors are transistor terms; UJTs use emitter and bases.

Common Pitfalls:

• Miswiring B1/B2—polarity matters for oscillator direction and thresholds.
• Assuming UJT is a standard BJT; its behavior and terminals differ significantly.

Final Answer:
one emitter lead and two base leads