In TTL (Transistor–Transistor Logic) families, which active switching device is used inside the basic gate structures?

Introduction / Context:
Different logic families are defined by their internal switching devices. TTL is historically important and uses bipolar technology. Recognizing the underlying transistor type helps you understand speed, power, and interfacing characteristics.

Given Data / Assumptions:

• TTL dates back to widespread bipolar processes.
• Gate cores employ multiple-emitter BJTs, Schottky clamping in some variants, and totem-pole outputs.
• Device physics of BJTs give TTL its switching behavior and currents.

Concept / Approach:
Because TTL is a bipolar logic family, its elementary gates (NAND as a primitive) use BJTs for input, switching, and output stages. This contrasts with CMOS, which uses complementary MOSFETs, and ECL, which uses differential BJT pairs biased in the active region.

Step-by-Step Solution:
Identify the family: TTL.Recall: “Transistor–Transistor Logic” refers to BJTs implementing both logic and amplification.Select “bipolar junction transistor (BJT).”

Verification / Alternative check:
Open any TTL gate schematic (e.g., 74xx NAND): you will find multi-emitter input BJT and BJT stages forming the totem-pole output.

Why Other Options Are Wrong:
FET/MOSFET are used in CMOS and NMOS/PMOS families, not TTL cores.Unijunction or tunnel diodes are not used as the active switch in TTL logic gates.

Common Pitfalls:
Confusing TTL-compatible CMOS (HCT families) with original TTL; compatibility refers to voltage levels, not the internal device type.

Final Answer:
bipolar junction transistor (BJT)