Lowest propagation delay among logic families Which logic family is known for the shortest propagation delay (i.e., the fastest switching) among the options below?

Introduction / Context:
Propagation delay defines how quickly a logic family transitions from input change to output change. For high-speed data paths and clock distribution, selecting a fast family is key. Emitter-Coupled Logic (ECL) is historically recognized for its very low delays due to its non-saturating operation.

Given Data / Assumptions:

• Comparison across common families: CMOS, BiCMOS, ECL, Schottky TTL (74S).
• Metric: shortest propagation delay.
• Typical operating conditions assumed (standard supply and load).

Concept / Approach:
ECL uses differential transistor pairs biased to avoid saturation, maintaining transistors in their active region. This minimizes charge storage delays and yields extremely fast switching, at the cost of higher static power and negative supply usage in some variants. CMOS and BiCMOS can be very fast in modern processes, but classic family comparisons still place ECL as the fastest among these choices.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for classic ECL (e.g., 10K/100K families) show sub-nanosecond to a few-ns delays, outperforming 74S and general-purpose CMOS of the same era.

Why Other Options Are Wrong:

• CMOS: efficient and scalable but not inherently the fastest among listed families.
• BiCMOS: blends strengths; still slower than classic ECL in propagation delay.
• 74SXX: Schottky TTL improves over standard TTL but trails ECL.

Common Pitfalls:
Confusing modern deep-submicron CMOS ASIC speed (process-dependent) with classic family characteristics.

Final Answer:
ECL