For Emitter-Coupled Logic (ECL), the switching speed is very high because

Emitter-coupled-logic (ECL):Emitter-coupled-logic (ECL) is a BJT logic family that is generally CONSIDERED as the fastest logic available.ECL achieves its high-speed operation by employing a relatively SMALL voltage swing and preventing the transistors from entering the saturation region. (Reduces the storage delay time).ECL transistor USES differential amplifier configuration as shown: Logic FamilyFull NameAdvantagesDisadvantagesCMOSComplementary metal-oxide-semiconductorLowest power consumptionUsed in all microcomputer chips today.Most common logic family.Easily DAMAGED by STATIC discharge and voltage spikes.TTLTransistor-transistor logicEarliest developed.Most rugged.Least susceptible to electrical damage.Consumes more power than CMOS – not suitable for battery-operated devices.ECLEmitter-coupled logicFastest available logic familyConsumes more power than CMOS. It requires extreme care in the wiring.