A 2-input TTL NAND gate usually consists of 5 transistors: Two multi-emitter transistors (counted as two separate transistors here) One phase splitter transist

A 2-input TTL NAND gate usually consists of 5 transistors: Two multi-emitter transistors (counted as two separate transistors here) One phase splitter transistor. One pull-down transistor. One output transistor. The multi-emitter transistors are easy to make on-chip and are a clever way of getting two inputs into one 'device'. For a 7400 that was 4 sections x 5 transistors per section = 20 transistors, total. NAND was the fundamental block in TTL; in the earlier RTL, NOR was the fundamental block - and used only 2 transistors, but needed resistors for the inputs and as the load pull-up. Power hungry and slow as a result.