The 74HC00 – 74HCT00 TTL IC contains four independent gates each of which performs the logic NAND function. The 74HC00; 74HCT00 are high-speed Si-gate CMOS devices that comply with JEDEC standard no. 7A. They are pin compatible with Low-power Schottky TTL (LSTTL).
The 74LS00 TTL IC contains four independent gates each of which performs the logic NAND function. The LS means Low Power Schottky. The low power means the IC has a lower power consumption than the original TTL device but with a slower response. The Schottky is used to reduce the response time. Schottky metal-semiconductor junctions are featured in the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector-base junctions of the bipolar junction transistor (BJT) to prevent their saturation, thereby greatly reducing their turn-off delays.