Answer: We all know that NAND and NOR are universal gates. we can realize any boolean expression with these two gates. And it is to be said that we prefer NAND than NOR gates.
NOR Gate NAND Gate
The reason to prefer NAND over NOR is the speed factor. holes current flows in pMOS transistors, and electron current flows in nMOS devices. And we know that mobility of electrons is grater than mobility of holes. In the NOR gate circuit the two pMOS transistors are connected in series and in the NAND gate it is parallel. in general mobility of electrons is approximately "2.3" times the mobility of holes. so the switching speed of NAND gate is 2 times faster than the NOR gate because the current should pass 2 pMOS transistors in the NOR gate, but in the case of NAND gate current has to pass only 1 pMOS transistor. this delay path made it use NAND gates over NOR.
