Universal Gates

In one of my interview, the interviewer asked why AND, OR gates are not called as universal gates. I couldn’t answer to his question. Let us know why NAND and NOR gates alone called has Universal gates.

Universal Gates
A logic gate which can implement any Boolean function without need of other logic gates is called Universal Gate. From the above explanation we can conclude NAND and NOR gates as universal gates, which means we can implement any logic gates using these two gates.

Implementation of logic gates using NAND gate
1.       Inverter

             

2.       AND GATE 

3.       OR GATE

Implementation of logic gated using NOR gate

1.       Inverter 

             

2.       AND GATE

             

3.       OR GATE

           

AND, OR ,NOT which are basic gates can be implemented by using NAND or NOR gates as shown above, similarly any logic can be implemented using NAND or NOR gates. And so these gates are called universal gates.

When you are asked to  design a gate using one of the universal gate, which gate is better to use? 
NAND gate is more commonly used compared to NOR gate because they offer less delay compared to NOR gate and also occupy less area.