In pass transistor logic,  nmos cannot be used in Passing

In pass transistor logic, NMOS can indeed be used for passing signals. However, there is a significant issue when it comes to passing a '1' or high signal. This is due to the 'threshold voltage drop' problem.

Here's a step-by-step explanation:

1. NMOS transistors are often used in pass transistor logic because of their relatively simple structure and low resistance when 'on' (conducting).

2. However, when an NMOS transistor is used to pass a '1' or high signal, the output voltage is not the same as the input voltage. Instead, it's the input voltage minus the threshold voltage of the NMOS transistor. This is known as the 'threshold voltage drop' problem.

3. This voltage drop means that the high signal passed by the NMOS transistor is not a full '1', but something less. This can cause problems in circuits where a full '1' is expected.

4. To overcome this issue, circuits often use a combination of NMOS and PMOS transistors. PMOS transistors do not have the threshold voltage drop problem when passing a '1', but they do have it when passing a '0'. By using both types of transistors, circuits can pass both '0' and '1' signals without any voltage drop.

5. So, while it's not accurate to say that NMOS cannot be used in pass transistor logic, it is true that there are significant issues that need to be addressed when they are used to pass high signals.