why is pmos better for passing logic 1?

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while n logic is good fr passing 0..............

plz help now...........
thanks

 

[Tolstoy]

while n logic is good fr passing 0..............

plz help now...........
thanks

If you try to pass a logic 1 with a npn transistor by connecting it as
an emitter follower and driving the base with that same logic 1, you
will get a full diode voltage drop across the transistor... the
transistor is "diode-connected" (base connected to collector).
The pnp will have a smaller voltage drop when you connect the emitter
to logic 1 and ground the base (through a current-limiting resistor,
of course). The voltage drop across the collector-emitter is much
smaller then.

For logic 0, the situation is reversed.

 

[Tolstoy]

If you try to pass a logic 1 with a npn transistor by connecting it as
an emitter follower and driving the base with that same logic 1, you
will get a full diode voltage drop across the transistor... the
transistor is "diode-connected" (base connected to collector).
The pnp will have a smaller voltage drop when you connect the emitter
to logic 1 and ground the base (through a current-limiting resistor,
of course). The voltage drop across the collector-emitter is much
smaller then.

For logic 0, the situation is reversed.

Oops, I just noticed you were asking about mos. The situation is even
more pronounced for mos than bipolar. How are you going to turn on an
nmos device when the drain is at the top rail? You would need another
voltage supply higher than your logic 1.