Not correct.
The original poster was correct in his description of the "problem."
The physical property that gives rise to that behavior is the threshold voltage of the MOSFET.
A very simplified way of thinking about this is that the MOSFET is off if its gate-to-source voltage (Vgs) is less than its threshold voltage (Vt.) Suppose you are trying to use an NMOS device as a switch to pass a logic 1 (Vdd.) What you are trying to accomplish is having the source voltage equal to logic 1, the gate voltage equal to logic 1 (to turn the NMOS device on) and the drain voltage equal to logic 1. This MOSFET will be off, because Vgs = 0 (Vgate - Vsource = Vdd - Vdd = 0.) In order for the MOSFET to bo on, the source voltage must be equal to at most Vdd - Vt. This is how the NMOS device will behave if you try to pass a logic 1 (Vdd) through it. If you repeat the same exercise with an NMOS device, trying to pass a logic 0, you will find that Vgs = Vdd => No problem. Also, if you repeat the exercise with a PMOS you will find that a PMOS can pass a logic 1 without any Drain-to-Source voltage drop, but not a logic 0. In a PMOS device, the source-to-gate voltage must be greater than the absolute value of Vt for the device to be on.
Sorry I didn't have time to give a better explanation, but I'm in a hurry. I hope that answers your question, but I'd be happy to answer any further questions that you might have.
Joe Andrew Holme wrote: : Mayank Kaushik wrote: :> Hi, :>
:> I read somewhere that pMOS degrades the logic 0 signal that passes :> through it and nMOS degrades the logic 1 signal that passes through :> it, while they both pass the opposite signals strongly. What is the :> physical property of these devices that gives rise to this behaviour?
: CMOS = Complementary MOS has push-pull outputs, using a pair of transistors : which can pull the output hard against either power supply rail. NMOS and : PMOS, in contrast, only have one logic-driven transistor, and a necessarily : weaker pullup or pulldown that is always on, working against it. The use : of the word "degrades" is misleading, since it implies a potentially : cummulative effect, which this is not. It would be better to say they have : different fan-outs for logic 1 and logic 0.