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
: 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.