Quote Originally Posted by Fuji View Post
i thought the mobility of the holes decreased due to the fact that it has to take the area of another hole...or something like that. I don't remember much of device physics.

The mobility also depends on the doping of the region...just thought i'd throw that in there.

So you said that the goal was to use the least amount of PMOS as possible.

However, CMOS is Complementary Metal Oxide Semiconductor which would imply PMOS and NMOS. The whole idea behind CMOS is that both PMOS and NMOS have their strengths. For an Inverter, the PMOS is better with the output high and the NMOS for the output low correct? (it might be reversed). But when you remove PMOS don't you start violating the mere definition of CMOS?

I've never done CMOS processing before so i don't really know it from a physical process standpoint rather more of theoretical from the exposure to it right now.
Understanding the physics of this is hard to explain in a text based forum, people often think of holes as a charged particle, say a positive charge like a proton. This is an incorrect way of thinking about it. The most simplistic form is to think of a hole as a 'missing electron', however, in metals (and semiconductors), electrons are delocalized over the lattice and here is where all heck breaks loose -- it is a difficult concept to wrap your brain around. To be more correct, a hole would be better thought of a somewhat localized region of deficient electron density.

From this concept we can then begin thinking about dispersion curves with the interaction of holes with the lattice, which because they are less localized than say the electron equivalent, interact differently. The end result is the hole mobility within any given material is less than that of the electron within the same material. Though the dispersion curve for holes lower the mobility, solid state physics has adopted collecting terms into an effective mass... holes have no true mass really, but the behavior of the mobility can be expressed (and naturally falls out) as a mass, which is always larger than electrons. It is easier to move a marble than a bowling ball.

There are many advantages that I have read as to why CMOS is preferred over just NMOS, part in due to what you rhetorical question above ... while I understand the physics, I am not completely well versed on the actual design side ... constructing an inverter in CMOS has an advantage that it consumes much less power overall. (NOTE: I had to cheat to make sure I got this right ... a good text on this subject is Solid State Electronic Devices by B. Streetman and S. Banerjee)

Jack