PMOS Body Diodes

Discussion in 'General Electronics Chat' started by newbie217, Feb 12, 2010.

  1. newbie217

    Thread Starter Active Member

    Apr 12, 2009
    Hi all,

    I had a simple question about the direction of a PMOS parasitic diode.

    First off, a PMOS is a PNP sandwitch. So, there are back to back diodes (substrate to source) and (substrate to drain). Shown in image (A)

    Usually, the substrate is connected to the highest potential supply, VDD / tied to the Source terminal. Making this connection shorts out diode D1 b/c its anode and cathode are at the same potential.

    This leaves the parasitic diode D2. Since the cathode is attached to the substrate, which is the Source, it can be redrawn, as shown in the image (B)

    So, when the PMOS is OFF, it can still conduct current from drain to source, but not the other way around. Is this correct? Do I have the right grasps of the above concepts?

    I'm looked through the literature and I usually see the PMOS device showing the parasitic diode drawn in the opposite direction. This is my main source of confusion.

    Thanks for the help!
  2. mik3

    Senior Member

    Feb 4, 2008
    I agree with the direction of the diode.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You have it right. Please provide some examples of literature which show the diode pointing the other way.