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About PMOS Diode connection (current direction)

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Apr 14, 2011
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Help! About PMOS Diode connection (current direction)


The figure above is a PMOS, when VL is higher than VR, there will be a current from VL to VR. It means that the diode turns on when it is forward-biased. On the other hand, when VR is higher than VL, the voltage of drain is higher than that of the substrate, so the drain-body junction starts to conduct.

Then my question is what is the diode direction? Base on the above statement it should be a bidirectional diode, right?


The figure above is voltage doubler and if it's a bidirectional thing how does it work?

PS: I notice if I change the voltage doubler to NMOS the output will become negetive instead of POMS positive. Why?

Thank you for your rely!
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I have difficulties to recognize a clear question from your post. The diode connected PMOS in the first schematic seems to be almost useless.

MOS transistors in analog IC design usually don't have a source-bulk short as assumed in the schematic, instead the PMOS bulk is connected to Vdd and NMOS bulk to Vss, so substrate diodes are reverse biased. This should be assumed for the second schematic as well.

Thanks for your rely! Yes, I know normally cmos bulk should be connected like what you say. However, diode is not avaliable to be integrated in IC design, right?. So I want to find a MOS diode, just function as an ideal diode(two port component) not the load as describe on the textbook. The schematic I post is quoted from a paper, I just confused about which port is the positive and which port is negative. And how to implement a diode if I want to use NMOS?

In a short, the MOS diodes will conduct current in the direction indicated by the source arrow. In the shown NMOS doubler circuit, the upper output terminal is positive.
Saying this, I presume that the substrate junction is reverse biased by suitable means.

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