Mosfet body diode question

[lcv]

Hi All
I am measuring a different voltage drop on the body diode of an n-ch mosfet depending on the connection of the gate (to source or gate).
E.g. on  a 2n7000,  i find the voltage drop on  the forward  diode from S to G+D to be constantly higher (about 40mV, depending on the current) than the one I measure on a S+G to D connection.
Can you confirm this  fact and is there an explanation for it?
Thanks
Regards,
Lcv

[Transmogrifox]

I haven't thought about this in depth, so disclaimer this response is just shooting from the hip...

Intuitively this seems to make sense.  When you connect gate to drain, the forward drop on the body diode pulls the gate lower, which then has an effect on the PN junction.  This would change the distribution of charge between N channel and P substrate, changing the property of the diode.

Again, I don't stand on this answer with 100% confidence, but I thought I would chime in that it does make sense to me.

FWIW I simulated this in LTSpice and the simulator shows both connections being equal.  That would mean that whatever causes this observed behavior is not modeled by the simulator, at least.

[lcv]

Interesting point, thank you.
Current wet weather we're having here encourages mosfets handling,  so i put together a little test to investigate further:



The idea is to get the gate even more negative vs. the source, adding two (switchable)  diodes in series to the drain, to see how that influences the body diode fw voltage.

The results are (assuming I didn't screw up something ) 
Vds=564mV when  G+S  (situation not shown in the picture)
Vds= 607mV  when G+D (switch closed)
Vds=609mV   when G to ground, D to 1.18 V vs ground (i.e. open switch)

So I'd say that further negative G has no or negligible influence, still your hypotheses can hold on the region Vgs=0 to -600mv
May be when Vgs=0 the mosfet is less "off" and some resistance is actually in parallel to the diode?
Thanks again
Regards,
Lcv

[Rob Strand]

The results are (assuming I didn't screw up something )
Vds=564mV when  G+S  (situation not shown in the picture)
Vds= 607mV  when G+D (switch closed)
Vds=609mV   when G to ground, D to 1.18 V vs ground (i.e. open switch)

Interesting effect.   Thanks for posting it.

May be when Vgs=0 the mosfet is less "off" and some resistance is actually in parallel to the diode?

I think this is the case.     The question is why.  BTW, If you were to connect the gate to the +rail the mosfet would switch on and there would be no diode effect (I wonder how that would sound maybe with another series diode).

I ran a spice sim with 1mA current and -1.2V between G and D, D held at 0V.
On three models I got the equal voltages.
On a fourth model I got these:
GS s/c: 553mV
DS s/c: 576mV
VG=VD-1.2V:  576mV
G to 9V:  3.7mV

*** However ***  I have a note against this model saying: The drain current is non-zero when Vgs=0.   In other words the model is not to be trusted because I didn't see that on real devices.    What that means is if you connect the MOSFET with the normal VD positive connection ie. with the source and gate to 0V and the drain to positive via a resistor there is current flowing.

So maybe you should do a check with the drain in the positive direction.