Why body-source diode is forward-biased in the picture?

[hanhan]

Hi,
I am reading about MOSFET in the page https://electronics.stackexchange.com/questions/72582/mosfet-why-the-drain-and-source-are-different and there is one thing I got stuck. Please help. Thank you.

 

[Jony130]

I don't know nothing about semiconductor physics. But if we connect the Source to GND and Drain to 1.2V. And if we assume that parasitic BJT base is at the middle of a length of a channel. The base voltage will be equal to 0.6V because it is on the middle of a MOS channel.
And also notice the bode to diode voltage is -0.6V. And to get rid one of these parasitic diodes. We short MOS body with the Source. And we end-up with this

 

[hanhan]

Thank you, Jony and nice image!

I don't know nothing about semiconductor physics. But if we connect the Source to GND and Drain to 1.2V. And if we assume that parasitic BJT base is at the middle of a length of a channel. The base voltage will be equal to 0.6V because it is on the middle of a MOS channel.

I see now.

And also notice the bode to diode voltage is -0.6V.

I don't get this. Which diode did you mean?

And to get rid one of these parasitic diodes. We short MOS body with the Source.

Interesting.
I think both diodes are parasitic and undesired. Is there a way to cancel both diode?
In 4-terminal MOSFET drain and source are the same. It is symmetric.
But in 3-terminal MOSFET drain and source are different.
Do you agree?

 

[Jony130]

I don't get this. Which diode did you mean?

body to drain diode

I think both diodes are parasitic and undesired.

Yes, those diodes are parasitic and undesired.

Is there a way to cancel both diode?

Add another MOSFET
**broken link removed**
http://bbs.dianyuan.com/bbs/u/48/1165249013.pdf

In 4-terminal MOSFET drain and source are the same. It is symmetric.
But in 3-terminal MOSFET drain and source are different.

Almost all discrete MOSFET has build-in parasitic diode (body diode with cathode on the Drain, anode on the Source for N-channel). This diode will not conduct until Source voltage will be greater then the Dource voltage. If Vsd < 0.6V body diode is OFF. So until the diode is OFF you can swap the drain and source. The MOSFET will conduct equally well in both directions.

 

[hanhan]

bode to drain diode

Sorry, I don't see the diode, can you tell me where? In your image, there are two diodes, body-drain and body-source diodes.

Add another MOSFET

As my understanding, in dual channel mosfet the voltage drop across drain and source each mosfet will cancel out that of the other mosfet. Is it right?

Almost all discrete MOSFET has build-in parasitic diode (body diode with cathode on the Drain, anode on the Source for N-channel). This diode will not conduct until Source voltage will be greater then the Dource voltage. If Vsd < 0.6V body diode is OFF. So until the diode is OFF you can swap the drain and source. The MOSFET will conduct equally well in both directions.

Got it.
Another question: in dual channel mosfet above, drain and source can be swapped, right?

 

[Jony130]

I made a stupid spelling error. I meant body-drain diode.

As my understanding, in dual channel mosfet the voltage drop across drain and source each mosfet will cancel out that of the other mosfet. Is it right?

No.

Got it.
Another question: in dual channel mosfet above, drain and source can be swapped, right?

Yes.

 

[hanhan]

Jony, I still don't know why we need a dual ch mosfet. In your picture, one thing I can get is about the symmetry, drain and source can be swapped. However, the parasitic diode, body to drain diode, is still there.

 

[Jony130]

Jony, I still don't know why we need a dual ch mosfet. In your picture, one thing I can get is about the symmetry, drain and source can be swapped. However, the parasitic diode, body to drain diode, is still there.

Yes, the parasitic diode is still there but now by changing Vg voltage we have a bidirectional switch.
https://forum.allaboutcircuits.com/showpost.php?p=588716&postcount=12

 

[hanhan]

Thanks