MOSFET source to drain Current

Question

Similar kinds of questions have been asked before it but they don't clarify how one can use this aspect in circuit.

I'm aware that a MOSFET is a bidirectional device and in one direction current can flow from Drain to Source upon giving a gate pulse of sufficient amplitude & through body diode in reverse direction.

Now, Lets say I have connected Drain to higher than Source and current is flowing through the body diode, But at the same time I want to turn on the MOSFET Source to Drain channel to allow current to flow from the channel from source to drain parallel with body diode. Is it possible ? In this case what should be the Gate potential will it be higher with reference to drain or source? I'm new to this domain & confused. Please Help!

Answer 1

A MOSFET always consumes power from the circuit. It has no mechanism to convert energy from some other form to electrical energy.

Therefore, the currents through a MOSFET always flow from a higher potential to a lower one.

This means, for an n-channel FET, if the drain is biased higher than the source, current will flow from drain to source (through the channel). If the source is biased higher than the drain, current will flow from source to drain (through the body diode).

Now, Lets say I have connected Drain to higher than Source and current is flowing through the body diode, But at the same time I want to turn on the MOSFET Source to Drain channel to allow current to flow from the channel from source to drain parallel with body diode.

If you turn on the FET, you may get parallel conduction through the channel and body diode, but both currents will flow from drain to source, because the drain is at a higher potential.

Answer 2

Normally, when MOS transistor is polarized properly, current flows only in one direction, that is from drain to source IDS current. I hope that by the term "bidirectional device" you mean that source and drain can be switched? This is true. But nowadays, MOS transistors are not symmetric devices due to not 90 degrees during manufacturing source and drains. I would post a picture to illustrate this, but I have problems with finding the proper one (it's strange as I saw many of these previously - maybe I'll add the picture later).

Let's focus on NMOS. Proper polarization is as follows: B - bulk - lowest possible potential in the circuit. Let's say ground.

so:

Vbulk <= Vsource <= Vdrain and Vbulk <= Vgate <= power supply

In other words, source potential should not be lower than bulk potential and not higher than drain potential. Gate potential should be between bulk potential and power supply potential.

Regarding, the body in NMOS. There are two p-n diodes in body: 1. p-n bulk-source 2. p-n bulk-drain

However, when NMOS is polarized properly, as discussed above, these diodes are never polarized forwardly.

Hope this helps.