Do MOSFETs conduct in both directions?

[tomasz1000]

If the gate of an N-channel MOSFET is properly positive wrt the source, will the MOSFET conduct when the source is positive wrt the drain?

 

[bhav]

it cannot conduct in both directions simultaneously, but it can conduct both ways as Source and Drain are interchangeable

 

[goldsmith]

Dear tomasz
Hi
If you use a jfet , of course it can work in both directions , but a mosfet can't ! although i have seen some special mosfets with high prices that could conduct in two directions . but i think you are talking about usual mosfets ?
Best Wishes
Goldsmith

 

[varunkant2k]

If the gate of an N-channel MOSFET is properly positive wrt the source, will the MOSFET conduct when the source is positive wrt the drain?

Yes if you use it in the triode region as a switch, it can conduct both way. any of higher voltage across source/drain will go to drain/source.

 

[godfreyl]

The problem is that MOSFETs normally have an internal diode between source and drain, so the reverse voltage is very limited.

 

[goldsmith]

The problem is that MOSFETs normally have an internal diode between source and drain, so the reverse voltage is very limited.

Hi all
I'm agree with Godfreyl , most of mosfets has an internally anti parallel diode . but there are some old types or some families without that diode but it is a bit hard to find them , these days . ( for linear signal amplification in both directions )
Respectfully
Goldsmith

 

[Baas Rietrot]

If you use the four terminal MOSFET (drain, base source and bulk) like in CMOS design you will see that the device
is symmetrical in layout. Thus you can flip it around or interchange the drain and the source terminals. But not with
the other layouts.

 

[goldsmith]

Hi Baas Rietrot
Can you introduce on of these mosfets , please ? ( part number ) .
Regards
Goldsmith

 

[FvM]

If the gate of an N-channel MOSFET is properly positive wrt the source, will the MOSFET conduct when the source is positive wrt the drain?

Yes. But for ususal power MOSFET with substrate connected to source, the reverse Vds is limited to a diode forward voltage. Active operation with reverse Vds is nevertheless important for synchronous switching.

As an example of a "four terminal" MOSFET

 

[Baas Rietrot]

Most of the time it is only used in IC design (CMOS) and layout,
but here is some symmetrical off the shelf components.

A few 4-terminal devices (drain, gate, source and bulk):

MIC94030	(p-channel MOSFET)
MIC94050	(p-channel MOSFET)
BSD22	(n-channel MOSFET)
BSS83	(n-channel MOSFET)

Some other Symmetrical devices:

BFT46	(n-channel FET)
BF862	(n-channel JFET)
PMBFJ620	(n-channel FETs)

Note: Some of these does contain internal diodes, but not the
same as high power MOSFETS.

 

[ZekeR]

As you're aware, when you purchase a discrete MOSFET, the body is usually electrically connected to the source. Some here have argued that if it weren't for this connection, the two would be entirely interchangeable. While this is true for low-voltage MOSFETs (which are typically used for logic gates in digital circuits or for pass transistors), for higher voltage ratings the drain and source are not usually made symmetrical.

For most purposes involving higher voltages (power switching, analog amplification, etc), engineers realize that the drain must be made to withstand more voltage, while the source will be maintained near the body voltage. Therefore, they customize the drain to be able to take a larger voltage without breaking (they make the drain bigger and decrease the doping concentration to increase the size of the depletion region). It would be wasteful to do the same for the source, since it won't be used the same way as the drain. Not to mention, the body must have the lowest potential within the transistor, so high voltage operation will place great stress on the gate-to-body dielectric (which is unfortunately the weak spot in standing off voltage; it is necessarily made thin for transistor operation).

For most discrete MOSFETs, even if you had access to the body pin separate from the source, it wouldn't do you much good. The transistor just wasn't engineered to work very well the other way; its breakdown voltage would be very low. Since they've already done lots of work assuming that the source will have the same potential as the body, they might as well connect them before selling the transistor to you (it saves them a pin, plus the headache of nagging customer questions on why the MOSFET broke).

MOSFETs will conduct equally in both directions when they are turned "on." An interesting consequence of the body-source connection is that, even if you turn the transistor "off," it will still conduct in the reverse direction. This is because of the PN junction formed between the body (P) and drain (N) (for N-channel). So if you apply a positive voltage to the drain and a negative voltage to the source, when the MOSFET is on, you will see current flowing with very little voltage; when the MOSFET is off, current will still flow, and the voltage will be ~0.7V (the diode on-voltage). Power MOSFETs generally have their source connected to their body, so if you wanted to prevent power flow in both directions, you would need to put two MOSFETs back-to-back (sources connected together).

For a little perspective, BJT's also conduct backwards, and they also have current gain when used the wrong way! Like the MOSFETs, however, their reverse capabilities are pretty much useless: the current gain hFE is typically around 4 (it's typically ~100 when used the right way), and the emitter-to-base breakdown voltage is typically around 5V (when the collector-to-base breakdown is more typically ~40V). You'll probably never see a BJT used backwards.

 

[varunkant2k]

For most discrete MOSFETs, even if you had access to the body pin separate from the source, it wouldn't do you much good.

Hi Zeker, yes you are right if you are talking about not generic MOSFET but DEMOS of DMOS (where drain is doped to bear the high voltage supply.
But generic MOSFET (generally 5V or lover devices) made equally symmetric and if their bulk(sub) is not connected internally, you can use it as an ideal switch (that conducts both way). Triode region makes the drain voltage equal to source voltage, while in saturation operation (as a gain device) it may show diode drop in one direction.

 

[grizedale]

If the gate of an N-channel MOSFET is properly positive wrt the source, will the MOSFET conduct when the source is positive wrt the drain?

...yes , just look at LLC convertes and fluorescent tube drivers.

 

[ZekeR]

Hi Zeker, yes you are right if you are talking about not generic MOSFET but DEMOS of DMOS (where drain is doped to bear the high voltage supply.

Thank you varunkant2k, but DMOS MOSFETs aren't the only variety of MOSFETs in which the drain is doped more lightly than the source in order to withstand higher voltage.

But generic MOSFET (generally 5V or lover devices) made equally symmetric and if their bulk(sub) is not connected internally, you can use it as an ideal switch (that conducts both way). Triode region makes the drain voltage equal to source voltage, while in saturation operation (as a gain device) it may show diode drop in one direction.

Bleh, this doesn't warrant a thorough response, other than to note that the vast majority of commercially available MOSFETs (which I will henceforth define as "generic") are made asymmetric to improve voltage breakdown. Final note, a MOSFET in saturation whose body is tied to the most negative potential in the system only behaves as a diode if its drain is attached to its gate. For a "generic" FET whose source is tied to its body, body diode conduction occurs during cutoff.

 

[FvM]

But generic MOSFET (generally 5V or lover devices) made equally symmetric and if their bulk(sub) is not connected internally, you can use it as an ideal switch (that conducts both way). Triode region makes the drain voltage equal to source voltage, while in saturation operation (as a gain device) it may show diode drop in one direction.

I'm under the impression, that several discussion points are still mixed - or even confused - in the discussion.

- Does a MOSFET conduct with reverse Vds?
- Can a reverse Vds voltage be switched-off? Or is it shorted by a reverse diode?
- Does the device show fully symmetric behaviour when exchanging D and S
- What's the voltage rating in reverse operation? (somehow related to Q3)

Because all these questions have been already addressed in the discussion, I don't feel a need to answer them once again.

The first question, which has been at least literally asked in the initial post can be clearly answered with "yes" for all MOSFETs, independent of their design details, respectively how Q2 to Q4 can be answered for it.

 

[markbool]

very useful information..