difference between gate-drain connection and gate-source connection

what is the difference between the gate-drain connected transistor or gate-source connected transistor which is supposed to act as a resistor.
I suppose, both should be the same as FET is a symmetric device.
But Am i wrong

help me


dan

Dear analog_curious ,
if u connect the gate of a mosfet to it's source,it'll be off.because their voltages are the same thing.
and if u connect the gate to the drain,it'll be like a diode and this diode has a dynamic resistance. (equal 1/gm)

Firstly, in many modern technologies you can choose which side is drain and which side is source with just biasing. However I do not think that they model them that way. So other than switching applications it is better to use them as they are, in simulations of course.

In layout they look mostly symmetrical and process vendor actually doesn't care which side is drain and which side is source. But there are specialized devices like LDMOS or some other. They are not symmetrical in any case.

Regarding your question I think the same as moottii but I must add that there are transistors that have negative threshold voltage. They will be conducting current even if their gate is connected to their source. I do not know why would anyone do that, it might work as a startup circuit.

But here is a suggestion; I believe if you bias your transistors properly you can see that the MOSFETs are symmetrical. First use a tech file (not a discrete mosfet they might not have symmetry), then simulate it with proper biasing for both cases. I've never done that myself if you achieve interesting results I will be eager to hear them.

analog_curious said:

I suppose, both should be the same as FET is a symmetric device.

Click to expand...

Hi Dan

Some JFETs are symmetrical but I'm not sure if they all are. Even with the symmetrical ones, there may be a difference in capacitances due to how it's physically constructed internally, layout of the bonding wires etc.

MOSFETs generally have a reverse-protection diode that makes it impossible to use them "backwards".

kemiyun said:

...there are transistors that have negative threshold voltage. They will be conducting current even if their gate is connected to their source. I do not know why would anyone do that...

Click to expand...

JFETs do that. They make quite good constant current sources with the gate connected to the source. They are even sold like that as two terminal devices called "constant current diodes" or "current limiting diodes". See here: https://en.wikipedia.org/wiki/Constant_current_diode

Cheers - Godfrey

godfreyl said:

MOSFETs generally have a reverse-protection diode that makes it impossible to use them "backwards".

Click to expand...

Complete nonsense!
A Mosfet used as inline reverse polarity protection is a prime example of "backwards" use.
The forward biased body diode and switched on mosfet proper, both provide for an even lower source-drain voltage drop than would be obtained with non "backwards" arrangement, where body diode is not conducting.

JFET's are symmetrical. Source and drain interchangeable.

Mosfets are a different story. Source and drain may or may not be interchangeable.

A search turned up one forum discussion where knowledgeable people said it depends.



I don't recall that all mosfets always had a body diode. I believe all (or practically all) have been manufactured with a zener diode to protect the insulated gate from temporary overvoltage.

analog_curious said:

what is the difference between the gate-drain connected transistor or gate-source connected transistor which is supposed to act as a resistor.

Click to expand...

If your main concern is the applicability of a JFET as a controlled resistor neither drain or source are connected to the gate,
The transistor behaves as a resistor for relatively small drain-source voltages (approx. < 0.5 volts).
When the source is grounded, the resistor value is controlled with the gate voltage.

ADI8421 said:

Complete nonsense!
A Mosfet used as inline reverse polarity protection is a prime example of "backwards" use.
The forward biased body diode and switched on mosfet proper, both provide for an even lower source-drain voltage drop than would be obtained with non "backwards" arrangement, where body diode is not conducting.

Click to expand...

what is non sense my pal

MOS specially DMOS can only be used in backward direction \

the common EMOS are not suiting that need

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analog_curious said:

what is the difference between the gate-drain connected transistor or gate-source connected transistor which is supposed to act as a resistor.
I suppose, both should be the same as FET is a symmetric device.
But Am i wrong

help me


dan

Click to expand...

only in the linear region of the JFET it will behave like a VVR in other regions it is not so

well, my little summary to the thread:
Integrated low voltage MOS are typically symmetrical, source and drain determined only by biasing.
Integrated high voltage MOS are typically not symmetric, because the high voltage capability is added only to the drain side (adds series resistance, not liked on source side, and costs floor space). You can make them symmetric, however, if you need.
Integrated DMOS are not symmetric do to space consuming high voltage capability on the drain side.
MOS devices with Vt<0 (depletion mode transistors) have been used as inverter load in NMOS enhancement depletion logic in the last century. Connecting gate and source may yield a nice 'conststant' current source, because you have one branch of MOS output characteristics, which is flat (nearly not depending on drain voltage) in saturation. They may have lower noise because the channel is buried somewhat in the substrate, away from the gate oxide Si interface. Typically they are symmetric.

Discrete MOSFETs: If you have four terminals and low voltage, they may be symmetric. You may tie source to bulk in the package and get a non symmetric three terminal device.
Power MOSFETs are non symmetric due to their voltage blocking capability on the drain side and vertical construction (source and gate on top of the chip, drain on the back).

Integrated JFETs (if you have any as special input devices for a CMOS process) are typically symmetric. Data sheets for discrete JFETs typically fix D, G, and S, so there must be some asymmetry.