Typical ON resistance of a FET

[Froskoy]

Hi,

I was just wondering what a typical value for the ON resistance of a FET was? I've calculated 333kΩ, which seemed very high - I wanted to check whether this sounds about right or not?

With very many thanks,

Froskoy.

 

[ronsimpson]

I do not know of a FET with 333k ohms on resistance. (or 33k or 3k)
I assume MOSFETs: I searched 26368 mosfet data sheets and found a range of 500 ohms for a very small device to 0.000,45 ohm for a very large part.
I have never used a part with more than 9 ohms and less than 0.004 ohms.

 

[dr peppers]

If the fet is operating in triode or saturation mode then I suspect something is wrong with your calculation, or the fet has been designed for a very particular application.

How did you work out the rds, it does sound like you ought to be saying ohms or even milli-ohms rather than k ohms.

 

[Sceadwian]

How are you measuring the resistance of the FET? There's probably something wrong with your measurement method. Either that or your FET is actually off or in it's saturation region as dr peppers said. Even 333 ohms is very high for anything other than a high voltage FET. Even on very small devices rds on should be bellow about 20 ohms.

 

[ronsimpson]

May he is measuring the off resistance. 333k ohms

 

[Sceadwian]

Only if it's a failed mosfet ron, typical off state resistance for FET's are in the meg range.

I'd like to hear Froskoy's test method first.

 

[lebevti]

if in the triode region, it should be low (10 Ohm)

if ur in saturation it would be high resistance

 

[WTP Pepper]

Hi,

I was just wondering what a typical value for the ON resistance of a FET was? I've calculated 333kΩ, which seemed very high - I wanted to check whether this sounds about right or not?

With very many thanks,

Froskoy.

Assuming it's an N channel MOSFET, anything less than 1 ohm is rubbish. For the off resistance, you are looking at tens of megaohms.
No FET I am aware of as more than 100ohm on resistance.

 

[ronsimpson]

Assuming it's an N channel MOSFET, anything less than 1 ohm is rubbish.

What???? Many parts have less than 1 ohm. The FDS5670 is a 10A part with 0.014 ohms of on resistance.

For the off resistance, you are looking at tens of megaohms.

Probably, if your fingers are on the probe then 333k is about right. If the MOSFET is hot then there is on current. (low resistance) I just look at a data sheet that shows 1mA of off current at 150 degrees C.

No FET I am aware of as more than 100ohm on resistance.

A quick search shows 24 MOSFETs with more than 100 ohms on resistance. ZVN1409 is 250 ohms.

 

[WTP Pepper]

A typo on my part. Most have an ON resistance of <1ohm

 

[MrAl]

Hi,

Is he talking FET as in JFET or FET as in MOSFET? Big difference.

 

[ronsimpson]

Is he talking FET as in JFET or FET as in MOSFET? Big difference.

With FET being used as JFET and MOSFET it is hard to know. Either way; 333k ohms on resistance???? I dough we will ever know the part number.

 

[Sudhir Hanumanthu]

Hi,
I too got the same problem as Froskoy.
I've got OFF resistance nearly 332 K Ohms and ON resistance as 11.98 K Ohms.
I've been calculating this resistance for a 45nm NMOS transistor my method is I've plotted Id vs Vds characteristics by sweeping Vgs from 0 to 1 and then I've calculated ON resistance using Ron=Vdd max(i.e at 1V)/Id (at vgs=1)
Roff= Vdd max/Id (at vgs<vth say at 0.3v)
I'm not sure whether this method of calculation is right or wrong.
So, Please kindly help me.
Thanks,

Sudhir.