Hi guys,

I saw in Malvino's Electronic Principles that it is stated that Idss
and gfs (the transconductance/gain) are easy to measure, whereas
Vgs(off) is not and that manufacturers calculate it from this formula
(hope I've remembered it right)

Vgs(off) = -2*Idss/gfs

I've just checked out this assertion by measuring Vgs - v - Id for a
bunch of assorted FETs and found that I could easily establish the
pinch off voltage to within about 0.1V either way. Contrary to what
the book says, I personally have found it a simple matter to measure
Vgs(off). So why do they make out it's a big deal?

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

"Paul Burridge" schrieb im Newsbeitrag
...
Hi guys,

I saw in Malvino's Electronic Principles that it is stated that Idss
and gfs (the transconductance/gain) are easy to measure, whereas
Vgs(off) is not and that manufacturers calculate it from this formula
(hope I've remembered it right)

Vgs(off) = -2*Idss/gfs

I've just checked out this assertion by measuring Vgs - v - Id for a
bunch of assorted FETs and found that I could easily establish the
pinch off voltage to within about 0.1V either way. Contrary to what
the book says, I personally have found it a simple matter to measure
Vgs(off). So why do they make out it's a big deal?

Hello Paul,
Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

How have you measured at exactly Id = 1nA +/-0.1nA ?

Best Regards,
Helmut

A typical datasheet:
http://www.fairchildsemi.com/ds/J3/J310.pdf

On Tue, 21 Sep 2004 20:56:00 +0200, "Helmut Sennewald"
wrote:

Hello Paul,
Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

How have you measured at exactly Id = 1nA +/-0.1nA ?

Hi Helmut,

Is it 1nA? I thought it was 5. No matter.
Yes, I'd expected someone to point out that the "negligible current"
point was the likely problem area. I can't honestly say that I have,
because my DVM drops out at 0.01mA! However, in the context of the
wide spread of parameters one encounters with FETs., I'm pretty
confident my 'drop-out' zone for current measurement is not too far
off the mark. But you've answered my question and as ever I'm grateful
to you for that.
I actually found it more difficult measuring Id as Vgs approached
zero. The negative tempco of these devices made that part more
challenging. Fortunately I've got a 'peak-hold' button on my meter and
by only connecting the drain circuit for a fraction of a second I was
able to get what I believe to be a valid reading. Certainly good
enough given the cushion one has to build into FET circuit design as a
matter of course, anyway.

A typical datasheet:
http://www.fairchildsemi.com/ds/J3/J310.pdf

Ah, I see they use a 300uS pulse, presumably to keep the device
temperature down?

Regards,

p.

In sci.electronics.design Paul Burridge wrote:
On Tue, 21 Sep 2004 20:56:00 +0200, "Helmut Sennewald"
wrote:

Hello Paul,
Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

How have you measured at exactly Id = 1nA +/-0.1nA ?

Hi Helmut,

Is it 1nA? I thought it was 5. No matter.
Yes, I'd expected someone to point out that the "negligible current"
point was the likely problem area. I can't honestly say that I have,
because my DVM drops out at 0.01mA! However, in the context of the

Turn it to voltage.
Most meters (the four I've measured) have a 1Mohm or so input impedance
on the 200mv range.
This is 200nA full scale.
Probably best to do the test twice, with the meter leads reversed though.

Ian Stirling wrote...

Paul Burridge wrote:
Helmut Sennewald wrote:

Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

Is it 1nA? I thought it was 5. No matter.
Yes, I'd expected someone to point out that the "negligible current"
point was the likely problem area. I can't honestly say that I have,
because my DVM drops out at 0.01mA! However, in the context of the

Turn it to voltage. Most meters (the four I've measured) have a 1Mohm
or so input impedance on the 200mv range. This is 200nA full scale.
Probably best to do the test twice, with the meter leads reversed though.

The multimeters on my bench have infinite input impedance on the 200mV
scale, although some let you turn on an internal resistor. We can use
external 10M or 100M if we like, which gives 1pA and 0.1pA measurement
resolution with a 4.5-digit meter.

The relevant "subthreshold" formula is Id = k e^(Vgs - Vt), which is
an exponential equation that clearly shows there's no sudden threshold
for FET current = negligible. Paul can take a look at AoE page 123,
and observe the measured gate-voltage to drain current relationship for
a typical MOSFET, which shows the standard smooth 100 or 150mV/decade
(p or n-type) gate-voltage change over a wide 7-decade current range.

1 or 5nA makes no difference? Nope, that apparently small detail makes
a predictably large 70 to 100mV difference. One must pay attention to
the specifications in this matter, 1nA, 1uA, whatever - it's a big deal.

BTW, as I've pointed out several times, power MOSFET spice models are
completely wrong in this region, just forget considering their results.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

Thanks, all, BTW.

On 21 Sep 2004 19:50:22 -0700, Winfield Hill
wrote:

The multimeters on my bench have infinite input impedance on the 200mV
scale, although some let you turn on an internal resistor. We can use
external 10M or 100M if we like, which gives 1pA and 0.1pA measurement
resolution with a 4.5-digit meter.

I admit I never thought of doing it this way. Incidentally, talking of
very low currents, on P.170 you've reproduced a picture of a static
damaged MOSFET. I assume from the commentary that this one was totally
trashed, but you've stated before that these devices can be partially
blown and still function, albeit to an impaired extent. Could one
feasibly measure any such 'minor damage' (through static) by checking
for picoamp range current leakage across the insulating layer?

The relevant "subthreshold" formula is Id = k e^(Vgs - Vt), which is
an exponential equation that clearly shows there's no sudden threshold
for FET current = negligible. Paul can take a look at AoE page 123,
and observe the measured gate-voltage to drain current relationship for
a typical MOSFET, which shows the standard smooth 100 or 150mV/decade
(p or n-type) gate-voltage change over a wide 7-decade current range.

1 or 5nA makes no difference? Nope, that apparently small detail makes
a predictably large 70 to 100mV difference. One must pay attention to
the specifications in this matter, 1nA, 1uA, whatever - it's a big deal.

Oh bugger.

BTW, as I've pointed out several times, power MOSFET spice models are
completely wrong in this region, just forget considering their results.

Yes, I think the same caveat applies for simulating FETs - and that's
just in the active region!

--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Tue, 21 Sep 2004 20:56:00 +0200, "Helmut Sennewald"
wrote:

"Paul Burridge" schrieb im Newsbeitrag
.. .
Hi guys,

I saw in Malvino's Electronic Principles that it is stated that Idss
and gfs (the transconductance/gain) are easy to measure, whereas
Vgs(off) is not and that manufacturers calculate it from this formula
(hope I've remembered it right)

Vgs(off) = -2*Idss/gfs

I've just checked out this assertion by measuring Vgs - v - Id for a
bunch of assorted FETs and found that I could easily establish the
pinch off voltage to within about 0.1V either way. Contrary to what
the book says, I personally have found it a simple matter to measure
Vgs(off). So why do they make out it's a big deal?

Hello Paul,
Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

How have you measured at exactly Id = 1nA +/-0.1nA ?

Best Regards,
Helmut

A typical datasheet:
http://www.fairchildsemi.com/ds/J3/J310.pdf


So, with not too much fault you may use a VTVM as load and see which
voltage it reads....


---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

International Rectifier has a white-paper on characterizing MOSFET's --
kind of like comparing a Caterpillar D-9 to a bicycle -- but all of the
standardized measurement methods are described in detail.

If you have to measure nano, pico or femto -- Bob Pease had this interesting
article on the NatSemi website about a half-dozen years ago:

http://www.national.com/rap/Story/0,1562,5,00.html


"Paul Burridge" wrote in message
...
Hi guys,

I saw in Malvino's Electronic Principles that it is stated that Idss
and gfs (the transconductance/gain) are easy to measure, whereas
Vgs(off) is not and that manufacturers calculate it from this formula
(hope I've remembered it right)

Vgs(off) = -2*Idss/gfs

I've just checked out this assertion by measuring Vgs - v - Id for a
bunch of assorted FETs and found that I could easily establish the
pinch off voltage to within about 0.1V either way. Contrary to what
the book says, I personally have found it a simple matter to measure
Vgs(off). So why do they make out it's a big deal?

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.