unexpected negative voltage from JFETs...

  • Thread starter Gerhard Hoffmann
  • Start date
On a sunny day (Wed, 2 Dec 2020 12:08:04 +0100) it happened Gerhard Hoffmann
<dk4xp@arcor.de> wrote in <rq7sik$nqi$1@solani.org>:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard
Click to expand...

What happens when you connect say a 10nF poly capacitor
between gate and source, short it for a moment, wait a while
and measure again?
That should rule out any external RF detection by the gate-source diode.
 
Am 03.12.20 um 04:36 schrieb Phil Hobbs:
On 12/2/20 6:08 AM, Gerhard Hoffmann wrote:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard



ATF38143 depletion pHEMTs self-bias to about -0.3V if you leave the gate
open.
Click to expand...

That seems to be the same effect.

I have looked into Cobbold, Theory & application of Field Effect
Transistors, Wiley-Interscience, but this is like drinking from a
fire hose. :) And in paper form, it\'s hard to search.

But it seems that gate current is dominated by recombination-generation
effects in the channel and not so much by diode leakage.

Effective Vdd in my case is abt. 2.5 V, there is absolutely no RF
or oscillation involved, at least below 2.5 GHz.
Seems to be a pure DC effect.

I know that even the BF862 can show nasty gate current effects
at Vdd > 5V. They can even spoil the noise behavior, but @ 2V5?

Cheers, Gerhard
 
On 12/3/20 6:07 AM, Gerhard Hoffmann wrote:
Am 03.12.20 um 04:36 schrieb Phil Hobbs:
On 12/2/20 6:08 AM, Gerhard Hoffmann wrote:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard



ATF38143 depletion pHEMTs self-bias to about -0.3V if you leave the
gate open.

That seems to be the same effect.

I have looked into Cobbold, Theory & application of Field Effect
Transistors, Wiley-Interscience, but this is like drinking from a
fire hose. :) And in paper form, it\'s hard to search.

But it seems that gate current is dominated by recombination-generation
effects in the channel and not so much by diode leakage.

Effective Vdd in my case is abt. 2.5 V, there is absolutely no RF
or oscillation involved, at least below 2.5 GHz.
Seems to be a pure DC effect.

I know that even the BF862 can show nasty gate current effects
at Vdd > 5V. They can even spoil the noise behavior, but @ 2V5?

Cheers, Gerhard
Click to expand...

In a JFET, you\'d think that in the presence of free carriers, the
built-in E field would forward-bias the junction the way it does in
photodiodes. It\'s the sign of the effect that\'s strange.

I imagine it\'s because there aren\'t many free holes, just electrons--the
chemical potential makes electrons diffuse against the junction
potential into the gate. In a photodiode there are the same number of
electrons and holes.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Thu, 3 Dec 2020 07:18:46 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 12/3/20 6:07 AM, Gerhard Hoffmann wrote:
Am 03.12.20 um 04:36 schrieb Phil Hobbs:
On 12/2/20 6:08 AM, Gerhard Hoffmann wrote:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard



ATF38143 depletion pHEMTs self-bias to about -0.3V if you leave the
gate open.

That seems to be the same effect.

I have looked into Cobbold, Theory & application of Field Effect
Transistors, Wiley-Interscience, but this is like drinking from a
fire hose. :) And in paper form, it\'s hard to search.

But it seems that gate current is dominated by recombination-generation
effects in the channel and not so much by diode leakage.

Effective Vdd in my case is abt. 2.5 V, there is absolutely no RF
or oscillation involved, at least below 2.5 GHz.
Seems to be a pure DC effect.

I know that even the BF862 can show nasty gate current effects
at Vdd > 5V. They can even spoil the noise behavior, but @ 2V5?

Cheers, Gerhard



In a JFET, you\'d think that in the presence of free carriers, the
built-in E field would forward-bias the junction the way it does in
photodiodes. It\'s the sign of the effect that\'s strange.

I imagine it\'s because there aren\'t many free holes, just electrons--the
chemical potential makes electrons diffuse against the junction
potential into the gate. In a photodiode there are the same number of
electrons and holes.

Cheers

Phil Hobbs
Click to expand...

What happens in a p-fet?



--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.
 
On 12/3/20 7:18 AM, Phil Hobbs wrote:
On 12/3/20 6:07 AM, Gerhard Hoffmann wrote:
Am 03.12.20 um 04:36 schrieb Phil Hobbs:
On 12/2/20 6:08 AM, Gerhard Hoffmann wrote:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative
supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard



ATF38143 depletion pHEMTs self-bias to about -0.3V if you leave the
gate open.

That seems to be the same effect.

I have looked into Cobbold, Theory & application of Field Effect
Transistors, Wiley-Interscience, but this is like drinking from a
fire hose. :) And in paper form, it\'s hard to search.

But it seems that gate current is dominated by recombination-generation
effects in the channel and not so much by diode leakage.

Effective Vdd in my case is abt. 2.5 V, there is absolutely no RF
or oscillation involved, at least below 2.5 GHz.
Seems to be a pure DC effect.

I know that even the BF862 can show nasty gate current effects
at Vdd > 5V. They can even spoil the noise behavior, but @ 2V5?

Cheers, Gerhard



In a JFET, you\'d think that in the presence of free carriers, the
built-in E field would forward-bias the junction the way it does in
photodiodes.  It\'s the sign of the effect that\'s strange.

I imagine it\'s because there aren\'t many free holes, just electrons--the
chemical potential makes electrons diffuse against the junction
potential into the gate.  In a photodiode there are the same number of
electrons and holes.
Click to expand...

And that\'s probably the explanation for Pease\'s effect too: the B-E
avalanche collects generates a lot of carrier pairs. In a saturated
BJT, the frozen-in field is the reason the collector goes below base
potential, but that field gets zeroed out before the collector goes
below ground.

In a B-E avalanche, the collector collects charge in the same way, but
because the base is grounded, the collector goes negative from there,
producing the negative open-circuit voltage.

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On 12/3/20 10:47 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 3 Dec 2020 07:18:46 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 12/3/20 6:07 AM, Gerhard Hoffmann wrote:
Am 03.12.20 um 04:36 schrieb Phil Hobbs:
On 12/2/20 6:08 AM, Gerhard Hoffmann wrote:

I have here an array of 16 On Semi CPH3910 JFETs, n-chan,
all in parallel. Sources are at GND, drains via 50 Ohm to +5.5V Vdd.

The gates are open and I measure there -2V, there is no negative supply.
Gate leakage seems also large, no way to use a 1 Meg gate resistor.

I would expect that from a 12AX7 array, but JFETs??

I remember Bob Pease\'s question about a negative voltage on an open
Collector when the BE junction Zeners.
That was photons from BE Zener glow into the BC junction.

Is that normal?

NSVJ5908 duals behave in the same way.


Cheers, Gerhard



ATF38143 depletion pHEMTs self-bias to about -0.3V if you leave the
gate open.

That seems to be the same effect.

I have looked into Cobbold, Theory & application of Field Effect
Transistors, Wiley-Interscience, but this is like drinking from a
fire hose. :) And in paper form, it\'s hard to search.

But it seems that gate current is dominated by recombination-generation
effects in the channel and not so much by diode leakage.

Effective Vdd in my case is abt. 2.5 V, there is absolutely no RF
or oscillation involved, at least below 2.5 GHz.
Seems to be a pure DC effect.

I know that even the BF862 can show nasty gate current effects
at Vdd > 5V. They can even spoil the noise behavior, but @ 2V5?

Cheers, Gerhard



In a JFET, you\'d think that in the presence of free carriers, the
built-in E field would forward-bias the junction the way it does in
photodiodes. It\'s the sign of the effect that\'s strange.

I imagine it\'s because there aren\'t many free holes, just electrons--the
chemical potential makes electrons diffuse against the junction
potential into the gate. In a photodiode there are the same number of
electrons and holes.

Cheers

Phil Hobbs

What happens in a p-fet?
Click to expand...

When you reverse polarity? The engines canna\' take it, Captain.

(There are still not a lot of minority carriers around, so it ought to
bias itself positive.)

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
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