2N3819 JFET - max Vds parameter

[Paul K.]

Hi,

I'm working on a PSPICE simulation of a crystal oscillator using a
2N3819. So far it's working great.

I will build the circuit, but before I do, I wanted to make sure I'm
not exceeding the maximum drain-source voltage (Vds) of the
transistor. I'm putting approximately 22V across it.

However, none of the datasheets I could find listed the maximum
voltage that can be placed across a 2N3819 (from drain to source).

E.g. http://www.fairchildsemi.com/ds/2N/2N3819.pdf

Looking at a JFET tutorial however...

http://cnx.rice.edu/content/m1030/latest/

I get the impression that the drain-gate voltage is really what I
should be interested in. According to the datasheet, anything beyond
25V will exceed the device rating.

However, I'm curious though, what happens to a JFET when that voltage
rating (drain-gate) is exceeded? Does it avalanche like a MOSFET?
Would it survive?

Also, is there a reason why the drain-source parameter of a JFET is
not specified in its datasheet?

Thanks,

Paul

 

[legg]

Hi,

I'm working on a PSPICE simulation of a crystal oscillator using a
2N3819. So far it's working great.

I will build the circuit, but before I do, I wanted to make sure I'm
not exceeding the maximum drain-source voltage (Vds) of the
transistor. I'm putting approximately 22V across it.

However, none of the datasheets I could find listed the maximum
voltage that can be placed across a 2N3819 (from drain to source).

E.g. http://www.fairchildsemi.com/ds/2N/2N3819.pdf

Looking at a JFET tutorial however...

http://cnx.rice.edu/content/m1030/latest/

I get the impression that the drain-gate voltage is really what I
should be interested in. According to the datasheet, anything beyond
25V will exceed the device rating.

However, I'm curious though, what happens to a JFET when that voltage
rating (drain-gate) is exceeded? Does it avalanche like a MOSFET?
Would it survive?

Also, is there a reason why the drain-source parameter of a JFET is
not specified in its datasheet?

As the gate on the NJFet is negative with respect to the source, when
the fet is pinched off (or even in linear operation), the greatest
voltage stress occurs between gate and drain rather than between
source and drain.

Pinched-off drain current is seldom specified at a Vds greater than
half the rated Vgs or Vgd.

RL

 

[Leon Heller]

Hi,

I'm working on a PSPICE simulation of a crystal oscillator using a
2N3819. So far it's working great.

I will build the circuit, but before I do, I wanted to make sure I'm
not exceeding the maximum drain-source voltage (Vds) of the
transistor. I'm putting approximately 22V across it.

However, none of the datasheets I could find listed the maximum
voltage that can be placed across a 2N3819 (from drain to source).

E.g. http://www.fairchildsemi.com/ds/2N/2N3819.pdf

Looking at a JFET tutorial however...

http://cnx.rice.edu/content/m1030/latest/

I get the impression that the drain-gate voltage is really what I
should be interested in. According to the datasheet, anything beyond
25V will exceed the device rating.

However, I'm curious though, what happens to a JFET when that voltage
rating (drain-gate) is exceeded? Does it avalanche like a MOSFET?
Would it survive?

Also, is there a reason why the drain-source parameter of a JFET is
not specified in its datasheet?

Probably because the drain-source connection is basically a resistor.

Leon

 

[Winfield Hill]

Paul K. wrote...

However, none of the datasheets I could find listed the maximum
voltage that can be placed across a 2N3819 (from drain to source).
E.g. http://www.fairchildsemi.com/ds/2N/2N3819.pdf
[ snip ]
I'm curious though, what happens to a JFET when that voltage
rating (drain-gate) is exceeded? Does it avalanche like a MOSFET?
Would it survive?

The Siliconix Vishay 2N3819 JFET datasheet has curves of gate leakage
vs drain-to-gate voltage. http://www.vishay.com/docs/70238/70238.pdf
You'll note that the gate leakage current starts increasing from its
normally low level at a Vdg of only 8 volts. At 20V it has increased
by 5 orders of magnitude (assume a drain current of 1mA is flowing).
At some point this increasing gate current becomes intolerable.

You mention 22V Vds, but no doubt you've got Vgs = -1V etc., so that's
23V Vdg, which is beyond the curves shown in the datasheet, but we can
extrapolate and estimate a gate current approaching 1uA. The maximum
Vgs spec for this JFET is 25V minimum (for Ig = 1uA) and 35V typical.
As you can see, this isn't a sudden breakdown, but an increasing gate
leakage with an arbitrary 1uA value called the "breakdown voltage."
This leakage increases more rapidly if drain current is flowing. And
as we point out in our JFET gate-current discussion (see AoE page 137),
and show in a curve, at some point full-scale avalanche will occur.

 

[Paul K.]

Thanks everyone for your help.

JFETs are pretty interesting devices. In some ways the're much easier
to work with than BJTs.

 

[Jim Thompson]

Thanks everyone for your help.

JFETs are pretty interesting devices. In some ways the're much easier
to work with than BJTs.

Not for some of us ;-)

...Jim Thompson

 

[Paul Burridge]

Thanks everyone for your help.

JFETs are pretty interesting devices. In some ways the're much easier
to work with than BJTs.

They are conceptually easier to understand, but only suitable for a
small range of applications where they're indeed preferable to BJTs.
The biggest drawback with them is the large spread of parameters one
encounters with any given type number and the limited amount of gain
they can provide.