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Vbe stuff

R

Robert Baer

Jan 1, 1970
0
Fred said:
I suppose it has more to do with a design goal minimizing conduction
losses in the high current diode thereby prohibiting the use of very
large ratios of majority doping material density as with the transistor.
This will cause a departure from the simplified minority carrier density
at the transition region boundaries as a function of forward bias
because the minority carrier density on both sides of the transition
region change significantly with a coupled dependence.
That would explain the large discrepancy noted for the 1N4006; thanks.
But the MPSA42????
 
J

Jim Thompson

Jan 1, 1970
0
The MPSA42 has a multi-milliamp rating,and should follow the log
relationship to at least 10mA or more; i measured at 100uA max - so
internal resistance is not significant.

Did you tie base to collector before measuring as a diode?

...Jim Thompson
 
J

Jim Thompson

Jan 1, 1970
0
That would explain the large discrepancy noted for the 1N4006; thanks.
But the MPSA42????

Cockpit error ?:)

...Jim Thompson
 
R

Robert Baer

Jan 1, 1970
0
Jonathan said:
But isn't forward resistance a valid part of this measurement being
discussed?

Jon
At 100uA, the IR drop is not significant.
 
R

Robert Baer

Jan 1, 1970
0
Eeyore said:
Jonathan Kirwan wrote:




I'm confident of it. I've never seen rbb specified for devices like the MPSAs
but I have a vague recollection of seeing a figure of ~ 1k somewhere once for
small signal parts. If so, the 100uA would account for 100mV in the resisitve
part of Vbe.

Graham
....and if so, that would make the measurement *larger*; the values seen
are !smaller! than what Mr Pease mentioned.
Besides,i stated that the DCT more was used, not a simple E-B
junction reading.
 
J

Jim Thompson

Jan 1, 1970
0
...and if so, that would make the measurement *larger*; the values seen
are !smaller! than what Mr Pease mentioned.
Besides,i stated that the DCT more was used, not a simple E-B
junction reading.

OK. I grabbed the MPSA42 model out of the PSpice libraries.

Result:

Logarithmic from ~1uA to ~10mA

Below 1uA, deviation due to ISE (recombination)

Above 10mA, deviation due to ?? Without me spending any time
thinking, could be IKF (current crowding around the edges of the
emitter) and/or resistance.

...Jim Thompson
 
J

Jonathan Kirwan

Jan 1, 1970
0
OK. I grabbed the MPSA42 model out of the PSpice libraries.

Result:

Logarithmic from ~1uA to ~10mA

Makes perfect sense. That's about the right range.
Below 1uA, deviation due to ISE (recombination)

That would be carrier recombination at the surface and in the space
charge layer, and the formation of surface channels. Each are
separate parts, lumped.
Above 10mA, deviation due to ?? Without me spending any time
thinking, could be IKF (current crowding around the edges of the
emitter) and/or resistance.

"The injection of minority carriers into the base region becomes
significant as compared to the majority carrier concentration. Since
space charge neutrality is maintained within the base, the total
majority carrier concentration is increased by the same amount as the
total minority carrier concentration."

Or so I'm told.

At high levels, the collector current asymptotes to e^(q*Vbe/(2*k*T))
according to W.M. Webster, "On the Variation of Junction-Transistor
Current-Amplification Factor with Emitter Current," Proc. IRE, Vol.
42, pp. 914-920, June 1954.

I can quote and understand just some. But I'm a hobbyist so I'll
leave the rest to you folks.

Jon
 
J

Jonathan Kirwan

Jan 1, 1970
0
At 100uA, the IR drop is not significant.

I was asking. In the case of high voltage _diodes_ (and this is one,
yes?), I was absorbing what I'm reading here and imagining the case
where the dopant levels are low and the effective resistance higher as
a result. How much higher, I've no idea. Keep in mind I'm merely a
hobbyist here learning from you folks.

You were talking about roughly 25mV (slightly more) than the usual
expectation of between 59mV and 60mV per decade of current at that
region. So about 25mV/90uA or about 270 ohms. That seems troubling
at first, but there is another place where this can appear in the
equation, which is:

I = Is*(e^(q(V-I*R)/kT)-1)

I hadn't worked that out for this case or any other, but I've seen it
present in some papers and I wondered.

Also, the emission coefficient of the 4006 is very close to 2. (1.984
in one model I see) and that definitely has an effect, I'd imagine.
Whether your actual diode has that value is another thing. But there
it is and that would _not_ be the resistance.

Anyway, I'd reading this with interest.

Jon
 
E

Eeyore

Jan 1, 1970
0
Robert said:
...and if so, that would make the measurement *larger*;
Yes.

the values seen
are !smaller! than what Mr Pease mentioned.

That's not what I read.

Besides,i stated that the DCT more was used, not a simple E-B
junction reading.

DCT ?

Graham
 
M

Michael A. Terrell

Jan 1, 1970
0
Jonathan said:
I took it to mean "diode coupled transistor."

Jon


Don't you mean "Diode Connected Transistor"?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

Jonathan Kirwan

Jan 1, 1970
0
Don't you mean "Diode Connected Transistor"?

Actually, I thought both phrases at the same time, but wrote only one
of them. The picture was the same. The words weren't.

Jon
 
F

Fred Bloggs

Jan 1, 1970
0
Jim said:
As someone else pointed out, the 1N4xxx devices are also very lightly
doped (almost PIN-diode-like) to get the "high-voltage" performance.

But, Fred, your dissertation sounds just like that... dissertation out
of the mouth of some _twisted_ PhD ;-)

...Jim Thompson

I am miles ahead of you, and this using the most elemental analysis of
the diode Is*(exp(V/VT)-1) diode equation. For low level injection,
which is certainly the case in the example measurements, the
volts-per-decade characteristic derives precisely from Vt and nothing
else. The measurement deviations are not about non-linearity due to Is
dependence on injection level or anything like that. So the explanation
of the behavior goes directly to the estimation of THE POTENTIAL DROP
ACROSS THE TRANSITION REGION AND THE ACCOMPANYING MINORITY CARRIER
CHARGE DENSITIES AT THE BOUNDARIES! Charge neutrality in steady state
requires that the majority carrier densities increase by the same amount
as the minority density at the boundaries, and this creates the
dependence of each minority carrier density upon the other. The
simplified model of say n-sub-P being in ratio to n-sub-N by the factor
exp(V/Vt) no longer holds when the doping densities in the two
complementary regions are of the same order. Anything having to due with
surface diffusion, recombination rates, and/or diffusion lengths all
influence an Is non-linearity and having nothing to do with transition
region fields and potentials. The transition region is the purest
structure in the device, and it is this that determines the
volts-per-decade current dependence.
 
J

Jim Thompson

Jan 1, 1970
0
I am miles ahead of you, and this using the most elemental analysis of
the diode Is*(exp(V/VT)-1) diode equation. For low level injection,
which is certainly the case in the example measurements, the
volts-per-decade characteristic derives precisely from Vt and nothing
else. The measurement deviations are not about non-linearity due to Is
dependence on injection level or anything like that. So the explanation
of the behavior goes directly to the estimation of THE POTENTIAL DROP
ACROSS THE TRANSITION REGION AND THE ACCOMPANYING MINORITY CARRIER
CHARGE DENSITIES AT THE BOUNDARIES! Charge neutrality in steady state
requires that the majority carrier densities increase by the same amount
as the minority density at the boundaries, and this creates the
dependence of each minority carrier density upon the other. The
simplified model of say n-sub-P being in ratio to n-sub-N by the factor
exp(V/Vt) no longer holds when the doping densities in the two
complementary regions are of the same order. Anything having to due with
surface diffusion, recombination rates, and/or diffusion lengths all
influence an Is non-linearity and having nothing to do with transition
region fields and potentials. The transition region is the purest
structure in the device, and it is this that determines the
volts-per-decade current dependence.

Fred, Thank you for the quote from some textbook, but an engineer
finds it more useful to think in terms of current crowding (IKF) and
dirty junctions (recombination, ISE and associated terms).

The "most elemental" equation doesn't demonstrate those conditions.

I've posted those two pages of equations before. If you missed it I
will post them again.

...Jim Thompson
 
E

Eeyore

Jan 1, 1970
0
Jonathan said:
I took it to mean "diode coupled transistor."

I assumed that was what was meant but couldn't figure a suitable
acronym.

Configured maybe ?

Graham
 
R

Robert Baer

Jan 1, 1970
0
Jim said:
Did you tie base to collector before measuring as a diode?

...Jim Thompson
"DCT" mode was used, as i mentioned - that is, the base was connected
to the collector during measurement.
 
R

Robert Baer

Jan 1, 1970
0
Jonathan said:
What's wrong with 59mV per decade??

Jon
Nothing.
But note the measured Vbe is significantly lower than what Mr. Pease
mentioned.
Hence the query.
 
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