Re: Q to Win re:oscillation in high-voltage MOSFETS

Winfield Hill wrote:

Robert Baer wrote...

I have tried the models for the FQD2N100TM FET and have some questions.
1) The log(Is) vs Vgs relationship does not hold below about 1mA, and i need that to work to at least 1uA; preferably to 10nA.
2) I see nonlinear *gate current*: 10uA at 0V, 0uA at 50V with "knees" near 5V and 9V. I had the mistaken impression that gates were insulated from the channel, and that there were no batteries inside FETs.
3) Could you please provide a resistive SPICE single-FET circuit that might oscillate?


You're asking about the model I posted, the second one? That model is
suppose to work correctly, even to the nA region. Actually, as I said,
I developed my own modified MOSFET model before Steven Sandler's paper
came out, so it's different from his style, which was what I posted. That is, I didn't run his model in my extensive tests, having completed
my 2500V amplifier analysis before his paper. So I'll have to go back
and test and evaluate his model to verify its proper operation, and to
observe the problems you mention above. This will take more time than
I have right now, so I'll see if I can do it this coming weekend. But
don't hold your breath, because now I'm getting into the intense work
rampup prior to my going away on vacation. This may therefore be a
subject visited far in the future. But you could email me the circuits
you played with and I'll put everything in a folder to examine later.


Using LTspice, i get an error message:
Fatal Error: Too many parameters for subcircuit type "fqd2n100" (instance: xm1)
TopSpice does not complain, and the following shows a problem:
Test Fet (TF.CIR)
..OPTIONS ACCT LIST NODE OPTS
..TEMP 27 ;90
..DC Vg 4.50 5.00 0.001
Rin 91 01 1E6 ;Vg VS Is Looks good if low value
Rdo 92 02 1E-3
Vd 92 00 100V
Vg 91 00 4.7V
* D G S
X1 02 01 03 FQD2N100
Rs 03 04 1E-3
V2 00 04 0V ;MEASURE Is
..PRINT I(Rs)
..PLOT I(Rs)
..SAVE

..SUBCKT FQD2N100  d  g  s       ;Note mod on designations - no help
Rg  g 1 0.04
M1 2 1 3 3 DMOS L=1u W=1u
..MODEL DMOS NMOS(VTO=4.66  KP=1.9  LEVEL=3)
Cgs 1 3 380p
Rd  d 4 3.5
Dds 3 4 DDS
..MODEL DDS D(BV=1050  M=0.42  CJO=35p  VJ=0.12)
Dbody 3  d DBODY
..MODEL DBODY D(IS=2.8E-13  N=1.00  RS=0.005  EG=1.10  TT=520n)
Ra 4 2 3.5
Rs 3 5 0.024
Ls 5  s 2.6n
M2 1 8 6 6 INTER
E2 8 6 4 1 2
..MODEL INTER NMOS(VTO=0  KP=10  LEVEL=1)
CGDMAX 7 4 380p
RCGD 7 4 1E7
DGD 6 4 DGD
RDGD 4 6 1E7
..MODEL DGD D(M=0.52  CJO=380p  VJ=0.12)
M3 7 9 1 1 INTER
E3 9 1 4 1 -2
..ENDS

..END
.

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