Step-by-step guide to digitizing and modeling Static Induction
Transistor characteristics
Overview
This tutorial guides you through creating a SPICE model for a Static
Induction Transistor (SIT) from published characteristic curves. SITs
are solid-state devices that exhibit triode-like characteristics.
The workflow consists of two main phases:
Digitization - Extract numerical data points from a
graph image using the Characteristic Curve Digitizer
Curve Fitting - Fit the digitized data to the
Rothacher SIT model using the SIT Curve Fitter
The result is a behavioral SPICE subcircuit that accurately models the
SIT's I-V characteristics.
About Static Induction Transistors
SITs are voltage-controlled devices with characteristics similar to
vacuum tube triodes:
Normally-on - Conduct current with zero gate
voltage
Triode-like curves - No distinct saturation region
like MOSFETs
Find an SIT output characteristics graph showing Id vs
Vd curves at various Vg values. Good sources
include:
Manufacturer datasheets (Sony, Tokin, Yamaha)
Application notes and technical papers
Curve Tracer Images
Save the graph as a PNG or JPG image file.
Note: SIT curves use different notation than triodes:
Vg (gate voltage) instead of Vgk, Vd
(drain voltage) instead of Vak, and Id (drain
current) instead of Ia.
2
Load the Image
Open the Characteristic Curve Digitizer and click "Choose File" to
load your image. Enter the device name as the project name (e.g.,
"2SK180").
3
Calibrate the Axes
Define three calibration points:
Origin - Click "Set Origin" then click on the
(0,0) point
X-Axis Point - Click "Set X-Axis" then click on
a known Vd value (e.g., 80V)
Y-Axis Point - Click "Set Y-Axis" then click on
a known Id value (e.g., 1.5A)
Enter the actual voltage and current values in the calibration
fields.
Important: Some SIT datasheets show current in
milliamps (mA) but the curve fitter expects Amps. Make sure to
convert: 1500mA = 1.5A. Set your Y calibration accordingly.
4
Create Data Series
Each curve at a different Vg value becomes a separate
series. Click "Add Series" and enter the gate voltage. For SITs,
Vg values are typically 0, -2, -4, -6, -8, -10, -12V,
etc.
5
Digitize Points
With a series selected, click along the curve to add data points:
Work from left to right along each curve
Add extra points in the "knee" region where current rises
sharply
Include points in the linear region at higher Vd
Press Z to undo mistakes
Use zoom for precision in crowded areas
Tip: SIT curves often have a distinct knee where the
device turns on. Capture several points in this transition region for
better model accuracy.
6
Repeat for All Curves
Create a series for each Vg curve shown in the
datasheet. Include curves from Vg = 0V down to the most
negative value shown.
7
Export the Data
Click "Download CSV" to save your digitized data. The file will be
named based on your project name (e.g., "2SK180.csv").
CSV File Format
The exported CSV uses the same format as the triode data:
Series 1: 0,
0.385,0.015
0.399,0.121
0.537,0.202
...
Series 2: -2,
1.280,0.023
1.927,0.091
...
Each series header contains "Series N: Vg" where Vg is the gate
voltage, followed by "Vd,Id" data pairs.
Click "Choose File" and select your exported CSV. The data summary
shows the number of curves and total data points loaded.
2
Review Initial Parameters
The default values work for most SITs. If you have prior knowledge
of the device, you can adjust:
K - Gain constant (affects overall current
magnitude)
X - Power law exponent (typically 1.5-2.5)
μ (mu) - Amplification factor (typically 5-50)
Vct - Threshold voltage offset
N - Logarithmic coefficient
3
Run the Curve Fit
Click "Run Curve Fit" to start optimization. The Nelder-Mead
algorithm will iteratively adjust parameters to minimize the error
between the model and your data.
4
Evaluate the Fit
Check the results:
RMSE - Should be small relative to typical
currents (< 0.01A is good)
Visual comparison - Fitted curves should
closely follow the data points
Knee region - Pay attention to how well the
turn-on behavior is captured
Troubleshooting: If the fit is poor, try adjusting
the initial μ value. SITs with very different characteristics may need
different starting points. You can also try running the fit multiple
times as the optimizer may find different local minima.
5
Adjust Plot Scale
Use the "Vd Max" and "Id Max" fields to adjust the plot range to
match your device's operating region. Click "Update Plot" to
refresh.
6
Export the SPICE Model
The SPICE Subcircuit Model panel shows a behavioral model using
the fitted parameters. Click "Copy to Clipboard" or "Download
.txt" to save.
