GmIDVisualizer

MOSFET and BJT gm/Id characterisation tool. Runs DC sweeps via ngspice or Xyce, then generates SVG plots of the standard analog design figures of merit.

Produces 6 plots per device:

MOSFET: gm/Id vs Jd, gm, gds, intrinsic gain; VGS vs gm/Id, Id BJT: gm/Ic vs Jc, gm, intrinsic gain, beta; VBE vs gm/Ic, Ic

Building

Requires C++23 (clang 19+ or GCC 14+), CMake 3.25+, and ninja.

nix develop            # or install deps manually
cmake -B build -G Ninja
cmake --build build

This produces two targets:

• libGmIDVisualizer.so — shared library (Schemify plugin or link into your own app)
• gmid_runner — standalone CLI

CLI usage

./build/gmid_runner \
  --model-file path/to/pdk_model.lib \
  --kind mosfet \
  --out-dir ./output

All flags:

Flag	Default	Description
--model-file	required	Path to SPICE model file
--kind	required	mosfet or bjt
--out-dir	required	Directory for SVG output
--device-name	auto-detect	Override .model name extraction
--vgs-start	0.0	VGS sweep start (V)
--vgs-stop	1.8	VGS sweep stop (V) — adjust for your PDK
--vds-start	0.05	VDS sweep start (V)
--vds-stop	1.8	VDS sweep stop (V)
--vgs-steps	181	VGS sweep points
--vds-steps	18	VDS sweep points
--width	10.0	Gate width (um)
--length	0.18	Gate length (um)
--temp	27.0	Temperature (C)

Library usage (C++)

#include <gmid/lib.hpp>

gmid::SweepConfig cfg;
cfg.model_file  = "sky130.lib";
cfg.device_name = "sky130_fd_pr__nfet_01v8";
cfg.kind        = gmid::ModelKind::mosfet;
cfg.vgs_stop    = 1.8;
cfg.vds_stop    = 1.8;
cfg.work_dir    = "/tmp/gmid_work";

auto result = gmid::characterise(cfg, "/tmp/gmid_out");
if (result.ok()) {
    for (auto& plot : result.plots) {
        // plot.svg  — path to SVG file
        // plot.lut  — vector of {x, y} data points
    }
}

C FFI

A C-linkage API is provided for use from Python, Zig, Rust, etc:

GmidCharResult* r = gmid_characterise(
    "model.lib", NULL, "mosfet", "./out", NULL,
    0.0, 1.8, 181,    /* vgs */
    0.05, 1.8, 18,    /* vds */
    10.0, 0.18, 27.0  /* W, L, T */
);
/* use r->plots[i].svg_path, r->plots[i].lut ... */
gmid_free_result(r);

Simulator backends

• ngspice (default) — must be on $PATH
• Xyce — used when available; accesses internal OP parameters directly

License

See repository for license details.