写一个VerilogA的mosefet模型

下面是一个简单的VerilogA MOSFET模型示例：

`timescale 1ns / 1ps

module mosfet_va(vd, vg, vs, id, w, l, tox, u0, vth, lamda, temp, cgs, cgd, cgb, pb, cgso, cgdo, cgbo);

  parameter REAL PI = 3.14159265358979323846;

  // Define variables
  electrical vd, vg, vs, id;
  real w, l, tox, u0, vth, lamda, temp;
  real cgs, cgd, cgb, pb, cgso, cgdo, cgbo;
  real vt, arg, beta, vgs, vds, vgb;
  real cgs0, cgd0, cgb0, id0;
  real cgs1, cgd1, cgb1, id1;
  real cgs2, cgd2, cgb2, id2;

  // Define constants
  real eps0 = 8.85e-12;
  real epsr = 3.9;
  real q = 1.6e-19;
  real kb = 1.38e-23;

  // Calculate threshold voltage
  vt = kb * temp / q * vth;

  // Calculate bulk charge capacitances
  cgs0 = w * tox * eps0 / l;
  cgd0 = w * tox * eps0 / l;
  cgb0 = w * tox * eps0 / pb;

  // Calculate overlap capacitances
  cgs1 = cgso * w * l;
  cgd1 = cgdo * w * l;
  cgb1 = cgbo * w * l;

  // Calculate parasitic capacitances
  cgs2 = cgs1 + cgs0;
  cgd2 = cgd1 + cgd0;
  cgb2 = cgb1 + cgb0;

  // Calculate current and capacitance coefficients
  beta = w / l * u0 * tox / 2 * (1 + lamda * vds);
  arg = vgs - vt;
  id0 = beta * arg * arg;
  id1 = beta * arg * (vds - 0.5 * lamda * vds * vds);
  id2 = beta * (vds - 0.5 * lamda * vds * vds) * (vds - 0.5 * lamda * vds * vds);

  // Calculate currents and capacitances
  if (vds < 0) begin
    vgb = vg - vs;
    vgs = vgb + vs;
    id = -id0 + id1 + id2;
  end else begin
    vgb = vg - vd;
    vgs = vgb + vs;
    id = -id0 + id1;
  end
  cgs = cgs2;
  cgd = cgd2;
  cgb = cgb2;

endmodule

这个模型包含了MOSFET的关键参数，包括宽度、长度、栅极氧化物厚度、迁移率、阈值电压、温度等等。它还考虑了各种电容，例如接触电容、重叠电容、寄生电容等。最后，它计算了MOSFET的电流和电容系数，以及计算了栅极、源极和漏极之间的电势差。