写一个VerilogA的mosefet模型

以下是一个简单的VerilogA MOSFET模型，仅供参考：

`timescale 1ns/1ps

module MOSFET_model (Drain, Gate, Source, Substrate, Vds, Vgs, Ids);

// Inputs
electrical Drain, Gate, Source, Substrate, Vds, Vgs;

// Outputs
electrical Ids;

// Parameters
parameter real L = 1e-6; // Channel length
parameter real W = 1e-6; // Channel width
parameter real tox = 1e-9; // Oxide thickness
parameter real u0 = 600e-4; // Mobility
parameter real Cox = 3.9 * 8.85e-14 / tox; // Gate oxide capacitance per unit area
parameter real Vt0 = 0.7; // Zero-bias threshold voltage
parameter real lambda = 0.02; // Channel-length modulation parameter
parameter real Rd = 1e-3; // Drain series resistance
parameter real Rs = 1e-3; // Source series resistance

// Local variables
real Vth; // Threshold voltage
real Vgs_eff; // Effective gate-source voltage
real Vds_eff; // Effective drain-source voltage
real Vov; // Overdrive voltage
real Ids_sat; // Saturation current
real gm; // Small-signal transconductance
real gds; // Small-signal output conductance
real Vdsat; // Drain-source saturation voltage
real Rd_eff; // Effective drain series resistance
real Rs_eff; // Effective source series resistance

// Compute effective drain and source resistances
Rd_eff = (Vds > 0) ? Rd : 0;
Rs_eff = (Vds < 0) ? Rs : 0;

// Compute threshold voltage
Vth = Vt0 + (2 * tox * 8.854e-12 * 1e-6) / Cox * (Substrate - 2 * Vt0) / (2 * tox);

// Compute effective gate-source voltage
Vgs_eff = Vgs - (Substrate - Source);

// Compute effective drain-source voltage
Vds_eff = Vds - Rd_eff * Ids + Rs_eff * Ids;

// Compute overdrive voltage
Vov = Vgs_eff - Vth;

// Compute saturation current
Ids_sat = W / L * u0 * Cox / 2 * (Vov) ^ 2;

// Compute small-signal transconductance
gm = W / L * u0 * Cox * (Vgs_eff - Vth);

// Compute small-signal output conductance
gds = W / L * u0 * Cox * (Vds_eff - Vdsat) + lambda * Ids_sat;

// Compute drain-source saturation voltage
Vdsat = (Vgs_eff - Vth - sqrt((Vgs_eff - Vth) ^ 2 - 2 * Cox * Ids_sat / W / L)) / lambda;

// Compute drain current
if (Vgs_eff < Vth) {
  Ids = 0;
} else if (Vds_eff < Vov) {
  Ids = gm * Vds_eff - gds / 2 * Vds_eff ^ 2;
} else {
  Ids = 0.5 * W / L * u0 * Cox * (Vgs_eff - Vth - Vov) ^ 2 * (1 + lambda * (Vds - Vdsat));
}

endmodule

该模型计算MOSFET的Drain电流（Ids）与Gate-Source电压（Vgs）和Drain-Source电压（Vds）之间的关系。它使用一些常见的MOSFET参数来计算电流，如门极氧化物厚度（tox）、载流子迁移率（u0）、栅极氧化物电容（Cox）等。还考虑了一些非理想效应，如栅极阈值电压（Vth）变化、沟道长度调制效应（lambda）、漏电阻（Rd）和源电阻（Rs）等。