Verilog-A Parameterization and Reusability

Introduction¶

Verilog-A has become the standard modeling language for analog behavioral modeling and compact device development in semiconductor design. One of its most powerful capabilities is parameterization, which allows the creation of flexible and reusable models. Parameterization ensures that a single Verilog-A model can describe entire device families — across different process corners, voltages, geometries, and performance ranges — simply by assigning new parameter values.

This article explores:

1. The role of parameters in Verilog-A models
2. How to use parameter and defparam effectively
3. Creating scalable and technology-portable models in a Process Design Kit (PDK)
4. Best practices for robust and reusable modeling

By mastering parameterization, engineers strengthen model portability, reduce maintenance effort, and improve simulation efficiency across large-scale design flows.

Why Parameterization Matters¶

Real semiconductor components vary widely in:

• Physical dimensions (W, L, area, perimeter)
• Temperature and environmental conditions
• Electrical performance (threshold voltage, mobility, resistance)
• Device configurations (multi-finger, stacked, high-voltage options)

Instead of writing a separate model for each variation, parameterization enables:

• A single model architecture
• Automated technology scaling
• Compact and clean design libraries
• Easy reuse across multiple foundries or PDK releases

Parameterization also accelerates simulation development, since engineers can tune behavior without rewriting the underlying model equations.

Using Parameters in Verilog-A¶

Parameters are declared at the module level and applied throughout the model. They support default values, valid ranges, and user overrides in netlists and schematic symbols.

Parameter Declaration Example¶

parameter real W = 1u from (0:inf);  // Width
parameter real L = 0.1u from (0:inf); // Length
parameter real Rsh = 10;  // Sheet resistance
````

Here:

* `real` data type ensures numeric precision
* `from (0:inf)` enforces physical validity

### Using Parameters in Equations

```verilog
Res = Rsh * (L / W);
V(p,n) <+ Res * I(p,n);

Changing W and L instantly creates different resistor values.

Parameter Validation¶

Engineers should always enforce legal ranges for physical parameters. Otherwise, the model may exhibit:

• Division-by-zero errors
• Negative resistance/capacitance
• Numerical overflow

Verilog-A range constraints prevent invalid geometry entry in schematic tools.

defparam Override in Netlists¶

Parameters may be overridden using:

• Instance parameter assignments
• defparam hierarchical overrides

Instance-Level Override¶

X1 (p n) device_model W=10u L=0.5u

Using defparam (netlist-level)¶

defparam X1.W = 10u;
defparam X1.L = 0.5u;

Although widely supported, modern modeling practice prefers instance-level overrides due to cleaner hierarchy and tool compatibility.

Scalable Modeling for PDKs¶

A Process Design Kit must support thousands of device variants, including:

Parameterization enables cross-device portability within a PDK.

Geometry-Dependent Scaling¶

Compact device behavior is strongly geometry-dependent. A MOSFET’s current is:

$$ I_D \propto \frac{W}{L}$$

Example:

Id = mu*Cox*(W/L) * f(Vgs, Vds);

Automatically updated as dimensions change.

Multi-finger Device Handling¶

PDKs require options for layout-driven models:

parameter integer NF = 1;
W_eff = NF * W;

Area-Based and Perimeter-Based Modeling¶

Used for:

• Varactors
• Junction leakage
• Parasitic extraction consistency

Irev = Js * Area + Jsw * Perim;

Where:

• Area = W × L
• Perim = 2 × (W + L)

Corner and Process Variability Parameterization¶

Models support:

• TT = Typical
• FF = Fast (Higher speed, lower Vth)
• SS = Slow (Lower mobility, higher Vth)

`ifdef CORNER_FF
    Vth = Vth_fast;
`endif

Corner control promotes more accurate silicon yield predictions.

Temperature-Dependent Scaling¶

Physical behavior changes with thermal effects. Models must integrate:

• Thermal voltage variation
• Carrier mobility temperature dependence
• Parasitic resistance increase

mu_eff = mu0 * pow($temperature/Tnom, alpha_mu);

This ensures the same model is valid across device operating conditions.

Behavioral Model Reuse¶

Models must be modular and adaptable across:

• System-level behavioral simulations
• Block-level transistor simulations
• Post-layout verification

Example: Parameterized Op-Amp¶

parameter real Gain = 60dB;
parameter real BW   = 10Meg;
parameter real Rin  = 1Meg;
parameter real Rout = 100;

V(out) <+ (Gain*(V(inp)-V(inn))) / (1 + s/BW);

Reusing this behavioral model accelerates architecture exploration.

Documentation and Standardization¶

Reusable models require:

Clear comments and parameter descriptions Default values consistent with silicon targets Naming standards aligned with PDK conventions

Good documentation ensures:

• Faster onboarding for new designers
• Easy integration with EDA tools

Best Practices for Parameterization & Reuse¶

Benefits for PDK Development¶

Parameterization enables long-term portability and supports automated model extraction workflows.