Selecting Models Automatically

The model selector uses the following criteria:

(If XLREF is not specified, XLREF is set to XL. If XWREF is not specified, XWREF is set to XW.)

The model selector syntax is based on a common model root name, with a unique extension for each model.

The following is an example of HSPICE syntax for models:

M1 drain gate source bulk NJ W=2u L=1u

.MODEL NJ4 NJF WMIN=1.5u WMAX=3u LMIN=.8u LMAX=2u

.MODEL NJ5 NJF WMIN=1.5u WMAX=3u LMIN=2u LMAX=6u

Automatic Model Selector Method illustrates the model selection method.

Figure D-1: Automatic Model Selector Method

For this example, there are several pch.x models, with varying drawn channel lengths and widths, in the model library. (The model root name is pch and the extensions are 1, 2, ..., 6). The NJ4 instance of the NJ Element (W=2 µ, L=1 µ) requires a model for which 1.5 µ <= channel width <= 3 µ, and 0.8 µ <= channel length <= 2 µ. The automatic model selector chooses the pch.4 model since that model satisfies these requirements. Similarly, the NJ5 transistor requires a model with 1.5 µ <= channel width <= 3 µ, and 2 µ <= channel length <= 6 µ. The pch.5 model satisfies these requirements. If a device size is out of range for all the models that exist, an error message is issued.

If a model within a subcircuit cannot be found, the automatic model selector searches the top level. If the automatic model selector fails to find a model, HSPICE terminates.

The following combination of conditions causes the automatic model selector to fail and terminates HSPICE:

1. In the element statement, a model name is used which contains a period (.).

2. The model library was not designed for use with the HSPICE automatic
model selector.

3. Either a multisweep specification or a .TEMP temperature analysis
statement is included in the HSPICE input.

The following example illustrates how a period in a model name can cause automatic model selection problems.

Case 1:

M1 d g s b N.CHN W=10u L=5u * Element statement

.MODEL N.CHN LMIN=1u LMAX=4u WMIN=2u WMAX=100u * .MODEL statement

Case 2:

.TEMP 25

.M1 d g s b N.CHN W=10u L=5u * Element statement

.MODEL N.CHN LMIN=1u LMAX=4u WMIN=2u WMAX=100u * .MODEL statement

In Case 1, since there is no multisweep or temperature analysis specified, the HSPICE model selector feature is not invoked, so the N.CHN model is used with no problems.

In Case 2, however, the presence of the .TEMP statement invokes the model selector feature. The model selector tries to find a model named N.nnn that fits within the length and width ranges given in the element statement. Because the length given in the element statement (5 µm) is not within the 1 to 4 µm range specified in the .MODEL statement, the model selector cannot find a model that matches the element statement, and HSPICE issues a "device `N' not found" error message.