Circuit Parameter Extraction

Circuit parameter extraction is identical to single component parameter extraction through the use of Transforms. However, because circuits are custom in nature, most of the extraction routines must also be custom designed. With the availability of the Program function and optimize transforms, this is simple and quick to evaluate and execute. The critical factor in a successful circuit level parameter extraction is the ability to make a measurement and subsequent extraction involving only the dominant component parameters.

For a full model extraction of a single component, you will attain more accuracy if that device is available without any additional components connected to it. For most functional block level circuits however, a subset of the transistor model parameters is usually sufficient for studying circuit behavior.

Extracting Transistor Parameters Using Library Functions

In the explanation of a selective measurement on a sub-portion of a circuit in the previous section, Q1 and its neighboring resistors were isolated in the ECL logic gate. The forward active model parameters can be extracted from this measurement using the model extraction functions in the function list or by setting up a custom optimization. To access the functions, add transforms that use them to a setup that contains the measurement. It is possible to use the provided transistor extraction functions to obtain model parameters for devices connected into a larger circuit.

Because all models in a circuit have model parameters in the Parameters table with the model's name as a prefix, IC-CAP must be told which model to use with the extraction transforms. This is easily done by setting a variable in the model level variable table. Enter a variable in the table called EXTR_MODEL and set its value to the name of the transistor whose parameters are being extracted. When the extraction transforms are executed, IC-CAP refers to the correct Parameters table entry as it writes the extracted value back to the table. For example, to use a function list transform on the model NPN1 mentioned above, add the following to the model level variable table:

 EXTR_MODEL     NPN1

Each time another transistor is used for an extraction, place its name in the value field. A more efficient method of extracting individual transistor models is to create an individual setup for each device. The variable table at the setup level can then include the EXTR_MODEL entry, keeping the transistor extraction local to that setup. This can also be done in an analogous way at the DUT level.

It is sometimes necessary to specify the particular DUT in a circuit that should be used in an extraction routine. For example, in a circuit that contains two MOSFETs there are two different sets of geometry parameters (L and W). For the extraction to work correctly, the EXTR_DUT variable must be set to the name of the transistor with the correct geometry parameters. Therefore, to characterize transistor M1, which uses model NMOS1, add the following to the model level variable table:

EXTR_DUT M1
EXTR_MODEL NMOS1

Situations where EXTR_DUT must be set can also arise when test circuits are defined. In this case, DUT parameters that normally appear without a prefix in the DUT Parameters table will include the transistor name from the Test Circuit as a prefix. For the extractions to use these parameters, EXTR_DUT must be set to the transistor name that is used in the test circuit.

Extracting Parameters Through Optimization

It is not always possible to adequately isolate a circuit component before using a standard extraction function. In these cases it is still possible to extract model parameters by using the optimize function. As with any extraction function, successful use of the optimizer requires that the parameters being optimized have a dominant effect over the simulation of the measured characteristics. Refer to Chapter 7, "Optimizing," in the IC-CAP User's Guide for more information regarding optimization.

In the OR/NOR gate shown in Figure 218, it is possible to use the optimizer to extract the values of NPN1.IS, NPN1.NF, NPN1.BF and RIEE. The following sequence of operations describes how to accomplish this.

1	Make an Ic and Ib versus V measurement between the NOR, IN1 and VEE terminals.

a

Connect the VEE, VREF and IN2 terminals to constant voltage sources of 0V. This keeps the base-emitter diodes of Q2 and Q0 in an off state. Disconnect VCC from the circuit.

b

Connect the NOR terminal to a voltage of approximately 1.0V.

c

Sweep the voltage on the IN1 terminal so that the measured currents at the NOR (collector) terminal are in the 1nA to 1A range.

2	Set up an optimization transform that optimizes the values of NPN1.IS, NPN1.NF, and NPN1.BF over the measured current. At low currents, RIEE has a minimal affect on the I-V relationship.

•

The target data is the measured Ic and Ib currents. The simulated data comes from the simulation of these currents.

•

The Parameters table contains NPN1.IS, NPN1.NF and NPN1.BF

3	Change the sweep voltage on the IN1 terminal so that the measured current at the NOR terminal is in the 10A to 1mA range.

•

The measured current should deviate from an exponential function due to the debiasing effect from RIEE.

4	Set up an optimization transform that optimizes the value of RIEE over the measured current.

•

The target data is the measured Ic current. The simulated data comes from the simulation of this current.

•

The Parameters table contains only the circuit element RIEE.

After each of these measurements and optimizations has been executed, the Model Parameters table is updated with the extracted values of these elements.