Measuring and Extracting

This section provides general information as well as procedures for performing measurements and extractions of MESFET devices.

Measurement and Extraction Guidelines

The following guidelines are provided to help you achieve more successful model measurements and extractions.

Setting Instrument Options

Before starting a measurement, you can quickly verify instrument option settings. Save the current instrument option settings by saving the example file to <file_name>.mdl from the model window. Some of the Instrument Options specify instrument calibration. For the most accurate results, calibrate the instruments before taking IC-CAP measurements.

Experiment with the other network analyzer options to obtain the best results with specific devices.

Measuring Instruments

Ensure that the measuring instruments (specified by unit names in the input and output tables) are correctly connected to the DUT. Refer to Table 86 for a list of nodes and corresponding measurement units. The quality of the measuring equipment (instruments, cables, test fixture, transistor sockets, and probes) can influence the noise level in the measurements.

Ensure that all characteristics of the measurement stimulus and corresponding measured response are specified in the respective input and output tables.

Calibration

For some measurements the instruments or test hardware must be calibrated to remove non-device parasitics from the DUT. For MESFET devices, stray capacitance due to probe systems, bond pads, and so on should be calibrated out prior to each measurement.

For high-frequency 2-port measurements with a network analyzer, the reference plane of the instrument must be calibrated out to the DUT. IC-CAP relies on the internal calibration of the instruments for full error-corrected data. It is critical that calibration using OPEN, SHORT, THRU, and 50 ohm loads be properly done.

Extracting Model Parameters

For a given setup, you can find the extraction transforms in the Extract/Optimize folder. IC-CAP's extraction algorithms exist as functions; choose Browse to list the functions available for a setup.

When the extract command is selected from the setup, all extractions in the setup are performed in the order listed in the setup. This order is usually critical to proper extraction performance. Extractions are typically completed instantly and the newly extracted model parameter values are placed in Model Parameters.

IC-CAP provides setups for two extraction methods. In general, you only need to perform one of the methods in order to extract parameters.

Simulating Device Response

Simulation uses model parameter values currently in the Parameters table. A SPICE deck is created and the simulation performed. The output of the SPICE simulation is then read into IC-CAP as simulated data.

SPICE3 is the only simulator fully compatible with the IC-CAP UCBGaas.mdl configuration file is. You can also use the HPSPICE simulator if you modify the parameter names to match it. DC simulations generally run much faster than cv and AC simulations.

If simulated results are not as expected, use the simulation debugger (in the Tools menu) to examine the input and output simulation files. The output of manual simulations is not available for further processing by IC-CAP functions such as transforms and plots.

Displaying Plots

The Display Plot function displays all graphical plots defined in a setup. The currently active graphs are listed under the Plots folder in each setup. View the plots for agreement between measured and simulated data. Measured data is displayed as a solid line; simulated data is displayed as a dashed or dotted line.

Optimizing Model Parameters

Optimization of model parameters improves the agreement between measured and simulated data. An optimize transform whose Extract Flag is set to Yes is automatically called after any extraction that precedes it in the transform list.

Optimizing AC parameters can be very time consuming because of the number of SPICE simulations required.

Extraction Procedure Overview

This section describes the general procedure for extracting model parameter data from the UCB GaAs MESFET. The general procedure applies to all types of parameters. Differences between extracting one type and another are primarily in the types of instruments, test setups, and transforms used.

Parameters are extracted from measured or simulated data. Measured data is data taken directly from instruments connected to the DUT inputs and outputs. Simulated data are results from the simulator. Once measured and simulated data have been obtained, each data set can be plotted and compared in the Plot window.

The general extraction procedure is summarized next, starting with the measurement process.

1	Install the device to test in a test fixture and connect the test instruments.

2	Ensure the test fixture, signal source and measuring instruments, and workstation are physically and logically configured for the IC-CAP system.

3	Choose File > Open > Examples. Select UCBGaas.mdl and choose OK. Select Open to load the file and open the model window. Choose OK.

When the UCBGaas model window opens you are ready to begin measurement and extraction operations.

4	Enter the variable name EXTR_PAR at the Model level and enter NMF1 as its value. This allows the extractions to find the model parameters for the model name NMF1 within the subcircuit of the model file. This concept is covered in more detail in Chapter 12, "Circuit Modeling."

5	Select the DUT and setup.

6	Issue the Measure command.

7	Issue the Extract command.

8	Issue the Simulate command.

9	Display the results.

10	Fine tune the extracted parameters if needed by optimizing.

Parameter Measurement and Extraction

The recommended method for extracting UCB GaAs model parameters is presented next. In this extraction, external resistances are extracted from AC data.

Parameter extractions are dependent on each other; to ensure accuracy extractions must be done in this order:

Inductances and resistances (AC)

External inductance and resistance parameters are extracted from an S-parameter measurement at a single bias setting. The gate of the device is strongly forward-biased to make the device look like a short circuit. The setup s_at_f is used to take the measurements and extract the parameters LD, LG, LS, RD, RG, and RS.

Diode parameters (DC)

Diode parameters PB, IS, and XN are extracted from data produced by the measurement of Ig versus Vg measured at zero drain voltage, with the source floating. The setup igvg_0vs or igvg_0vd is used to make the measurements and extractions, depending on whether the gate-source or gate-drain junction is preferred.

Other DC parameters (DC)

The remaining DC parameters are extracted using two setups: idvd_vg and idvg_hi_vd. Use idvd_vg to measure Id versus Vg at different gate voltages, then use idvg_hi_vd to measure Id versus Vg at a constant drain voltage. Parameters VTO, BETA, ALPHA, LAMBDA, and B are then extracted from the resulting data.

AC (capacitance parameters)

The capacitance parameters CGD and CGS are extracted from an S-parameter measurement using the setup s_vs_f. The measured data is first corrected using the inductances and resistances extracted in the initial step, then the capacitances are extracted from the corrected data.

Use a network analyzer to make the next set of measurements. S-parameter measurements are highly sensitive—the instrument must be properly calibrated.

1	Place the device to be measured in the test fixture.

2	Select the ac/s_at_f DUT/setup and choose Measure.

3	In the ac/s_at_f DUT/setup choose Extract to extract inductance and resistance parameters.

4	Select the ac/s_vs_f DUT/setup and choose Measure. Do not extract the parameters for this setup yet.

5	Disconnect the device from the network analyzer and directly connect it to the appropriate units for DC measurements.

6	Select the dc/ igvg_0vs or igvg_0vd DUT/setup and choose Measure.

7	Repeat Step 6, but choose Extract to extract diode parameters.

8	Repeat Step 6, but choose Optimize.

9	Select the dc/idvg_hi_vd DUT/setup and choose Measure to measure Id versus Vg at a constant Vd.

10	Select the dc/idvd_vg DUT/setup and perform the measure and extract steps to measure and extract the other DC parameters.

11	Repeat Step 10, but choose Optimize.

12	Select the ac/s_vs_f DUT/setup and choose Extract to extract the capacitances from the data that was measured in step 4.

All model parameters are extracted and their values added to the Parameters table; they can be viewed in the Model Parameters folder.

Alternate Extraction Method

If AC data is not available, IC-CAP supports an alternate method for extracting UCB MESFET model parameters. This procedure uses the Fukui technique [3]; external resistances are extracted along with the diode parameters from DC data—this differs from the recommended method. Use this method only if the AC data is not available—this alternate method produces parameters that are less precise than those of the recommended method.

Parameter extractions are dependent on each other; to ensure accuracy extractions must be done in the following order.

Resistance and diode parameters (DC)

Using DC measurements only, this procedure uses the Fukui algorithm to extract the resistance parameters RD, RG, and RS from DC data (refer to Table 84). Diode parameters PB, IS, and XN are also extracted. The extraction requires the setups listed in the following table.

Table 87 Resistance and Diode Measurement and Extraction Setups for the Alternative Method

idvg_low_vd	Id versus Vg	Small Vd
igvg_0vs	Ig versus Vg	Vs=0, with Drain floating
igvg_0vd	Ig versus Vg	Vd=0, with Source floating

The extraction is performed from the setup igvg_0vd. To use the Fukui algorithm, the following inputs must be added to the function GAASDC_lev1 (extract transform).

VG (low Vds)     idvg_low_vd/vg
VD (low Vds)     idvg_low_vd/vd
ID (low Vds)     idvg_low_vd/id.m
VG (D Flt)     igvg_0vs/vg
IG (D Flt)     igvg_0vd/ig.m

Other DC parameters (DC)

Use the recommended method described previously.

Inductance parameters (AC)

Use the recommended method described previously. Parameters LD, LG, and LS are extracted from the S-parameter measurement. The same transform also extracts the resistance parameters, overwriting the existing ones as it does so.

Capacitance parameters (AC )

Use the recommended method described previously.

The alternate extraction procedure follows.

1	Connect the NWA to extract inductances and capacitances.

2	Place the device to be measured in the test fixture.

3	Select the ac/s_at_f DUT/setup and choose Measure.

4	In the ac/s_at_f DUT/setup, choose Extract to extract inductance and resistance parameters.

5	Select the ac/s_vs_f DUT/setup and choose Measure.

6	In the ac/s_vs_f DUT/setup, choose Extract to extract the capacitances from the data that was measured in step 3.

7	Select dc/igvg_0vs DUT/setup and choose Measure.

8	Repeat Step 7 for dc/idvg_low_vd and dc/igvg_0vd.

9	In the dc/igvg_0vd DUT/setup, choose Extract to extract the resistance and diode parameters from the measured data for the three DC setups.

10	Repeat Step 9 but choose Optimize.

11	Select the dc/idvg_hi_vd DUT/setup and choose Measure to measure Id versus Vg at a constant Vd.

12	In the dc/idvd_vg DUT/setup, repeat the Measure and Extract steps to measure and extract the other DC parameters.

13	Repeat step 13 but choose Optimize.

Simulating

To simulate any individual setup, choose Simulate with that setup active. Simulations can be performed in any order once all of the model parameters have been extracted.

For more information on simulation, refer to Chapter 6, "Simulating," in the IC-CAP User's Guide.

Displaying Plots

To display plots issue the Display Plot command from a DUT to display the plots for all setups in that DUT. The Plots use the most recent set of measured and simulated data. Viewing plots is an ideal way to compare measured and simulated data to determine if further optimization would be useful. For more information on Plots, refer to Chapter 10, "Printing and Plotting," in the IC-CAP User's Guide.

Optimizing

The optimization operation uses a numerical approach to minimize errors between measured and simulated data. As with the other IC-CAP commands, optimization can be performed at either the DUT or setup level. Optimization is more commonly performed from setups—optimization for all setups under a DUT is rarely required.

Optimization is typically interactive in nature, with the best results obtained when you specify the characteristics of the desired results.

For more information on optimization, refer to Chapter 7, "Optimizing," in the IC-CAP User's Guide.