Measuring and Extracting

This section 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 options settings. Save the current instrument option settings by saving the model 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 91 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

IC-CAP's extraction algorithms exist as transforms in the function list, under Extractions. Extraction transforms for a given setup are listed in the transform tile for the setup.

When the extract command is issued from the setup level, all extractions in the setup are performed in the order listed in the setup; this order is usually critical to proper extraction performance. The extractions are typically completed instantly and the newly extracted parameter values are placed in Model Parameters.

The configuration file supplied with IC-CAP contains the setups for two different extraction methods, and two different sets of model parameters (level 1 and 2). In general only one set of these parameters is important, and you need to perform only one of the methods in order to extract model parameters. Set the parameter MODEL to the desired number (1 or 2) before starting the extraction.

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.

HPSPICE is the only simulator fully compatible with the IC-CAP Curtice GaAs model configuration file. This simulator uses the JFET convention for calling the model. The figure below is an example of the circuit definition for the Curtice GaAs MESFET model to be used with this simulator. JCGAAS is the device name; NMF1 is the model name; RCAY specifies to use the Curtice GaAs model; NJF specifies the N type FET, which is the only type supported in this model. MODEL = 1 specifies the level 1 model.

Simulations vary in the amount of time they take to complete. 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.

Figure 216 Example of the circuit definition for HPSPICE

.SUBCKT CGAAS 1=D 2=G 3=S JCGASS 11 22 33 NMF1 LD 1 11 1n LG 2 22 1n LS 3 33 1n .MODEL NMF1 RCAY NJF + MODEL = 1 + BETA = 100u + VBI = 0.7 + ..... + ..... .ENDS

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 Curtice GaAs MESFET. The procedure applies to all types of parameters. The differences between extracting one type and another lie primarily in the types of instruments, setups, and transforms used.

Parameters can be 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, both data sets can be plotted and compared.

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 CGaas2.mdl and choose OK. Select Open to load the file and open the model window. Choose OK.

When the CGaas2 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	Execute the Measure command.

7	Execute the Extract command.

8	Execute 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 Curtice GaAs model parameters is presented next. In this extraction, external resistances are extracted from AC data.

The Curtice GaAs MESFET model is a 2-level model. IC-CAP supports and extracts parameters for both levels of this model. The following procedure extracts parameters for level 1 or 2 depending on the value of the parameter MODEL.

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

Inductance and resistance parameters (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 s_at_f setup is used to take the measurements and extract the parameters LD, LG, LS, RD, RG, and RS.

Diode parameters (DC)

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

Threshold parameters (DC)

Parameters that describe the threshold characteristics are extracted using Id versus Vg measurement at a high drain voltage. The idvg_hi_vd setup is used for this extraction. For the level 1 model, VTO will be extracted; for the level 2 model, A0, A1, A2 and A3 will be extracted.

Linear and saturation parameters (DC)

Parameters that control the linear and saturation regions of device operation are extracted using Id versus Vd measurement at different gate voltages. The idvd_vg setup is used for this extraction. BETA, LAMBDA and ALPHA are extracted for the level 1 model; BETA and GAMMA are extracted for the level 2 model.

Capacitance parameters (AC)

Capacitance parameters CGDO and CGSO, and AC parameters A5, CDS, RDSO and RIN are extracted from an S-parameter measurement using the s_vs_f setup. Measured data is corrected using the inductances and resistances extracted in the initial step; capacitance and other AC parameters are then extracted from corrected data.

By defining IC-CAP system variables LINEAR_CGS, LINEAR_CGD, and CONSTANT_TAU and setting their values to true, CGS, CGD, and TAU, respectively, can be extracted.

Use a network analyzer to make the next set of measurements. S-parameter measurements are highly sensitive—it is important that the instrument 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 select Extract to extract the 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 the 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 repeat 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 Curtice 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.

Resistance and diode parameters (DC)

Using DC measurements only, this procedure uses the Fukui algorithm to extract resistance parameters RD, RG, and RS from DC data. Diode parameters VBI, IS, and N are also extracted. The extraction requires the setups listed in the following table.

Table 92 Resistance and Diode Measurement and Extraction Setups

igvg_0vd	Ig versus Vg	Vd=0, with Source floating
idvg_low_vd	Id versus Vg	Small Vd
gvg_0vs	Ig versus Vg	Vs=0, with Drain floating

This extraction is located in the igvg_0vd setup. To use the Fukui algorithm, add the following inputs to the function GAASDC_lev1.

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_0vs/ig.m

Threshold Parameters ( )

Use the recommended method for measuring and extracting threshold parameters.

Linear & Saturation Parameters ( )

Use the recommended method for measuring and extracting linear & saturation parameters.

Inductance Parameters (AC)

Use the recommended method for measuring and extracting inductances. The parameters LD, LG, and LS are extracted from the S-parameter measurement. In addition, the Transform also extracts and overwrites the resistance parameters.

Capacitance Parameters ( )

Use the recommended method for measuring and extracting capacitance parameters.

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	Repeat Step 2, but select Extract to extract the inductance and resistance parameters

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

6	Select the ac/s_vs_f DUT/Setup and 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 8 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	Select the dc/idvd_vg DUT/Setup and repeat the Measure and Extract steps to measure and extract the other DC parameters.

13	Repeat Step 10, but choose Optimize.

14	Select the dc/idvd_vg DUT/Setup and repeat the Measure and Extract steps to measure and extract the other DC parameters.

15	Repeat Step 10, but choose Optimize.

Simulating

To simulate any individual setup, choose Simulate with that setup active. Simulations can be performed in any order after 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 of measured and simulated data issue the Display Plots command from a DUT to display the plots for all setups contained in that DUT.

Viewing plots is an ideal way to compare measured and simulated data to determine if further optimization would be useful. For more information on displaying plots, refer to Chapter 10, "Printing and Plotting," in the IC-CAP User's Guide.

Optimizing

The optimization operation uses a numerical approach to minimizing 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.