Measuring and Extracting

Bipolar parameter extraction is divided into four categories: DC, capacitance, parasitic resistance, and AC. These categories correspond to the required supporting measurements described under Test Instruments.

The bjt_npn.mdl file provides DUTs and setups that correctly bias a typical device for the measurements needed to perform the associated parameter extractions.

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 the instrument options settings. Save the current instrument option settings by saving the model file to <file_name>.mdl. Some of the Instrument Options specify instrument calibration. For the most accurate results, calibrate the instruments before taking IC-CAP measurements.

Typical DC and cv instrument options are:

When taking AC measurements with a network analyzer, several instrument settings are critical. And, calibration must be performed on structures that have impedances similar to the stray parasitics of the device under test (DUT).

Typical AC instrument options are:

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 inputs and outputs) are correctly connected to the DUT. Refer to Table 83 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.

The series resistance in test fixtures can also be critical when making high current measurements. For example, a 1-ohm resistance in series with the emitter at an Ic = 20 mA can cause a factor-of-two error in the measured versus simulated DC performance. Series resistances that are not accounted for in the device model can be included by adding them to the test circuit for the DUT.

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

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

In making high-frequency two-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. Calibration using OPEN, SHORT, THRU, and 50-ohm LOADS must be correctly performed.

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.

Simulating Device Response

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

Select a simulator from Tools > Hardware Setup or define a SIMULATOR variable. 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 (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). For more information refer to "Using the Simulation Debugger" in the IC-CAP User's Guide.

Displaying Plots

The Display Plot function displays all graphical plots defined in a setup. The currently active graphs are listed in the Plots folder in each setup.

Measured data is displayed as a solid line; simulated data is displayed as a dashed or dotted line of the same color. After an extraction and subsequent simulation, view the plots for agreement between measured and simulated data. Plots are automatically updated each time a measurement or simulation is performed.

Optimizing Model Parameters

Optimization of model parameters improves the agreement between measured and simulated data. The bipolar model typically requires very little optimization because most of the extraction algorithms have some optimization built into them.

Capacitance parameter extractions are actually done through optimization. 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.

PNP Transistors

In the bjt_pnp.mdl file included with IC-CAP, PNP transistors are measured, extracted, and simulated in a manner similar to NPN transistors. The critical difference with a PNP device is that the bias voltages are of opposite polarity from an NPN device.

To extract the two models using the same algorithms, set a variable in Model Variables; POLARITY should have the value PNP. The extraction default NPN will result in incorrect parameter values or extraction errors on PNP data.

Another variable convenient for displaying PNP Plots is inv_plot. This variable can invert the plots in bjt_pnp.mdl so they plot in the same direction as NPN plots. Set inv_plot to -1 to do this.

Extracting Parameters

This section describes the general process for extracting model parameter data from the UCB bipolar transistor. The process applies to all types of parameters: DC, capacitance, and AC. The differences between extracting one type of parameter and another are primarily in the types of instruments used to measure the data and the specifications within the DUTs and setups.

Parameters are typically extracted from measured data but can also be extracted from simulated data. To extract from measured data, ensure that the outputs specified in the extraction transforms use the .m suffix. For example, IS and NF are extracted using the BJTDC_is_nf function. To extract from measured data, IC-CAP uses log10(ic.m) as the specification of the forward collector current. (Use the .s suffix when extracting from simulated data.)

When performing an extraction, accurate results depend on the sequence of steps. The top-to-bottom order of DUTs and setups in a model file is the suggested order of measurements and extractions. In the bjt_npn.mdl file, the large signal DC and junction capacitance parameters are independent of each other. However, for the parasitic resistances and AC parameters to be accurately extracted, the preceding two groups must be successfully extracted first. Setups in bjt_npn.mdl are designed for use with a typical bipolar transistor. You may be able to improve results with your own devices by modifying these setups to more closely conform to your needs.

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 measuring instruments.

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

3	Load the model.

4	Select the DUT and setup.

5	Issue the Measure command.

6	Issue the Extract command.

7	Issue the Simulate command.

8	Display the results.

9	Fine tune the extracted parameters if needed by optimizing.

DC Large-Signal Parameters

Setups are provided for measuring and extracting the properties of the internal transistor (not including parasitic resistances); these are fearly, rearly, fgummel, and rgummel.

The fearly and rearly setups measure forward and reverse Early voltage characteristics, respectively. The Early voltage parameters VAF and VAR are extracted simultaneously in the rearly setup, using measurements taken from both fearly and rearly.

The fgummel and rgummel setups perform the forward and reverse Gummel plot measurements. Parameters IS, BF, NE, IKF, ISE, and NF are extracted by the fgummel setup, while the extractions in the rgummel setup produce the BR, NR, IKR, ISC, and NC parameters. The model uses the saturated current parameter IS to simulate current flow in both directions.

Junction Capacitance Parameters

Measuring bipolar transistor capacitance characteristics requires three DUTs. This is because each p-n junction is a physically different one-port connection. The base-emitter, base-collector, and collector-substrate junctions each have a different DUT and setup. While you will perform all three measurements on the same physical device, each measurement requires different instrument connections for the corresponding DUT and setup. The DUTs and instrument connections for each measurement are listed in Table 83.

Each p-n junction is measured from a small forward bias to a large reverse bias. The extractions are performed using the transform set_CJ to find the initial value of CJ0, then optimizing the parameters of the general p-n junction capacitance equation to the measured data. This produces the capacitance, built-in voltage, and grading factors for each DUT: CJE, VJE, MJE; CJC, VJC, MJC; CJS, VJS, MJS. For the most accurate extractions, calibrate out stray capacitance from cables, probes, and bond pads before taking each p-n junction capacitance measurement.

DC Parasitic Resistance Parameters

Three parasitic resistances are connected to the bipolar transistor: RE, RC, and RB. RE and RC are constant value components, while RB is a function of base current. RE is measured by the setup reflyback. This setup saturates the transistor, then measures the differential voltage drop from collector to emitter (with Ic = 0) versus the differential base-to-emitter current.

RC is measured by the setup rcsat, which measures the parameter as the DC resistance from collector to emitter at the onset of saturation. Alternately, the rcactive setup can be used to measure the collector resistance in the active region of device operation. However, this extraction is dependent on the operating point, which must be specified by manually placing a box on the Plot contained in the setup. For complete information on using this extraction, refer to HP Application Note Advanced Bipolar Transistor Modeling Techniques[1].

The setup rbbib does not actually measure or extract RB. Instead, it produces a characteristic curve of base-to-emitter bias versus DC base current. The resulting curve is used when the base resistance is measured and extracted using S-parameters.

Base Resistance and Transit Time Parameters

The AC DUT uses setups that measure S-parameters with a network analyzer. The quality of the measured S-parameters depends on the calibration of the network analyzer. IC-CAP does not perform error correction; it relies totally on the measuring instruments for the correction of errors.

Making high-frequency measurements on packaged transistors can lead to unexpected results. This is because of the stray capacitance and inductance that are a part of the package. Measure S-parameters with a high-quality microwave wafer probe.

The AC setup rbbac measures H11 of the transistor in the common emitter mode. This input impedance is then used in the extraction to produce model parameters RB, IRB, and RBM.

The AC setup h21vsvbe measures H21 of the transistor in the common emitter mode. The measured current gain is then used to extract a small signal model that produces the parameters TF, ITF, VTF, XTF, and PTF.

The AC setup h21vsvbc measures H21 of the transistor in the common collector mode. The measured current gain is then used to extract the parameter TR.