Hints, Tips and Solutions

Volume 12, Number 1, January 2002

BSIM4 RF CMOS modeling with UTMOST III.

The BSIM4 model provides a RF CMOS modeling capability. Prior to the BSIM4 model the RF modeling was accomplished by using macro models. The macro models were complex, contained many external elements and exhibited poor geometry scaling.

The BSIM4 RF modeling environment has been implemented in UTMOST III's s-parameter measurement and modeling module. The "s_meas" routine in MOS module of UTMOST III can control Network Analyzers for measurement and data acquisition. The measured s-parameters can be viewed using the s_plots, z_plots, y_plots and h_plots routines. These routines can also be used for simulation and RF model parameter optimization purposes.

The first step towards RF CMOS modeling starts with designing RF structures for the testchip. The calibration structures (open pad structures for de-embedding) and actual transistors should have special pads for signal-ground or ground-signal-ground RF microprobes.

Assuming that the proper structures exist in the testchip, a DC model should be extracted prior to the RF modeling. The user should obtain a good DC model since the DC parameters greatly effect the RF characteristics. Especially the output conductance modeling is very important for good RF modeling.

The measurements of the standard calibration structures and open pad devices can be accomplished using the AC Calibration Screen which is located in the hardware configuration screen's "AC ANALYZER" column. (Figure 1.)

Figure 1. AC Calibration Screen of HP8753 Network Analyzer.

After calibration and de-embedding is completed the s-parameters of the actual device can be measured using the "s_meas" routine of UTMOST III MOS module. The measured results will be displayed in the H21 (AC Gain) versus Frequency format (Figure 2.)

Figure 2. Measured H21 (AC Gain) versus Frequency.

The measured data can be displayed in y_parameter format using the "y-plots" routine. In order to display four types of y-parameters in one screen the "ALL" option in the "Routine Control" screen's "Multiple Select." menu should be highlighted. The real and imaginary parts of y11, y12, y21 and y22 will be displayed in the graphics screen. (Figure 3.)

Figure 3.Measured vs. Simulated data of a RF modeling
project accomplished using the BSIM4 RF model.

The "External SPICE" option will be used for simulations.Here are some of the RF parameters which can be used for modeling the y-parameters:

The complete list of RF parameters are available in the BSIM4 User's Manual. These parameters can be used to optimize the RF model until good matching between the measured and simulated data is obtained. (Figure 3.)