HINTS & TIPS

Q. What is the recommended noise model in SPICE?

A. NOISE MODEL EXTRACTION AND ANALYSIS OF NOISE MODELS IN SPICE:

The traditional Noise models in SPICE used two parameters (AF and KF) to model the noise level and the frequency dependency of flicker noise in MOS devices. There are three different standard models and they can be activated using the noise level selector parameter NLEV in SPICE.

The models are:

NLEV=0

 

 

NLEV=1

 

NLEV=2

 

Among these three models the NLEV=2 model is widely used. The NLEV=2 model incorporates the gm of the device into the noise model equation and provides a better fit compared to the NLEV=0 and NLEV=1 models. However none of these three standard models can model the DC bias dependency of flicker noise very well. Therefore a table with AF and KF values at different DC bias points is provided as a solution.

In order to extract the AF and KF for NLEV=2 model the flicker noise data should be collected using the S3245A noise amplifier and UTMOST III noise routine as explained in the last Simulation Standard issue.

After the data collection is completed the Fit Variable "select_model" (Figure 1) should be set to 2 (selecting the NLEV=2 model) and the "Fit" button from the "Options" menu of the graphics screen should be pressed. (Figure 2) The AF and KF will be extracted and copied into the parameters screen. The parameter NLEV in the parameter screen should be set to 2 prior to the parameter extraction.

Figure 1. Noise Routine Fit Variable Screen.

 

Figure 2. Measured data for NLEV =2 Noise model parameter extraction.

 

Another attempt to model the flicker noise was made by the BSIM3v3 model. In addition to the standard approach (NOIMOD= 1) the BSIM3v3 model also has a DC bias dependent noise model (NOIMOD=2). The NOIMOD = 1 model will not be discussed in this article because it is very similar to the standard NLEV=0 model. However the NOIMOD=2 model introduces four new parameters named NOIA, NOIB, NOIC and EM.

NOIMOD=2 model is given as:

NOIMOD=2

 

 

The NOIMOD =2 model seems to provide better fits (Better than NLEV=2 standard SPICE model) for modeling flicker noise at different DC bias conditions. However the NOIMOD=2 model requires the collection of more data points. In order to find the bias dependency of the model UTMOST III will automatically collect noise data at four different DC bias conditions. The first three bias conditions will be in the linear region of operation and the fourth one will be in the saturation region of operation. For the first linear bias UTMOST will supply: VGS=VGS_start and VDS=0.5V. For the second linear bias UTMOST will supply: VGS=VGS_start and VDS=0.7V. For the third linear bias, UTMOST will supply: VGS=VGS_start+0.5 and VDS=0.5V. For the fourth saturation region bias, UTMOST will supply: VGS=VGS_start and VDS=2V.

In order to activate NOIMOD=2 model data collection the "select_model" in the Fit Variable screen should be set to 4. After the data collection, noise characteristics at four different DC bias points should be displayed in the graphics screen. (Figure 3). To extract the NOIMOD=2 noise model parameters the BSIM3v3 SPICE parameter NOIMOD in the parameter screen should be set to 2 and the "Fit" button from the "Options" menu should be pressed. The extracted parameter values will be copied into the parameter screen.

 

Figure 3. Measured data for NOIMOD =2 Noise model parameter extraction.

 

 

Prior to the noise model parameter extraction the DC model of the MOS device should be present in the parameter screen. The simulation of the noise characteristics can be accomplished using the "External SPICE" option in UTMOST III. The global optimization of the noise parameters is available in UTMOST III and the optimization method is identical to the DC model parameter optimization.

Q. How can I plot a single curve in ALL_DC routine of MOS or SOI technologies?

A. ALL_DC routines were structured to display different type of characteristics of multiple devices together. This flexible structure enables UTMOST III users to view model fits and globally optimize certain parameters for different geometries or characteristics.

However this feature does not prevent users to display or simulate or optimize a single ID/VG, ID/VD, RDS/VDS or GM/VGS characteristics. In order to display a single device characteristic a single geometry should be selected from the strategy screen.

The ALL_DC routine measurement setup screen has a distinct curve-type selection button called "To model". The characteristic types are listed in this buttons selection menu. (Figure 1.) A single characteristic should be selected and the "Measure" button in the extraction screen should be pressed to retrieve the single curve from the log file.

 

Figure 1. The ALL_DC routine Set Measurement Screen with single
characteristic ID/VD selected for display