Q: How can I make BSIM3 extraction routine to provide defined VGS steps for IDS vs VDS characteristics?

A: The BSIM3 extraction routine in UTMOST performs four different types of measurements for each selected geometry. There are two IDS vs VGS (low and high VDS) and two IDS vs VDS (0V and high VBS) measurements. The first measurement of the BSIM3 parameter extraction routine is the IDS vs VGS measurement at low VDS. The BISM3 routine will extract the VT from the IDS vs VGS measurements and VT + VG_strt_offset will define the first VGS step for the following IDS vs VDS characteristics. The user can also specify the maximum VGS bias using the VGS_stop variable. However the intermediate VG steps will be calculated based on the VGS_start and VGS_stop values. The threshold voltage (VT) will vary for different geometries and as a result each device will have different VGS_steps. In addition, the VGS_steps may sometimes have 3 digits after the decimal point depending on the extracted VT and the maximum VGS. This methodology is essential for the BSIM3 routine to extract proper model parameters. However, if the user would like to define the VGS_start bias rather than allowing UTMOST to calculate it or maintain uniform VGS steps for every geometry, then the Fit Variables can be utilized to provide this option.

To open the Fit variables screen for BSIM3 routine press the "setups" button from the main UTMOST screen and select the BSIM3 routine. Press the "Set Measurement" button to open the set measurement screen for the BSIM3 routine. The Firt Variable button is located on the top right corner of the BSIM3 set measurement screen. The last four Fitting variables in the Fit variables screen are used to define boundaries for the VT extraction. This feature was implemented to prevent accidental destruction of devices when the VT was incorrectly extracted due to the bad IDS vs VGS data.The variables VTH_low_VB0, VTH_highVB0, VTH_low_VBB and VTH_high_VBB can also be utilized to force a specific VGS_start value (Figure 1).

For example: If the user would like to define VGS_start to be 1V for the IDS vs VDS characteristics at VBS=0V and 2V at VBS=high,then the variables VTH_low_VB0 and VTH_highVB0 should be equal to 1 and the variables VTH_low_VBB and VTH_high_VBB should be equal to 2.The VGS_strt_off variable in the Measurement setup screen should be set to 0. In this setup, (assuming that VGS maximum is equal to 5V and and the #_of VG steps is 5) the VGS steps will be 1V and 0.75V even for the IDS vs VDS characteristics at VBS=0 and VBS=high respectively.

Figure 1. BSIM3 Fit Variables Screen.