The Bsim Spice Model

By Ajay Kumar Gautam P08ec901 SVNIT, Surat 

The Berkeley Short Channel IGFET Model, BSIM, is a SPICE model for both n- and p-channel MOS transistors.  The BSIM1 SPICE model is adequate for modeling MOSFETs with channel lengths down to 1 micro mt.  Shorter channel lengths require the use of the BSIM2 or BSIM3 model.  Here, we will discuss, how to determine threshold voltage, transconductance parameter, and other related parameter, using the BSIM model. 

All electrical parameters, P', used in the BSIM model are determined using

-(1)

where

L and W are the drawn channel length and width, DL and DW are the deltalength and deltawidth . L - DL and W - DW are Leff and Weff. P, LP, and WP are the electrical parameters associated with the electrical parameter P’.

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It give the equations and examples for determining the a) threshold voltage, b) drain current, and c) subthreshold drain current

2.1 The Threshold Voltage 

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The threshold voltage is calculated in the BSIM1 model using

-(2) VSB and VDS are the substrate-to-source and drain-to source voltages. ETA is zero-bias drain-induced barrier-lowering coefficient The model parameters are calculated using Eq.(1). ETADB is given by

-(3)

It is not practical to calculate the thershold voltage by using Eq.(2).  So, assume device is large so that second & third term in Eq.(1) are negligible.  Thershold voltage is -(4) Example 1 Compare the exact equation (2) for VTHN with the approximate equation (4) when the substrate potential is -5 V, the source is grounded, and the drain is at 2V. Use the BSIM1 model parameters for the CN20 process with W/L =3/2. 

 SOLUTION-

First, calculate the simple approximation for the threshold voltage using Eq. (4). This is given by Or a general equation in terms of VSB for this process is

Now we substitute these values into Eq. (2), We see a difference of 40 mV for this minimum-size device between the exact calculations using Eq. (2) and the hand calculations using Eq. (4). CONCLUSION—  These results are not representative of submicron device sizes where the BSIM1 parameters may lose their physical significance. They may behave like curvefitting parameters. 

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Example 2-

Using the CN20 BSIM1 model parameters, plot the drain current of a minimum-size MOSFET against gate-source voltage when VSB varies from 0 to 5 V and the VDS of the MOSFET is 5 V.  SolutionThe plot is shown below.

Figure 2 Threshold voltage change with substrate bias.

NOTE a) The threshold voltage increases as the substrate potential becomes more negative (the body effect). b)The threshold voltage tends to change less as VSB is decreased.  This fact is used to get better threshold matching in analog CMOS design.  The body effect shown in Fig. 2