Transcript
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’.
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 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 curve-fitting parameters.
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.
Solution- The 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
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