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7/30/2019 verilog-a mos model
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By:
AVIRUP DASGUPTA (Y9227159)
VAIBHAV GOYAL (Y9635)
RAHUL JHA (Y9450)
Threshold Voltage Based Modelling
Course: Integrated Circuit Technology
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Introduction
Threshold voltage is basically the value of gate-source voltage to turn on the transistor.
Threshold voltage is very important parameter in
the modeling of MOSFET device.
Depending on Threshold voltage, the MOSFET
operation can be divided into three major regions: Weak Inversion Region: Gate voltage less than threshold
voltage. Here inversion charge density is less than doping
concentration Transition Region: Gate voltage comparable to Threshold
voltage.
Strong Inversion Region: Gate voltage higher compared to
threshold voltage. Inversion charge density higher than doping
concentration.
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Standard Model
The threshold voltage is governed by thefollowing equation
Vth : Threshold voltage, VFB : Flatband Volatge
,,,,,,,,,,,,,,,,,,,,,,,
However the standard model applies only to long uniformly doped channel
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Vertical Non-Uniform Doping
More concentration near Si/SiO2 interface anddecreases throughout the interface
Modeling governed by following equations:
Where gamma is the body effect coefficient for doping, Vbm is maximumbody bias and Vbx is body bias for depletion width = Xt such that
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Other Effects
DIBL Effects: Drain voltage control over channelbecause of high drain voltage. Modeling is
governed by:
where KDIBL is given by
Mobility Model
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Effective Vgs-teffand Vdseff Current equation is different in three regions
(mentioned earlier). Hence single smooth
equations of Vgs-teffand Vdseffhave been
developed that work perfectly in all three regions.
Here delta is the smoothening factor.
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Current Calculation
Having smoothened equations for Vgsteffand Vdseff, we now derive the current equation valid for all
the regions:
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Channel Length Modulation and DIBL Effect
The modified equations once we include channel lengthmodulation and DIBL effect can be given as
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Voltage Saturation
Source-Drain Resistance
Impact Ionization Current
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Charge Calculations The inversion charge density from a distance x from the source
can be calculated by the following formula:
Hence Total Inversion Charge is the integration from 0 to Length
of the channel which gives:
In Accumulation region: Qacc= Cox(Vgb - Vfb) = QG
In Depletion region: Qg = Qdep
In Inversion region: Qg= -QI
Charge equation valid in all regions should be of form:
max(min(Qacc, Qdep),QI) in max and min functions can be
considered as below:
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Simulation Results
Ids versus Vds plot for different Vg
500
400
300
200
100
00 0.5 1 1.5 2
2.5
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Simulation Results
Ids versus Vgs on semilog axis Plot. DIBL effectcan be seen
0 0.5 1 1.5 2
2.5
-2.5
-5.0
-7.5
-10.0
-12.5
-15.0
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Simulation Results Impact Ionisation Current versus Vg on semilog axis for different
Vds.
0 0.5 1 1.5 2
2.5
2.5
0
-2.5
-5.0
-7.5
-10.0
-12.5
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Simulation Results
3rd order derivatives are continuous
0 0.5 1 1.5 2
2.5
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0
-5.0
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THANK YOU
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