Post on 27-Jul-2020
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Electrical
Characteristics of
MOSFETs - 6Lecture# 17
VLSI Design
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Delay in CMOS Devices
We know that the MOSFETs normally behave like a switch
There is a delay present in switching the MOSFET from one state
to another
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
tr = Rise Time of Output
tf = Fall Time of Output
tphl = Propagation Delay high-to-low
tphl = Propagation Delay low-to-high
Vi Vo
Delay in CMOS Devices
For any digital VLSI Design the important factor is the delay
between input and output
The delay of devices which are dependent on other device in
processing chain
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Delay in CMOS Devices
For any digital VLSI Design the important factor is the delay
between input and output.
The delay of devices which are dependent on other device in
processing chain.
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
A1
A2
A3
A1
A2
A3
Output 1 0 0 1 0
Delay in CMOS Devices
For any digital VLSI Design the important factor is the delay
between input and output.
The delay of devices which are dependent on other device in
processing chain.
The delay factor is important as it determines the maximum
frequency of a device.
To understand the delay it is necessary to understand two
important properties which are
Resistance of MOSFETs
Capacitance of MOSFETs
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Delay in CMOS Devices
To understand the delay it is necessary to understand two
important properties which are
Resistance of MOSFETs
Capacitance of MOSFETs
Resistance: If the resistance is high, the current of device will be
low, so we require more time to charge a capacitor resulting
delays in our design
Capacitor: If larger capacitances in MOSFETs, large amount of
time is required to charge and discharge the capacitors
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
It is very important to study the Resistance and
Capacitance of your design
MOSFET RC Model
A MOSFET has non-linear IV characteristics
Making the design and analysis, a complex task
Usually simpler linear models are used for hand calculations
Realistic models are used for spice simulations
Design process involves creating/building a new system
To contain the complexity, simpler models are used
Once the design is ready it is analyzed using complex models
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Electrical
Characteristics of
MOSFETs - 7Lecture# 17
VLSI Design
8
MOSFET RC Model
A MOSFET has non-linear IV characteristics
Making the design and analysis, a complex task
Usually simpler linear models are used for hand calculations
Realistic models are used for spice simulations
Design process involves creating/building a new system
To contain the complexity, simpler models are used
Once the design is ready it is analyzed using complex models
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Resistance in MOSFETs
In MOSFETs there are different sources of resistances
Two Major components:
Drain / Source Resistance
Channel Resistance
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
n+ n+
Source Drain
W
L
RS RDRn
Resistance in MOSFETs
Drain / Source Resistance
A parasitic component that effects the performance of MOSFETs
It is more pronounced in smaller size MOSFETs
The formula is:
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
n+ n+
Source Drain
W
L
RS RDRn
𝑅𝐷/𝑆 = 𝑅𝑠ℎ × 𝑁𝐶 + 𝑅𝐶
𝑅𝐷/𝑆 Drain/Source Resistance
𝑅𝑠ℎ Sheet Resistance
𝑁𝐶 No. of Contacts
𝑅𝐶 Contact Resistance
Resistance in MOSFETs
Drain / Source Resistance
𝑅𝑠ℎ is sheet resistance and is given by
𝑅𝑠ℎ =𝜌
𝑡
here 𝑡 is thickness of the metal
𝑁𝐶 is given by 𝐿𝑚
𝑊𝑚, where 𝐿𝑚 and 𝑊𝑚 are length and width of metal
conductor connected to contact
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
𝑊𝑚
p-substrate
n+
Source
Drain
n+
𝐿𝑚
Resistance in MOSFETs
Drain / Source Resistance
At Contact and Semiconductor junction there will be a drop in potential
So, 𝑅𝐶 is contact resistance that is because of Metal and Semiconductor
contact junction
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
p-substrate
n+
Source
Drain
n+
n+
𝑅𝐷/𝑆 = 𝑅𝑠ℎ × 𝑁𝐶 + 𝑅𝐶
Resistance in MOSFETs
Channel Resistance (𝑅𝑛)
Generally, the channel resistance is given by
𝑅𝑛 = 𝑉𝐷𝑆𝑛/𝐼𝐷𝑛
𝐼𝐷𝑛 varies with 𝑉𝐷𝑆𝑛, which makes 𝑅𝑛 a function of 𝑉𝐷𝑆𝑛
As discussed, these are not the only factors which results in non-linearity
Channel Resistance depends on the region of operation
This can be explained by all regions separately
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Resistance in MOSFETs
Channel Resistance (𝑅𝑛)
Active Region - Point ‘a’
𝐼𝐷𝑛 ≈ 𝛽𝑛 𝑉𝐺𝑆𝑛 − 𝑉𝑇𝑛 𝑉𝐷𝑆𝑛
𝑅𝑛 ≈1
𝛽𝑛 𝑉𝐺𝑆𝑛−𝑉𝑇𝑛
In above equation
𝛽𝑛 = 𝑘𝑛′ 𝑊
𝐿,
𝑘𝑛′ = 𝜇𝑛
𝜖𝑜𝑥
𝑡𝑜𝑥,
𝐶𝑜𝑥 =𝜖𝑜𝑥
𝑡𝑜𝑥
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Resistance in MOSFETs
Channel Resistance (𝑅𝑛)
Non-saturation Region - Point ‘b’
𝐼𝐷𝑛 =𝛽𝑛
22 𝑉𝐺𝑆𝑛 − 𝑉𝑇𝑛 𝑉𝐷𝑆𝑛 − 𝑉𝐷𝑆𝑛
2
𝐼𝐷𝑛 = 𝑉𝐷𝑆𝑛𝛽𝑛
22 𝑉𝐺𝑆𝑛 − 𝑉𝑇𝑛 − 𝑉𝐷𝑆𝑛
𝑉𝐷𝑆𝑛
𝐼𝐷𝑛= 𝑅𝑛 =
2
𝛽𝑛 2 𝑉𝐺𝑆𝑛−𝑉𝑇𝑛 −𝑉𝐷𝑆𝑛
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Resistance in MOSFETs
Channel Resistance (𝑅𝑛)
Saturation Region - Point ‘c’
𝐼𝐷𝑛 =𝛽𝑛
2𝑉𝐺𝑆𝑛 − 𝑉𝑇𝑛
2
𝑉𝐷𝑆𝑛
𝐼𝐷𝑛= 𝑅𝑛 =
2 𝑉𝐷𝑆𝑛
𝛽𝑛 𝑉𝐺𝑆𝑛−𝑉𝑇𝑛2
In saturation region the current is constant. To achieve this
behavior
𝑅𝑛 ∝ 𝑉𝐷𝑆𝑛
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad
Resistance in MOSFETs
Observations
𝑅𝑛 ∝1
𝛽𝑛while 𝛽𝑛 = 𝑘𝑛
′ 𝑊
𝐿 𝑛
Important Factor is aspect Ratio of MOSFET which is (W/L)
Increasing width ‘W’ results in increased 𝛽𝑛 and decreased Rn
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Dr. Sohaib Ayyaz Qazi | COMSATS University Islamabad