Electrical Characteristics of MOSFETs - 6saqazi.com/EEE434/...Electrical_Characteristics_of_MOSFETs_6_7.pdf · MOSFET RC Model A MOSFET has non-linear IV characteristics Making the

Electrical

Characteristics of

MOSFETs - 6Lecture# 17

VLSI Design

1

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

2

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


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

3




between input and output.


processing chain.

4


A1

A2

A3

A1

A2

A3

Output 1 0 0 1 0



between input and output.


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

5



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

6


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

7


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

9


Resistance in MOSFETs

In MOSFETs there are different sources of resistances

Two Major components:

Drain / Source Resistance

Channel Resistance

10


n+ n+

Source Drain

W

L

RS RDRn



A parasitic component that effects the performance of MOSFETs

It is more pronounced in smaller size MOSFETs

The formula is:

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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



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

12


𝑊𝑚

p-substrate

n+

Source

Drain

n+

𝐿𝑚



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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p-substrate

n+

Source

Drain

n+

n+

𝑅𝐷/𝑆 = 𝑅𝑠ℎ × 𝑁𝐶 + 𝑅𝐶


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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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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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

16




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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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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Electrical Characteristics of MOSFETs - 6saqazi.com/EEE434/...Electrical_Characteristics_of_MOSFETs_6_7.pdf · MOSFET RC Model A MOSFET has non-linear IV characteristics Making the