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EE603 EE603 –– CMOS IC CMOS IC DESIGN DESIGN

TOPIC 3 TOPIC 3 –– THE DEVICETHE DEVICE

Faizah Amir

POLISAS

TEKNOLOGI

TERAS

PEMBANGUNAN

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LESSON LEARNING OUTCOME

At the end of this session, you should be able to:

1. explain the diode basic concept.2. explain the design abstraction level.3. explain MOS transistor theory.4. explain the issues related to submicron

MOS transistor.5. apply MOS transistor theory in MOS

switch model.

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Electrons and Holes

• Atoms with one extra valence electron than silicon are used to produce "n-type" semiconductor material.

•Atoms with one less valence electron result in "p-type" material.

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• P-type material has large amount of acceptor ions and corresponding number of holes.

• N-type material has large amount of donor ions and corresponding number of electrons.

Electrons and Holes

N-type (negative) P-type (positive)

DopantGroup V

(e.g. Arsenic, Antimony and Phosphorus)

Group III(e.g. Indium, Gallium

and Boron)

Bonds Excess Electrons Missing Electrons

(Holes)

Majority Carriers Electrons Hole

Minority Carriers Holes Electrons

Properties of extrinsic semiconductor in silicon

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N-Type & P-Type Semiconductor

• P-type material has majority of holes and a few electrons.

• Holes are called majority carriers and thermally generated electrons are called minority carriers.

• N-type material has majority of electrons and a few holes.

• Electrons are called majority carriers and holes are called minority carriers.

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P-N Junction Diode

• When the p-type and n-type semiconductors are joined , there will be a flow of electrons from n-type material to the p-type material, due to the diffusion process.

• Some of the free electrons in the n-type that diffuse across the junction and combine with holes to form negative ions in p-side and leaves behind positive ions on the n-side.

• The combining of electrons and holes depletes the holes in the p-type and electrons in the n-type near the junction.

• The region near the junction acts as a barrier which blocks movement of carriers. This region which blocks the movement of charge carriers, is called depletion region.

Formation of a P-N junction

Diode SymbolP-N Junction

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Biasing The P-N Junction Diode

If a reverse bias voltage is applied across the P-N junction, the depletion region expands, further resisting any current through it.

If a forward bias voltage is applied across the P-N junction, the depletion region will collapse and become thinner, so that the diode becomes less resistive to current through it.

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Diode in IC

p

n

n

p

A

B

Al

One-dimensional representation

Diode Symbol

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Ideal Diode Equation

φT - thermal voltage and is equal to 26 mV at room temperature.

IS - saturation current of the diode (constant value) and determined empirically

The ideal diode equation relates the current through the diode (ID) to the diode bias voltage VD.

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Ideal Diode Equation

Simplification:

VD / φφφφT

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Ideal Diode Equation

An ideal diode would show an abrupt transition from perfectly conducting (diode is short circuit) to perfectly non-conducting (diode is open circuit) states at 0V.

I

V

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Ideal Diode Equation

Diode acts as switch since:In the ON state, diode is short circuit and in the OFF state, diode is open circuit.

I

V

ONOFF

Symbol for ideal diode

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Real Diode

Diodes formed in different materials have different cut-in voltages.

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MOS Transistor

• MOS transistor is known as MOSFET (metal oxide semiconductor field-effect transistor).

• MOSFET that is widely used in integrated circuit is the CMOS technology (combination of NMOS and PMOS).

• MOSFET is widely used nowadays in electronic equipment, e.g. mobile phone, computer, medical electronic equipment, etc.

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MOSFET

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MOS TransistorMOS Transistor Cross-Section :

MOS Transistor Symbol :

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p-substratep-substrate

N-well

Or VG < VT

MOSFET Operation :Cutoff

The channel is not established and the device is in a non-conducting state (also called cutoff or sub-threshold).

Depletion region

VD Small

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Channel Existence

• At VGS = VT, the positive gate voltage causes positive charge to accumulate on the gate electrode and negative charge on the substrate side.

• When VGS > VT and small VDS is applied, the semiconductor surface below the gate inverts to n-type material.

• The n-type material that exist between source and drain regions is called n-channel.

• At this point, current ID starts to flow from drain to source through the channel.

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MOSFET Operation :

• Threshold Voltage (VT ) defines the voltage at which a MOS transistor begins to conduct.

• The value of threshold voltage is in a range of 0.5 Volts.

• Factor that affects the threshold voltage:i. Substrate doping

ii. Oxide thicknessiii. Gate material

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Threshold Voltage

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Resistive or Linear Operation

• VD is increased until VD < VG - VT, a continuous conductive channel still exist between source and drain regions.

• The transistor is in the resistive or linear region. • At that point, the conducting channel disappears or is pinched off near

the drain.• In resistive or linear operation, the current ID increase proportionally to

VD.

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Saturation Region

• VDS is increased until VDS > VGS – VT, the channel thickness gradually is reduced from source to drain until pinch-off increased until the channel length is decreased.

• Under those circumstances, the transistor is in the saturation region and consequently, the current remains constant (or saturates).

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NMOS Transistor I-V Characteristics

I-V Characteristics of NMOS Transistor

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T

NMOS & PMOS Transistor I-V Characteristics

I-V Characteristics of NMOS vs. PMOS

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NMOS Operation

For further references :

www.circuitstoday.com/mos-transistors-operation

claymore.engineer.gvsu.edu/~streiana/214/NMOS.pdf

www.doe.carleton.ca/~tjs/21-MOS

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Submicron MOS Transistor

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Channel Length

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Channel Length

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Issues Related to Submicron MOS Transistor

i. Channel Length Modulationii. Velocity Saturationiii. Sub-threshold Conductioniv. Threshold variationsv. Parasitic Resistancesvi. Latch-up

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Channel Length Modulation

As VDS increases aboveVDSAT, the length of the depleted channel beyond pinch-off point, ∆∆∆∆L, increases and actual L decreases.

ID increases slightly with VDS instead of being constant.

For long-channel transistors, the effect of channel length variation is not prominent. With the decrease in channel length, however, the variation matters.

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Velocity Saturation

The behaviour of short - channel transistors deviate considerably from the resistive and saturated models. This deficiency is the velocity saturation effect.

• The carrier mobility of semiconductor is a constant. However, at high field strengths, the carriers fail to follow this linear model.

• When the electrical field along the channel reaches a critical value, the velocity of the carriers tends to saturate due to scattering effects (collisions suffered by the carriers).

• This means that in short-channel devices, the saturation point is reached with only a couple of volts between drain and source.

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Sub-Threshold ConductionIn short-channel transistor, the current does not drop abruptly to 0 at VGS = VT , meaning that MOS transistor is already partiallyconducting for voltages below the threshold voltage. This effect is called sub-threshold or weak-inversion conduction. The transition from the on- to the off-condition is thus not abrupt, but gradual.

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Threshold Variations

• Threshold voltage is only a function of the manufacturing technology and the applied body bias VSB . The threshold can therefore be considered as a constant over all NMOS (PMOS) transistors in a design.

• As the device dimensions are reduced, this model becomes inaccurate, and the threshold potential becomes a function of L, W, and VDS.

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Parasitic ResistanceParasitic resistance – uninvited resistances in series with the drain and source regions in

scaled down transistors. How? The junctions become shallower and smaller contact openings.Effect: The resistances cause a deterioration in the device performance,

as it reduces the drain current for a given control voltage. The resistance must be kept as small as possible.

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RC - the contact resistanceW - the width of the transistorLS,D - the length of the source or drain region R is the sheet resistance per square of the drain source diffusion, and ranges from 20 to 100 Ω/.

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Latch-up

Definition

A type of short-circuit which can occur in an improper designed circuit.

The existence of a low-impedance path between the power supply rails of a MOSFET circuit will trigger a parasitic structure which will disrupt the proper functioning of the part, possibly even leading to its destruction due to over current.

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Latch-up

• The MOS technology contains a number of intrinsic bipolar transistors. The combination of wells and substrates results in the formation of parasitic resistance.

• The n-p-n-p structure is formed by the source of the NMOS, the p-substrate, the n-well and the source of the PMOS.

Origin of Latch- up

NMOS PMOS

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Latch-up

Latch-up Equivalent Circuit

• The n-p-n-p structure is formed by thesource of the NMOS, the p-substrate, the n-well and the source of the PMOS. The equivalent circuit form a thyristor-like devices.

• Triggering the thyristors leads to a shorting of the VDD and VSS lines, usually resulting in a destruction of the chip, or at best a system failure that can only be resolved by power-down.

• When one of the two bipolar transistors gets forward biased (e.g., due to current flowing through the well, or substrate), it feeds the base of the other transistor. This positive feedback increases the current until the circuit fails or burns out.

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Latch-up

• Place numerous well and substrate contacts close to the source connections of the NMOS/PMOS devices ( to minimized the resistances Rnwell and Rpsubs ) .

• Place guard rings around the transistors to reduce the resistance.

How to avoid latch-up phenomenon?

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SPICE MODELS

Level 1 : Long Channel Equations – Very Simple

Level 2 : Physical Model – Includes Velocity Saturation and Threshold Variations

Level 3 : The formulations are derived from the model 1 and take into account a set of physical limitations in a semi-empirical way (based on curve fitting to the measured devices.

Level 4 (BSIM) : Berkeley Short-channel IGFET Model recommended for ultra-deep submicron technology simulation

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MOS Switch Model

NMOS Transistor Switch Model

SYMBOL

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Switch is ON Switch is OFF

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MOS Switch Model

PMOS Transistor Switch Model

SYMBOL

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Switch is ON Switch is OFF

Design Abstraction Level

As the IC design becomes more complex, how to design chips with more and more functions?

Efficient design method is needed: different levels of abstraction.

Design abstraction :The complexity of the circuit is reduced by successively replacing detail with simplifications at higher levels of abstraction.

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Design Abstraction Level

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Design Abstraction Level

Level Explanation

1. Specification/System The chip function and specification is stated clearly (as

stated in data books).

2. Module The function/specification identified is represented in

register form. The function blocks are divided into

smaller blocks such as counter, register and

combinational logic.

3. Logic Gate The blocks earlier are divided into logic gates such as

NAND gate, NOR gate, XOR gate, etc.

4. Transistor Circuit Logic gates are represented in transistor form such as

NMOS and PMOS.

5. Device/Layout The layout of each transistor is produced and sent to

the IC fabrication lab.

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