Basic MOS Device Physics

Lecture 16MSE 515

Topics

• MOS Structure• MOS IV Characteristics• CCD

Revolution and Evolution in Electronics

Source: IntelSource: Intel

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Source: IntelSource: Intel

1,000,0001,000,000

100,000100,000

10,00010,000

1,0001,000

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1 Billion 1 Billion TransistorsTransistors

80868086

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PentiumPentium®® IIII

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NMOS Structure

LD is caused by side diffusion

Source: the terminal that provides charge carriers. (electrons in NMOS)Drain: the terminal that collects charge carriers.

Substrate contact--to reverse bias the pn junctionConnect to most negative supply voltagein most circuits.

• Although no current should ideally conduct before threshold, a small percentage of electrons with energy greater than or equal to a few kT have sufficient energy to surmount the potential barriers!

Subthreshold Characteristics

Short-Channel MOSFETs

• As a result, there is a slight amount of current conduction below VT

Potential contours in a long channel MOSFET.

In a long channel MOSFET, the potential is uniform and parallel to the gate.

Short-Channel MOSFETs

Narrow Width Effect

• If the Polysilicon gate is atop the region of a LOCOS isolation where the oxide is increasing in thickness.

• It is possible to form a channel under LOCOS away from the thin gate oxide! This is quite important for devices with L < 1 mm.

Short-Channel MOSFETs

CMOS Structure

PMOSNMOS

Reverse bias the pn junction

Reverse bias the pn junction

Connect to most positive supply voltage in most

circuits.

MOS IV Characteristics

• Threshold Voltage• Derivation of I/V Characteristics

– I-V curve– Transconductance– Resistance in the linear region

• Second Order Effect– Body Effect– Channel Length Modulation– Subthreshold conduction

Threshold Voltage

1. Holes are expelled from the gate area2. Depletion region (negative ions) is

created underneath the gate.3. No current flows because no

charge carriers are available.

MOSFET as a variable resistor

The conductive channel between S and D can be viewedas resistor, which is voltage dependent.

Threshold Voltage (3)When the surface potential increases to a critical value, inversion occurs.1. No further change in the width of the

depletion region is observed.2. A thin layer of electrons in the depletion

region appear underneath the oxide.3. A continuous n-type (hence the name

inversion) region is formed between the source and the drain. Electrons can no be sourced from S and be collected at the drain terminal. (Current, however, flows from drain to source)

4. Further increase in VG will fruther incrase the charge density.

The voltage VG required to provide an inversion layer is called the threshold voltage.

Implantation of p+ dopants to alter the threshold

Threshold voltage can be adjusted by implantingDopants into the channel area during fabrication.E.g. Implant p+ material to increase threshold voltage.

Formation of Inversion Layer in a PFET

The VGS must be sufficient negative to produce an inversion layer underneath the gate.

I-V Characteristics

Channel Charge

A channel is formed when VG is increased to the pointthat the voltage difference between the gate and the channel exceeds VTH.

Application of VDS

What happens when you introduce a voltage at the drain terminal?

Channel Potential Variation

VX the voltage along the channel

VX increases as you move from S to D.

VG-VX is reduced as you move from S to D.

E.g. VS=0, VG=0.6, VD=0.6At x=0, VG-VX=0.6 (more than VTH)At x=L, VG-VX=0 (less than VTH)

Pinch Off

Small VDS

Large VDS

No channelElectrons reaches the Dvia the electric field in the depletion region

Saturation Region

Linear Region

MOSFET as a controlled linear resistor

1. Take derivative of ID with respect to VDS

2. For small VDS, the drain resistance is

Transistor in Saturation Region

• I-V characteristics• Transconductance• Output resistance• Body transconductance

Saturation of Drain Current

Transconductance

Analog applications:How does Ids respond to changes in VGS?

IDS vs VGS

0.13 um NMOSVDS=0.6 VW/L=12um/0.12 umVB=VS=0Y axis: Ids

X axis: Vgs

Different Expressions of Transconductance

Channel Length Modulation

As VDS increases, L1 will move towards the source, sincea larger VDS will increase VX .

L is really L1

ID will increase as VDS increases.The modulation of L due to VDS is called channel length modulation.

Controlling channel modulation

For a longer channel length, the relative change in L andHence ID for a given change in VDS is smaller.

Therefore, to minimize channel length modulation, minimumlength transistors should be avoided.

Output resistance due to gds

MOS Device Layout

MOS Capacitances

Detector zoologyX-ray Visible NIR MIR

l [mm]

Silicon CCD & CMOS

0.3 1.1

0.9 2.5 5 20

HgCdTe

InSb

STJ

0.1

Si:As

In this course, we concentrate on 2-D focal plane arrays. • Optical – silicon-based (CCD, CMOS)• Infrared – IR material plus silicon CMOS multiplexer

Will not address: APD (avalanche photodiodes)STJs (superconducting tunneling junctions)

Step 2: Charge Generation

Silicon CCD

Similar physics for IR materials

33

CCD Introduction

• A CCD is a two-dimensional array of metal-oxide-semiconductor (MOS) capacitors.

• The charges are stored in the depletion region of the MOS capacitors.

• Charges are moved in the CCD circuit by manipulating the voltages on the gates of the capacitors so as to allow the charge to spill from one capacitor to the next (thus the name “charge-coupled” device).

• An amplifier provides an output voltage that can be processed.

• The CCD is a serial device where charge packets are read one at a time.

34

Potential in MOS Capacitor

35

CCD Phased Clocking: Summary

36

123

CCD Phased Clocking: Step 3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3

37

CCD output circuit

38

Charge Transfer Efficiency

• When the wells are nearly empty, charge can be trapped by impurities in the silicon. So faint images can have tails in the vertical direction.

• Modern CCDs can have a charge transfer efficiency (CTE) per transfer of 0.9999995, so after 2000 transfers only 0.1% of the charge is lost.

good CTE bad CTE

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Threshold Voltage

• VG=0.6 V• VD=1.2 V• CMOS: 0.13 um• W/L=12um/0.12

um• NFET

I-V characteristic Equation for PMOS transistor

More on Body Effect

• Example• Analysis• gmbs

Variable S-B Voltage

constant

gm as function of region

saturation

0.13 um NMOSVGS=0.6 VW/L=12um/0.12 umVB=VS=0Y axis: gmX axis: vds

linear

gds

saturation

0.13 um NMOSVGS=0.6 VW/L=12um/0.12 umVB=VS=0Y axis: gmX axis: vds

linear

Slope due to channel length modulation

Body Effect

The n-type inversion layer connects the source to the drain.The source terminal is connected to channel. Therefore, A nonzero VSB introduces charges to the Cdep. The math is shown in the next slide.

A nonzero VSB for NFET or VBS for PFET has the net effectOf increasing the |VTH|

Experimental Data of Body Effect

W/L=12 um/0.12umCMOS: 0.13 um processVDS=50 mVSimulator: 433 mVAlternative method: 376 mV

Subthreshold current

Subtresholdregion

As VG increases, the surface potential will increase.

There is very little majority carriers underneath the gate.

There are two pn junctions. (B-S and B-D)The density of the minority carrierdepends on the difference in the voltage across the two pn junction diode.A diffusion current will result the electron densities at D and S are not identical.

Conceptual Visualization of Saturation and Triode(Linear) Region

NMOS

PMOS

I-V Characteristic Equations for NMOS transistor

(Triode Region:VDS<VGS-VTH)

Saturation: VDS>VGS-VTH

To produce a channel (VGS>VTH)

VTH as a function of VSB

(VTH0: with out body effect)

Body effect coefficient

VSB dependent

Sensitivity of IDS to VSB

(chain rule)

gm

η=1/3 to 1/4, bias dependent

Bias dependent CGS and CGD

Complete NMOS Small Signal Model

Complete PMOS Small Signal Model

Transconductance in the triode region

(Triode region)

For amplifier applications, MOSFETs are biased in saturation

Small signal model of an NMOS

Small Signal Model

• If the bias current and voltages of a MOSFET are only disturbed slightly by signals, the nonlinear amd large signal model an be reduced to linear and small signal representation.