ece520 – vlsi designpzarkesh/ece520/lectures/lecture2.pdf · ece520 - lecture 2 university of new...

ECE520 - Lecture 2 Slide: 1University of New Mexico

Office: ECE Bldg. 230BOffice hours: Wednesday 2:00-3:00PM or by appointment

E-mail: [email protected]

Payman Zarkesh-Ha

ECE520 – VLSI Design

Lecture 2: Basic MOS Physics


Review of Last Lecture Semiconductor technology trend and Moor’s law

Benefits of transistor scaling:● More functionality in the same foot print● Faster device● Devices with less switching energy● Less cost/function

Challenges of transistor scaling:● Device size reaching quantum level● Power dissipation and heat removal concerns● Interconnect worsen by scaling● Manufacturing yield issues


Today’s Lecture

Overview of Diode Physics

BASIC MOS Physics:● Understanding of device operation● Basic device equations for long channel MOSFET● Long channel MOS models for manual analysis


Reading Assignment

Today we will review Chapter 3 (MOS Physics)● Skim through Diodes but focus on Section 3.2.3 (diode transient behavior)● Study Section 3.3 (MOS transistor) thoroughly


The Diode

n

p

p

n

B A SiO2Al

A

B

Al

A

B

Cross-section of pn -junction in an IC process

One-dimensionalrepresentation diode symbol


Depletion Region

hole diffusionelectron diffusion

p n

hole driftelectron drift

ChargeDensity

Distancex+

-

ElectricalxField

x

PotentialV

W2-W1

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.

Built-in potential


Forward Bias Diode

x

pn0

np0

-W1 W20p n

(W2)

n-regionp-region

Lp

diffusion

Typically avoided in Digital ICs


Reverse Bias Diode

x

pn0

np0

-W1 W20n-regionp-region

diffusion

The Dominant Operation Mode


Diode IV Curve


Junction Capacitance

2

i

DA0 n

NNLnq

KT10

DA

DAsiD0j NN

NN2qAC

Built-in potential


Diffusion Capacitance

qKT

T

Thermal Potential


What is a Transistor?

VGS VT

RonS D

A Switch!

|VGS|

An MOS Transistor


MOSFET Top & Cross Section View

Metal Oxide Semiconductor Field Effect Transistor


NMOS Device Cross-Section

Source Drain

Gate

Bulk

IDS is Defined as “from Drain to Source” Current● Majority carriers are electrons● NMOS device conducts when “gate-to-source” voltage is positive

IDS is as a function of:● Channel width (W)● Inverse of channel length (1/L)● Gate-to-source potential (VGS)


PMOS Device Cross-Section

Source Drain

Gate

Bulk

Complement of NMOS

Built inside an N-well implant in substrate

Majority carriers are holes, not electrons

Conducts when gate-source voltage is negative


Device Operation: Cutoff

Cutoff region (VGS = 0 )● The Source to Drain connection looks like two back to back

series connected diode

Therefore ideally IDS = 0● 1st order approximation only

VGS=0


Polysilicon gate forms a conductive top plate of a capacitor● Gate oxide forms the dielectric of a parallel plate capacitor● P-doped substrate forms the conductive bottom plate of a capacitor

Gate Oxide Capacitance


Device Operation: Depletion

As gate potential increases● Positive majority carriers (holes) in the substrate repelled

from the surface (depleting the material of carriers)● A depletion region is formed under the surface of the gate● This depletion region is formed as potential at the silicon

surface underneath the gate reaches φF

i

AF n

NLnq

KT

small VGS


Device Operation: Inversion As the surface potential beneath the gate increases

beyond φF● Electrons from heavily doped source and drain are attracted to

the gate and move into the channel● When the surface potential reaches 2φF the charge density of

electrons in the channel equals the original doping density of the P-substrate

● At this time the channel is inverted● Therefore, a conductive path is formed between source and drain

Large VGS


Device Operation: Inversion

Inversion region is simply a resistor

We need an applied VDS to get current flow

When drain voltage is applied the depletion region grows at the drain junction

Large VGS


MOSFET Band Diagram


MOSFET Threshold Voltage

– VGS component accounted for threshold adjustment implant

five


MOSFET Threshold Voltage Components

i

AF n

NLnq

KT

?

ox

I

ox

SS

ox

BFmsT C

QCQ

CQV 20



Qox


ox

I

ox

SS

ox

BFmsT C

QCQ

CQV 20


?

i

AF n

NLnq

KT



ox

FsiAB C

qNV

22

QB =



ox

ox

ox

FsiAFmsT C

QC

qNV

2220

ox

oxox t

C

i

AF n

NLnq

KTWhere: and

But what happen Bulk and Source are at different potential?


Body Effect

ox

SBFsiAB C

VqNV

22


Body Effect

-2.5 -2 -1.5 -1 -0.5 00.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

VBS

(V)

VT (V

)

Key Point: Body effect raises VT

Why does this matter?● In a stack (such as NMOS in a NAND

gate) the sources of higher devices in the stack do not equal 0V due to resistance of the lower transistors - this results in lower current drive (lower Ids) due to higher apparent VT

● Single polarity pass gates can only bring the drain to VDD-VT

● Body bias can be purposely created to lower standby power by modulating IOFF

FBSFTT VVV 220


ID = -vnQ(x)W

Current Voltage Relation


Current Voltage Relation

W


Device Operation: Linear Region


Device Operation: Saturation


Linear into Saturation


Saturation Region


Saturation Region Analogy


MOSFET Parameter Measurement


Channel Length Modulation


Device Operation: I-V curves

DSDS

DSTGSnDS VVVVVL

WKI

1

2

2

TGSDS VVV

DSTGSn

DS VVVL

WKI

12

2

TGSDS VVV

I DS

[mA]

VDS [V]

ece520 – vlsi designpzarkesh/ece520/lectures/lecture2.pdf · ece520 - lecture 2 university of new...

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