physics of mos structures part i

8/2/2019 Physics of MOS Structures Part I

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IHP - MicroelectronicsIm Technologiepark 2515236 Frankfurt (Oder)

Germany

IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved

Physics ofMetal-Oxide-Semiconductor

(MOS) Structures(Part I)

Thomas Schrder


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Overview

2) Physics of MOS - Structures

1) MOS structures in important semiconductor device classes


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Overview of MOS structures in important device applications

Logic devices: Metal - Oxide Semiconductor Field Effect Transistor

Besides isolation (see image) of the individual transistorsfrom each other, thermally grown silicon dioxide (SiO2)

plays a crucial role in the performance of the transistoras the gate dielectric layer.

transistor physics and the role of dielectricswill be discussed in the lectures held on

12/2/ 2005


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Volatile Memory devices: Dynamic random access memory (DRAM)

Dielectrics play a crucial role to guarantee the storage ofsufficient charge despite continuously

shrinking device dimensions

Volatile memories and the role of dielectrics

will be discussed in the lectures held on12 / 9 / 2005

Q is S CC CC

C V A V d

= =

Yesterday:

planar cells

Today:

3D cells


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Nonvolatile Memory devices: Flotox and Flash Concepts

Charge is injected (write) and driven out (erase)by high voltage pulses

Nonvolatile memories and the role of dielectricwill be discussed in the lectures held on

12 / 16 / 2005

Today`s market leader:

Q /T S CG

V C =


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Nonvolatile Memory devices: Ferroelectric random access memory (Fe-RAM)

Ferroelectrics allow to store informationwhen the power is switched off

Volatile memories and the role of dielectricswill be discussed in the lectures held on

12 / 16 / 2005

Today and more to come in future:


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flash FLOTOX

DRAM

StorageREQUIREMENTS

Electrically Stable Interface with

Few Defect ChargesImportant Important

Standard

CMOS

Nonvoltatile Memories

-- Negligible

High Mobility for High Speed Vital Important Negligible Negligible

Negligible

Low Leakage &

Few Dielectrics BreakdownsImportant Important Important Vital

High Reliability against Hot Carriersinjected from substrate

Important Important Negligible

Important

Thermal & Chemical Stability

high temp. (850-1000 C)Important Important -- --

High Reliability against Carriersflowing through films (high QBD, etc.)

Important Vital Vital

Important

High Dielectric Constant Negligible Negligible Negligible Important

Good Coverage on Steps Negligible Negligible Negligible

Gate Dielectrics

in Active FET's

used as

Passive Elements

Overview of MOS strucures in important device applications

Scaling makes the development of advanced dielectric layersa hot topic in materials research

Ongoing scaling requires tothin out the dielectric layer

Dielectric layers need to meet very differentrequirements specific for each application


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Overview

2) Physics of MOS - Structures

1) MOS structures in important semiconductor device classes


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( )/ 2 0ms m g F x E q + + =( / )

F t A i In N n = for p-type( / )

F t D i In N n = for n-type

Ideal MOS - Structure

Very simple system geometry .. but quite complicate physics

Let`s start with some idealizations:

1) Work function difference is zero

Thermal voltage: kt/q

2) No charged defects in the insulator 3) No current over the dielectric (perfect insulator)

Ideal MOS structuredrawn for p-type Si

Fermi Potential


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Ideal MOS Structure at different biases VG

If you bias the metal gate at different gate voltages VG, threedifferent situations can in principle arise:(discussion is based here on p-type Si)

Accumulation;majority carriers accumulate in semiconductor surface region

Depletion;

negative charge appears in semiconductor surface regionbecause majority carriers are chased away(uncompensated acceptor atoms rest behind)

and minority carriers are attracted

Inversion;

high negative charge is induced insemiconductor surface region

more minority than majority carriers(Ei crosses over EF in surface region (intrinsic condition) )


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/ /

D in(x) N e =n et t =

/ /

A i p(x)=N e =n et t

Derive a relationship between the charge in thesemiconductor and the surface band bending

Band bending:

zero inside the semiconductor;

Band bending

Surface Potential

Surface potential:

referenced to Fermi potential

Note:Arrow downwards (upwards):

positive (negative) band bending

Fermi Potential

Result:Band bending is the sum of surface plus Fermi potential

Using the band bending (or surface potential) as a function of the distance x from the interface, the chargecarrier concentration can be described as a function of x:

intrinsic carrier concentrationFor specific doping level NA and ND


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[ ]2

D A2

s s

(x) q=- =- p(x)-n(x)+N -N

x

tA)

D

- = 2 F( ,Nx L

s tD

AL = qN

From these relations, the 1 D Poisson equation is solved under the following assumptions:

1) NA is uniform over x ; 2) Boltzmann statistics; 3) no surface charge quantization

First Integration:Electric field is givenby potential gradient

1 D Poisson equation:non-uniform volume charge

density causes curvatureof potential

t- / /2

A t i A tF( ,N )= e + / -1+(n /N ) (e - / -1)t

Debye Length

just forcompleteness



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s ts s s s A

D

Q - = 2 F( ,N )L

m

The relationship between the charge in thesemiconductor surface and the surface band bending then reads:

Solid Line: NA = 4*1015 cm-3 ; Dotted Line: NA = 2*10

17 cm-3

Influences of doping concentration:1) certainly, Fermi level is a function of doping

2) the more p-type the wafer, the higher thepotential for inversion

Accumulation:From the sign of surface band bending:

semiconductor charge is positive

Depletion:From the sign of surface band bending:

semiconductor charge is negative

Inversion:From the sign of surface band bending:semiconductor charge is negative

(a) L = 100 nm Transistor; (b) L = 10 nm Transistor:dielectrics has to survive higher electric fields


physics of mos structures part i

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