Physics of MOS Structures Part I
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Transcript of Physics of MOS Structures Part I
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8/2/2019 Physics of MOS Structures Part I
1/13
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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8/2/2019 Physics of MOS Structures Part I
2/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
Overview
2) Physics of MOS - Structures
1) MOS structures in important semiconductor device classes
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8/2/2019 Physics of MOS Structures Part I
3/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
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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8/2/2019 Physics of MOS Structures Part I
4/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
Overview of MOS structures in important device applications
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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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
Overview of MOS structures in important device applications
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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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
Overview of MOS structures in important device applications
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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8/2/2019 Physics of MOS Structures Part I
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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
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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8/2/2019 Physics of MOS Structures Part I
8/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
Overview
2) Physics of MOS - Structures
1) MOS structures in important semiconductor device classes
-
8/2/2019 Physics of MOS Structures Part I
9/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
( )/ 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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8/2/2019 Physics of MOS Structures Part I
10/13
IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
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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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
/ /
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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8/2/2019 Physics of MOS Structures Part I
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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
[ ]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
Derive a relationship between the charge in thesemiconductor and the surface band bending
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8/2/2019 Physics of MOS Structures Part I
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IHP Im Technologiepark 25 15236 Frankfurt (Oder) Germany www.ihp-microelectronics.com 2005 - All rights reserved
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
Derive a relationship between the charge in thesemiconductor and the surface band bending