Chapter 18. NONIDEAL MOS

NONIDEAL MOS

Sung June Kimkimsj@snu.ac.kr

http://helios.snu.ac.kr

Chapter 18.

Chapter 18. NONIDEAL MOS

CONTENTS

• Metal-Semiconductor Work function Difference• Oxide Charges• MOSFET Threshold Considerations

Chapter 18. NONIDEAL MOS

Physics of Non-ideal MOS-C

1 fms ¹ 0

2 charges exist in SiO2

“ real ”

Chapter 18. NONIDEAL MOS

Workfunction Difference

• Workfunction :– the minimum energy required to bring an electron

from the Fermi level to the vacuum level.– for Al, qf

m= 4.3 eV.

• Electron affinity :– the energy difference between conduction band edges

of the semiconductor and the vacuum level.– for silicon, qc = 4.05 eV.

Chapter 18. NONIDEAL MOS

Equilibrium

EF

eV3.4q m =f

Silicon

eV05.4q =c

sqf

Al

sm ff ¹

msqf

msFBV f=

Chapter 18. NONIDEAL MOS

Heavily-doped polysilicon gate

gate bulk

EF = EC

at equilibrium

Chapter 18. NONIDEAL MOS

Heavily-doped polysilicon gate

Gate SiliconOxide

Chapter 18. NONIDEAL MOS

÷÷ø

öççè

æ--=

--=

i

Ag

g

nNln

qkT2/E

2/EFms

ff

~ 560mV ~ 350mV

약 -900 mV

Chapter 18. NONIDEAL MOS

Oxide Charges and Traps

• Qtot : total oxide charges (per unit area)

Si

SiOx

SiO2

metal

Chapter 18. NONIDEAL MOS

( )ò-= ox0 ox

ooT dxxx

K1V re

D

Chapter 18. NONIDEAL MOS

Mobile Ionic Charges

• Na+, K+

• Very high diffusivities in the oxide even below 200 °C– VT instability

Chapter 18. NONIDEAL MOS

• Test : MOS-C CV

High T High T

- VGB + VGB

VG

C

after +BT stress

after -BT stress

ox

mCQV =D

Chapter 18. NONIDEAL MOS

Fixed Oxide Charge

1. Located very close to the interface2. Associated with the structure of the interfacial between Si and

SiO2

3. Function of substrate orientation, oxidation temperature (anneal condition)

Chapter 18. NONIDEAL MOS

( ) ( ) mV 2.5 =´

´´-= -

--

29

21019

T F/cm10690cm101 C106.1VD

2nF/cm cm

F/cm , 6901050

10854.89.3t

KC50t)100( 8

14

ox

ooxoxox =

´´´

=== -

-e

(111) > (110) > (100)

1´1010 cm-2 \ (100) wafers are used for MOS IC

ox

f

ox

fT C

qNCQ

V-

=-

=D

Å

Chapter 18. NONIDEAL MOS

Interface Trapped Charge1. Arises from allowed energy states in the forbidden gap of

the Si, very close to Si-SiO2 interface.2. Physical origin unknown yet.

® “dangling bond” (?)Dangling bond

surface

electron can be trapped

O O OSi

Si

Chapter 18. NONIDEAL MOS

3. The surface state acts like a recombination center.4. Function of VGB ®가장나쁜 charge.5. Distributed throughout the entire band gap

Filled Empty

donor-like traps “0” “+” below Ei

acceptor-like traps “-” “0” above Ei

filled

empty

Interfacialtraps

Ev

Ec

EF

Chapter 18. NONIDEAL MOS

- accumulation : mostly empty

Qit

+3

- inversion :mostly filled

donor-like

acceptor-like

Qit

-3

Chapter 18. NONIDEAL MOS

6. Distribution of trap level vs. energy

7. (111) > (110) > (100) , Dit(111) : Dit(100) » 3:1

Nit( #/cm2 )

Ev

Ei

Ec

E

Chapter 18. NONIDEAL MOS

8. Annealing with hydrogen at relatively low temperature (£500 °C) minimize Dit.

0.1 mm´ 0.1 mm® 1 trap

1010 traps/cm2 ® 100 traps/mm2

D S[H] diffusion

Chapter 18. NONIDEAL MOS

Oxide Trapped Charge Qot

• Due to holes and electrons trapped in the bulk of oxide.

Chapter 18. NONIDEAL MOS

Effect of Oxide Charge on VT

) interface at the Q If ( totox

totT C

QV -=D

Chapter 18. NONIDEAL MOS

Threshold Voltage Equation

• Flat band voltage º VFB ºox

totms C

Q-f

ox

FA0sFFBT C

NqK42VV

fef

++= ~ 0.6 V

potential drop in Si potential drop in oxide

Chapter 18. NONIDEAL MOS

Back Biasing• Body effect: Reverse biasing the back contact relative to

the source Ô VT change

If VBS=0, inversion occurs when fs=2fF

If VBS<0, inversion layer carriers migrate laterally into the S/D. The inversion occurs when fs=2fF+|VBS|.The maximum depletion width

( )A

BSFosT qN

|V|2K2W +=

fe

Chapter 18. NONIDEAL MOS

• At threshold,

( )

ox

BSFAosBSFFBGB C

|V|2NqK2|V|2VV

++++=

fef

( )

( )FBSFox

AosBST

ox

BSFAosFFBBSGBGS

2|V|2C

NqK2)0(VV

C|V|2NqK2

2VVVV

ffe

fef

-++==

+++=+=\

=

VT

Body Factor