1. Sung June Kim kimsj@snu.ac.kr http://nanobio.snu.ac.kr
Chapter 18.
NONIDEAL MOS
Sung June Kim

2. CONTENTS Metal-Semiconductor Work function Difference Oxide Charges
MOSFET Threshold Considerations

3. Physics of Non-ideal MOS-C
“ real ”
1 ms  0
2 charges exist in SiO2

4. Workfunction Difference
the minimum energy required to bring an electron from the Fermi level to the vacuum level.
for Al, qm = 4.3 eV.
Electron affinity :
the energy difference between conduction band edges of the semiconductor and the vacuum level.
for silicon, q = 4.05 eV.

5. Silicon Al
Equilibrium EF

6. Heavily-doped polysilicon gate
at equilibrium
gate bulk
EF = EC

7. Heavily-doped polysilicon gate
Oxide

8. ABOUT mV
~ 560mV
~ 350mV

9. Oxide Charges and Traps
Qtot : total oxide charges (per unit area) Si
SiOx
SiO2
metal

10. 왜 이것이 VG 가 아닌가

11. Mobile Ionic Charges
Na+, K+
Very high diffusivities in the oxide even below 200 C
VT instability

12. High T - VGB + VGB after -BT stress C VG after +BT stress
Test : MOS-C CV
High T
- VGB
+ VGB
after -BT stress C VG
after +BT stress

13. Fixed Oxide Charge 1. Located very close to the interface
2. Associated with the structure of the interfacial between Si and SiO2
3. Function of substrate orientation, oxidation temperature (annealing condition)

14. Varies as a function of the Si surface orientation
(111) > (110) > (100)
11010 cm  (100) wafers are used for MOS IC Å

15. Interface Trapped Charge
1. 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 Si Si O O O
surface
electron can be trapped

16. Ec EF Ev Interfacial traps empty filled
3. The surface state acts like a recombination center.
4. Function of VGB  charge in the worst case
5. Distributed throughout the entire band gap Ec
Interfacial
traps EF
empty Ev
filled

17. acceptor-like Qit Qit +3 -3 donor-like - inversion : mostly filled
- accumulation :
mostly empty
Qit +3

18. E Ec Ei Ev Nit ( #/cm2 ) 6. Distribution of trap level vs. energy
7. (111) > (110) > (100) , Dit(111) : Dit(100)  3:1 Ev Ei Ec E
Nit ( #/cm2 )

19. 0.1 m 0.1 m  1 trap 1010 traps/cm2  100 traps/m2 [H] diffusion D
8. Annealing with hydrogen at relatively low temperature ( 500 C) minimize Dit.
1010 traps/cm2  100 traps/m2
0.1 m 0.1 m
 1 trap
[H] diffusion D S

20. Oxide Trapped Charge Qot
Due to holes and electrons trapped in the bulk of oxide.

21. Effect of Oxide Charge on VT

22. Threshold Voltage Equation
Flat band voltage  VFB 
~ 0.6 V
potential drop in Si
potential drop in oxide

23. Back Biasing
Body effect: Reverse biasing the back contact relative to the source → VT change
If VBS<0, inversion layer carriers migrate laterally into the S/D. The inversion occurs when s=2F+|VBS|.
The maximum depletion width
If VBS=0, inversion occurs when s=2F

24. At threshold, = VT
Body Factor