Effect of small-channel geometry in MOSFET

1. Short Channel Effect
in MOSFET
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
1/17/2021 1Arpan Deyasi, RCCIIT, India

2. Q. What is meant by short-channel?
Leff≃(zD+zS)
Leff
zDzS
Dielectric
1/17/2021 2Arpan Deyasi, RCCIIT, India
DrainSource
Channel

3. 1/17/2021 3Arpan Deyasi, RCCIIT, India
I. Electric field tends to increase because of improper
scaling of source potential
II. Built-in potential is neither negligible nor scalable
III. Vertical depth of source/drain junctions can’t be reduced
IV. Mobility decreases with increase of substrate doping
V. Subthreshold slope is not scalable
Why small geometry effects arise?

4. 1/17/2021 4Arpan Deyasi, RCCIIT, India
Short-channel effects in MOSFET
1. Hot electron effect
2. Dielectric breakdown
3. Output impedance variation
4. Mobility degradation
5. Threshold voltage shift

5. 1/17/2021 Arpan Deyasi, RCCIIT, India 5
A few related
physical phenomena

6. 1/17/2021 6Arpan Deyasi, RCCIIT, India
Hot electron
Coldelectron
Warmelectron
Hotelectron
3 3
2 2
B L B ek T k T>>
3 3
2 2
B L B ek T k T≈
3 3
2 2
B L B ek T k T<<
E
v

7. 1/17/2021 Arpan Deyasi, RCCIIT, India 7
Drain-Induced Barrier Lowering

8. 1/17/2021 Arpan Deyasi, RCCIIT, India 8
Surface Scattering

9. 1/17/2021 Arpan Deyasi, RCCIIT, India 9
Channel Length Modulation

10. 1/17/2021 Arpan Deyasi, RCCIIT, India 10
Impact Ionization

11. 1/17/2021 Arpan Deyasi, RCCIIT, India 11
Effects of
Short-Channel Geometry

12. 1/17/2021 12Arpan Deyasi, RCCIIT, India
Hot electron effect
Substrate (p)
S (n+) D (n+)
Gate
Channel
Dielectric

13. 1/17/2021 13Arpan Deyasi, RCCIIT, India
Dielectric Breakdown
Substrate (p)
S (n+) D (n+)
Gate
Channel
Dielectric

14. 1/17/2021 14Arpan Deyasi, RCCIIT, India
Output Impedance Variation
CLM+DIBL
VDS
Z0
DIBL

15. 1/17/2021 15Arpan Deyasi, RCCIIT, India
Mobility Degradation

16. 1/17/2021 16Arpan Deyasi, RCCIIT, India
Mobility Degradation
0
1
1 ( )
surf
GS ThV V
µ µ
θ
=
+ −
μ0: mobility at threshold voltage
θ: mobility reduction factor

17. 1/17/2021 17Arpan Deyasi, RCCIIT, India
Threshold Voltage Variation
Dielectric
S D
Gate
zSm zDm
ΔLS ΔLD
zj
zj

18. 1/17/2021 18Arpan Deyasi, RCCIIT, India
ΔLS: lateral extent of depletion region in source
ΔLD: lateral extent of depletion region in drain
zS: depth of depletion region in source
zD: depth of depletion region in drain
zSm: vertical extent of bulk depilation region in source
zDm: vertical extent of bulk depilation region in drain
Threshold Voltage Variation

19. 1/17/2021 Arpan Deyasi, RCCIIT, India 19
22 2
( ) ( )j D Dm j Dz z z z L+ = + + ∆
Threshold Voltage Variation
2
2 2
2
0
2
D j D
Dm D j D
L z L
z z z z
 ∆ + + ∆
  =
 + − − 

20. 1/17/2021 Arpan Deyasi, RCCIIT, India 20
Threshold Voltage Variation
2 2 2
( ( ) 2 )
D j
j D D j Dm
L z
z z z z z
∆ =− +
− − +
1 2 1D
D j
j
z
L z
z
 
∆ + − 
  


21. 1/17/2021 Arpan Deyasi, RCCIIT, India 21
Threshold Voltage Variation
Similarly,
1 2 1S
S j
j
z
L z
z
 
∆ + − 
  


22. 1/17/2021 Arpan Deyasi, RCCIIT, India 22
where
Threshold Voltage Variation
2
( )D DS F
A
z V
qN
ε
φ
 
= + 
 
2
S F
A
z
qN
ε
φ
 
=  
 

23. 1/17/2021 Arpan Deyasi, RCCIIT, India 23
( )TO short channel TO TOV V V− = − ∆
Threshold Voltage Variation
VTO: zero-bias threshold voltage
ΔVTO: threshold voltage shift

24. 1/17/2021 Arpan Deyasi, RCCIIT, India 24
Threshold Voltage Variation
1
2 2
2
S D
TO A F
D
L L
V qN
C L
ε φ
∆ + ∆
∆ =−
2 2
2
1 2 1 1 2 1
j
TO A F
D
SD
j j
z
V qN
C L
zz
z z
ε φ∆ =− ×
    
 + − + + −   
        