Influence Of Ballistic Effects In
Ultra-Small MOSFETs
J. SAINT MARTIN, V. AUBRY-FORTUNA, A. BOURNEL, P. DOLLFUS,
S. GALDIN, C. CHASSAT
[email protected]
INSTITUT D'ÉLECTRONIQUE FONDAMENTALE
IEF, UMR CNRS 8622, Université Paris Sud,
Centre scientifique d'Orsay - Bât. 220 F-91405 ORSAY cedex FRANCE
Contents
 Introduction
 Channel length and ballistic transport
 Bias dependence on ballisticity
 Ballisticity and long-channel effective mobility
Contents
 Introduction
 Channel length and ballistic transport
 Bias dependence on ballisticity
 Ballisticity and long-channel effective mobility
Transport in decanano MOSFET
Front gate
0
Source
Entrance
Scattering
counting
region
Back gate
y
Drain
Exit
Quasi-ballistic transport: electron mean free path is similar to Lch
Monte Carlo
simulation
• Bint = % of ballistic electrons at the drain-end
• Beff =
Ion
Ion_with_a_ballistic_channel
Contents
 Introduction
 Channel length and ballistic transport
 Bias dependence on ballisticity
 Ballisticity and long-channel effective mobility
Studied devices
LG
SiO2
Tox = 1.2 nm
TSi = 5 nm
Fm = 4.46 eV
VDD = 0.7 V
5×1019
x
cm-3
y
5×1019 cm-3
0
Undoped
Tox = 1.2 nm
SiO2
Tspacer = 5 nm
Toverlap = 5 nm
Lch
Ts pacer = 5 nm
Toverlap = 5 nm
2000
Fraction of electrons (%)
Drain current ID (µA/µm)
From quasi-stationary to quasi-ballistic
VGS = 0.7 V
Lch = 15 nm
1500
Lch = 25 nm
1000
Lch = 50 nm
Lch = 100 nm
500
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Source-drain voltage VDS (V)
0.7
100
Drain-end of the channel
VGS = VDS = 0.7 V
10
Lch = 200 nm
1
25 nm
0.1
15 nm
0
10
20
50 nm
30
100 nm
40
50
Number of scattering events
Transition for Lch ≈ 50 nm in undoped channels
60
Ballisticity and channel length
Fraction of electrons (%)
Intrinsic ballisticity Bint (%)
100
80
60
40
Bint
20
0
0
50
100
150
Channel length Lch (nm)
200
100
Drain-end of the channel
VGS = VDS = 0.7 V
10
Lch = 200 nm
1
25 nm
0.1
15 nm
0
10
20
50 nm
30
100 nm
40
50
Number of scattering events
Bint strongly increases for Lch < 100 nm
60
Ballisticity and current
100
Ballistic Si channel
2000
Ballisticity (%)
Drain current Ion (A/m)
2500
1500
Standard Si channel
1000
80
Beff
60
40
Beff - Bint
20
Bint
500
0
50
100
150
Channel length Lch (nm)
200
0
0
50
100
150
200
Channel length Lch (nm)
Gap between Bint and Beff no more constant for Lch < 30 nm
Beff(Bint) in undoped channels
Effective ballisticity Beff (%)
Channel length Lch (nm)
100 50 35 25 20 15
100
80
60
40
20
0
0
20
40
60
80
Intrinsic ballisticity Bint (%)
100
Strong impact of Bint on Beff for Bint < 20 %
Beff(Bint): quasi linear correlation for Bint > 20%
Decreasing impact of Bint on Beff
Contents
 Introduction
 Channel length and ballistic transport
 Bias dependence on ballisticity
 Ballisticity and long-channel effective mobility
Bint as a function of VDS for different VGS
Ballisticity Bint (%)
50
VGS = 0.3 V
45
VGS = 0.5 V
40
35
VGS = 0.7 V
Lch = 25 nm
30
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Source-drain voltage VDS (V)
Bint decreases when VGS increases
Bint decreases when VDS increases
‘Sta-bal’ and ‘bal-sta’ architectures
sta
or
bal
sta
‘sta-bal’
sta
or
bal
bal
‘bal-sta’
Bint (VDS): ‘sta-bal’ vs. ‘bal-sta’
Ballisticity B int (%)
65
sta-bal
60
55
50
bal-sta
45
40
35
standard
30
25
VGS = 0.7 V (Same VT)
0
0.2
0.4
0.6
0.8
1
1.2
Source-Drain voltage VDS (V)
Bint decrease is due to scatterings in the 2nd half of the channel
Higher phonon scattering rate > driving field
Contents
 Introduction
 Channel length and ballistic transport
 Bias dependence on ballisticity
 Ballisticity and long-channel effective mobility
Studied strained SGMOS
SiO2
Tox = 0.7 nm
2×1020
TSi = 10 nm
2×1020
LG = 18 nm
cm-3
cm-3
Strained Si
1.2×1019 cm-3
1.2×1019 cm-3
Si1-xGex (P) Pseudo substrate
Mobility enhancement (%)
LG = 25 nm
V. Aubry-Fortuna et al.,to be published
120
Long channel
‘x’ increase
 Strain increase
mt population
increase
 eff
enhancement
100
80
60
40
20
0
0
0.05
0.1
0.15
0.2
Ge content (x)
28
VGS - VT = 0.8 V
VDS = 1 V
3400
26
24
3200
22
3000
20
18
2800
2600
16
0
0.05
0.1
0.15
Ge content (x)
14
0.2
Ballisticity Bint (%)
Current Ion (A/m)
3600
eff saturation for x > 0.15
Ion and Bint increase similarly
Ion(Bint) vs. Ion(eff)
x = 0.10 0.15 0.20
2.2
Bint/Bint(Si)
2
µeff/µeff(Si)
1.8
1.6
1.4
1.2
1
1
1.1
1.2
Ion/ Ion(Si)
1.3
1.4
Bint more relevant than µeff to account for Ion
(Bint(Ion) linear for Bint  [ 15%, 30%])
Conclusions
 Connections between Bint and:
•
channel length
•
strain
•
bias
 High quasi ballistic influence for Bint < ≈ 20 %
 Bint more relevant than µeff to account for Ion
Role of MOS architecture (cf. paper)