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Device Design Considerations for
Ultra-Thin Body Non-Hysteretic
Negative Capacitance FETs
Chun Wing Yeung, Asif Khan
Sayeef Salahuddin, Chenming Hu
EECS, UC Berkeley
3RD BERKELEY SYMPOSIUM ON
ENERGY EFFICIENT ELECTRONIC SYSTEMS
1
• Introduction
• Background of Negative Capacitance
FET (NCFET)
• Design considerations
• Simulation of UTB NCFET
• Conclusion
Outline
2
Node
(nm)
250 180 130 90 65 32 14
Vdd
(V)
2.5 1.8 1.3 1.2 1.1 0.9 0.8
Vdd scaling is slowing down
3
IC Vdd scaling history and ITRS projection
Boltzmann statistics lead to 60mV/decade limit.
Source: C. Hu, IEDM 2010
Tunnel FET
Band-To-Band
Tunneling
• Reverse-biased
P-I-N
Feedback
FET A Padilla, IEDM 2008
Internal Feedback
• Forward-biased
P-I-N
Negative
Capacitance FET S. Salahuddin,
Nano lett. 2008
Negative
Capacitance
Amplification
Sub-60mV/dec Swing FETs
4
SWITCHING MECHANISM STRUCTURE
N+i-Si
Gate
P+
BOX
VG
Gate
Lg
VD>0V
Gate
N++
P++
i-Si
Gate
VD > 0
The Negative Capacitor
5
Khan et al. APL 99, (2011) Landau Theory of
Ferroelectric
The Negative Capacitance
Region
6
V (v)
Q (C/cm^2)
𝑵𝒆𝒈𝒂𝒕𝒊𝒗𝒆 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒂𝒏𝒄𝒆
𝑹𝒆𝒈𝒊𝒐𝒏
𝑷𝒐𝒔𝒊𝒕𝒊𝒗𝒆 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒂𝒏𝒄𝒆
𝑹𝒆𝒈𝒊𝒐𝒏
|CFE|
Capacitance Model for
Negative Capacitance FET
Schematic of NCFET
CMOS
VG
Underlying
MOSFET
CFE
Cox
Cdep
ᵠs
VMOS
Capacitance Model of NCFET
Capacitive Divider
8
VG
CFE
CMOS
VMOS
𝑽𝑴𝑶𝑺 = 𝑽𝑮
𝑪𝑭𝑬
𝑪𝑭𝑬 + 𝐶𝑴𝑶𝑺
𝑽𝑴𝑶𝑺 = 𝑽𝑮
|𝑪𝑭𝑬|
𝑪𝑭𝑬 − 𝐶𝑴𝑶𝑺
Since CFE can be negative:
A𝑽 =𝜕𝑉𝑀𝑂𝑆
𝜕𝑉𝐺=
𝐶𝐹𝐸
𝐶𝐹𝐸 − 𝐶𝑀𝑂𝑆
𝑽𝑴𝑶𝑺 = 𝑽𝑮 ∗? ?
Condition 1:
For large amplification,
|CFE| should be close to CMOS
Use Negative Capacitance to Reduce the
Subthreshold Swing
Schematic of Negative Capacitance FET VG
Underlying MOSFET
CMOS
CFE
Cox
Cdep
ᵠs
𝑺𝑺 𝒐𝒇 𝑵𝑪𝑭𝑬𝑻 = 𝟔𝟎𝒎𝑽/𝒅𝒆𝒄 ∗ 𝟏 + 𝑪𝒅𝒆𝒑
𝑪𝒐𝒙∗
𝟏
𝑨𝑽
Subthreshold Swing of NCFET
𝑺𝑺 𝒐𝒇 𝑵𝑪𝑭𝑬𝑻 = 𝟔𝟎𝒎𝑽/𝒅𝒆𝒄 ∗ 𝟏 + 𝑪𝒅𝒆𝒑
𝑪𝒐𝒙−
𝑪𝒅𝒆𝒑
|𝑪𝑭𝑬|
Condition 2:
For swing < 60mV/dec
|CFE| < Cox
(sets the maximun value for |CFE|)
VG
Cox’
CFE
Cox
Cdep
ᵠ Cox’ = 𝑪𝒐𝒙∗|𝑪𝑭𝑬|
𝑪𝑭𝑬
−𝑪𝒐𝒙
Condition for no hysteresis
𝑺𝑺 𝒐𝒇 𝑵𝑪𝑭𝑬𝑻 = 𝟔𝟎𝒎𝑽/𝒅𝒆𝒄 ∗ 𝟏 + 𝑪𝒅𝒆𝒑
𝑪𝒐𝒙−
𝑪𝒅𝒆𝒑
|𝑪𝑭𝑬|
For stable operation (no hysteresis): 𝑪𝒅𝒆𝒑
𝑪𝒐𝒙−
𝑪𝒅𝒆𝒑
|𝑪𝑭𝑬| must be larger than -1
𝑪𝒅𝒆𝒑
𝑪𝒐𝒙−
𝑪𝒅𝒆𝒑
|𝑪𝑭𝑬| > -1
𝟏
|𝑪𝑭𝑬| <
𝟏
𝑪𝒐𝒙 +
𝟏
𝑪𝒅𝒆𝒑
|CFE| > CMOS
Condition 3:
For no hysteresis, |CFE| must be
larger than CMOS
(sets the minimum value for |CFE|)
A𝑽 =𝐶𝐹𝐸
𝐶𝐹𝐸 − 𝐶𝑀𝑂𝑆
The “window” of CFE
12
-2 -1 0 1 2
0
4
8
Ca
pa
cita
nce
(a
u)
VG (V)
CMOS
COX Condition 2:
If swing < 60mV/dec
=> |CFE| < Cox
Condition 3:
For no hysteresis, |CFE|
must be larger than CMOS
Condition 1:
For small swing, CFE should
be ~ CMOS
Choosing the |CFE| value
13
-2 -1 0 1 2
0
4
8
Capacitance (
au)
VG (V)
𝐶𝐹𝐸
CMOS
The Capacitance Mismatch
14
-2 -1 0 1 2
0
4
8
Capacitance (
au)
VG (V)
Av=𝐶𝐹𝐸
𝐶𝐹𝐸 −𝐶𝑀𝑂𝑆
𝐶𝐹𝐸
VG
CMOS
CFE
CMOS
0.0 0.2 0.4 0.6 0.8 1.00
5
10
15
20
V
CMOS
/CFE
What do we need?
15
-2 -1 0 1 2
0
4
8C
MO
S (
au
)
VG (V)
We need a underlying MOSFET with
a larger depletion capacitance.
Device Optimization:
UTB with Extremely Thin BOX
16
VG
CMOS
CFE
Cox
Cs
ᵠ
CBOX
“Cdep”
p+ Si
i- Semiconductor N+N+High-K Oxide
Metal
Ferro-electric
VD
VG
Buried Oxide
By using a thin body and a thin
BOX, Cdep can be increased
p+ Si
i- Semiconductor N+N+High-K Oxide
Metal
Ferro-electric
VD
VG
Buried Oxide
Simulation of Negative
Capacitance FET Structure Coupled 2D Electrostatics-1D Landau Simulation Ferroelectric: 1-D Landau Simulation. PbZr0.5Ti0.5O3. Anisotropy Constants from Haun et al., Ferroelectrics 99, 63 (1989) Intrinsic MOSFET (2D): TCAD Sentaurus Simulation. Interlayer Metallic Electrode: To screen out non-uniformity in potential profile as well due to domain formation.
Effect of varying TS
18
0.0 0.1 0.2 0.3 0.4 0.510
-6
10-3
100
103
I DS (
µA
/µm
)
VGS
(V)
Ts=0.5nm
Ts=1nm
Ts=2nm
VG
CMOS
CFE
Cox
Cs
ᵠ
CBOX
“Cdep”
Simulated Id-Vg of
UTB NCFET
19
0.0 0.2 0.4 0.6 0.8 1.010
-6
10-3
100
103
~18mV/dec
~22mV/dec~235mV/dec
WIthout FE
Vds=0.05
Vds=0.5
WIth FE
Vds=0.05
Vds=0.5
I DS (
µA
/µm
)
VGS
(V)
Lg=32nm, Ts=0.7nm, Tox=3nm, Tbox=1nm
Summary
20
200 300 400 50015
20
25A
vg
.SS
(m
V/d
ec
)
VDD
(mV)
LG=32nm
tS=0.7nm
tBOX
=1nm
tOX
=3nm
100
200
300
400
IO
N (µA
/µm
)
• NCFET can operate hysteresis free.
• For sub-60mV/dec and no hysteresis,
there is a maximum and minimum
constraint for CFE.
• UTB design can reduce the issue of
capacitance mismatch.
• Simulation shows NCFET can achieve
swing of sub-30mV/dec from pA/μm to
μA/μm with estimated ION of 0.250mA/μm
at 0.3V VDD
Conclusion
21
Acknowledgements
This work was partially supported by Office of Naval
Research, the FCRP MSD center, Qualcomm, and
NSF E3S Center at Berkeley.
22
• Thank you!
23
•𝜕𝜑𝑠
𝜕𝑉𝐺=
𝜕𝜑𝑠
𝜕𝑉𝑀𝑂𝑆∗
𝜕𝑉𝑀𝑂𝑆
𝜕𝑉𝐺=
𝐶𝑜𝑥
𝐶𝑜𝑥+𝐶𝑑𝑒𝑝∗
𝐶𝐹𝐸
𝐶𝐹𝐸 −𝐶𝑀𝑂𝑆
•𝜕𝜑𝑠
𝜕𝑉𝐺=
𝐶𝑜𝑥
𝐶𝑜𝑥+𝐶𝑑𝑒𝑝∗
𝐶𝐹𝐸
𝐶𝐹𝐸 −𝐶𝑜𝑥𝐶𝑑𝑒𝑝
𝐶𝑜𝑥+𝐶𝑑𝑒𝑝
• SS=𝜕𝑉𝐺
𝜕𝑉𝑀𝑂𝑆∗
𝜕𝑉𝑀𝑂𝑆
𝜕𝜑𝑠∗
60𝑚𝑉
𝑑𝑒𝑐
• =60𝑚𝑉
𝑑𝑒𝑐∗ (1 +
𝐶𝑑𝑒𝑝
𝐶𝑜𝑥−
𝐶𝑑𝑒𝑝
|𝐶𝐹𝐸|)
Backup Slide
24
• Ec ~ 30k to 300kV/cm
• ~ 0.003 to 0.03V/nm
25
Material Exploration for Ferroelectric
Negative Capacitance
substrate
SrRuO3
Au
. . .
BSTO
LAO
BSTO
LAO
BSTO
LAO
Ba0.8Sr0.2TiO3-LaAlO3 superlattice
(layer thickness <10nm)
Unpublished
26
Three Different Ferroelectric
Negative Capacitance Model
Systems has been identified so far.
1. Pb(Zr0.2Ti0.8)O3, > 225 0C
2. PbTiO3, > 110 0C
3. (Ba0.8Sr0.2)TiO3 > 110 0C
Negative Capacitance in sub-10nm Films:
Ba0.8Sr0.2TiO3-LaAlO3 superlattice
With three different negative capacitance material system identified,
the stage is set for making the first crystalline NCFET.
MOSFET Scaling: The Negative
Capacitance Approach
Negative capacitance can give S < 60 mV/decade
Moore’s Law Negative Capacitance FET
Salahuddin et al., Nanoletters 8, 405 (2008).
27
Fabrication of Negative Capacitance FET for simultaneous High Performance-Ultra Low Power Operation for Mobile Computing
Ferroelectric Negative
Capacitance
Positive Capacitance Ferroelectric
Negative Capacitance
V C
Capacitor
Landau-Devonshire Theory of Ferroelectrics tells that Ferroelectric capacitors can give rise to negative capacitance
under certain conditions.
28
Epitaxial Growth of
Nanoscale Ferroelectric
Heterostructure
29 Excellent Epitaxial Quality, Interface Roughness 2-3
unit cells
Pulsed Laser Deposition
Material Type a-axis
param
eter
Pb(Zr0.2Ti0.8)
O3
Ferroelec 3.953
A
SrTiO3 Dielectric 3.905
A
SrRuO3 Metal 3.92 A STO substrate
SRO
Dielectric STO
Ferro PZT
Au
Interface
roughness less
than 2-3 unit cell.
TEM Courtesy:
Prof. X. Pan,
UMich
Material Exploration for
Ferroelectric Negative
Capacitance
substrate
SrRuO3
Au
. . .
BSTO
LAO
BSTO
LAO
BSTO
LAO
Ba0.8Sr0.2TiO3-LaAlO3 superlattice
(layer thickness <10nm)
Unpublished
30
Three Different Ferroelectric
Negative Capacitance Model
Systems has been identified so far.
1. Pb(Zr0.2Ti0.8)O3, > 225 0C
2. PbTiO3, > 110 0C
3. (Ba0.8Sr0.2)TiO3 > 110 0C
Negative Capacitance in sub-10nm
Films: Ba0.8Sr0.2TiO3-LaAlO3 superlattice
With three different negative capacitance material system identified,
the stage is set for making the first crystalline NCFET.
31
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