Transcript Document 7612123

Doping Profile Dependence of the Vertical
Impact Ionization MOSFET’s (I-MOS)
Performance
Ulrich Abelein, Mathias Born, Markus Schindler,
Andreas Assmuth, Peter Iskra, Torsten Sulima, Ignaz Eisele
Nano and Giga Challenges in Electronics and Photonics
NGC 2007
Phoenix, Arizona, USA
16 March 2007
Overview
• Motivation
• Vertical Impact Ionisation MOSFET (IMOS):
– Device Concept
– Influence of Doping Profiles
• Electrical Characterization
• Summary and Outlook
Ulrich Abelein
NGC 2007
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Motivation
Conventional MOSFET:
Subthreshold slope S = dVG/d(logID) is diffusion limited.

min S = kT/q · ln10 = 60 mV/dec @ 300 K
Minimum static leakage current ILEAK:
ILEAK = ID(VT) · 10-VT/S
Shrinking the feature size according to Moore‘s Law makes a reduction of
VT necessary.

ILEAK 
Solution

Ulrich Abelein

Reducing S below the kT/q limit!

Achievable by gate controlled impact ionisation
Impact Ionisation MOSFET (IMOS)
NGC 2007
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Device Concept – Device Structure
Drain contact
Gate oxide (4.5 nm)
Spacer
n+ Poly
Spacer
n+ Si drain
i- Si
p+ delta layer
Gate
oxide (4.5 nm)
Gate
contact
i- Si
n+ Poly
n+ Si source
Source contact
Schematic drawing of the vertical IMOS (above) and SIMS
profile of the mesa layer stack (left hand side)
Ulrich Abelein
NGC 2007
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Device Concept – Simulation Results
 p+ delta barrier lowered
by gate field
 High field between p+
delta layer and drain
causes impact ionisation
Drain contact
Spacer
n+ Si drain
Energy in eV
1
-1
-2
0
80
Distance in nm
p+ delta layer
Gate oxide
i- Si
Drain
0
Spacer
Gate
contact
n+ Poly
i- Si
VGS=VDS=0 V
VGS=0 V; VDS=2 V
VGS = VDS=2 V
n+ Si source
-
Source
1010
1020
1030
Ionisation rate in pairs / (cm3s)
Source contact
Simulations of the electric field and the ionisation rate in the channel region
Ulrich Abelein
NGC 2007
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Device Concept – Operating Modes
VDS < 1.25 V
 Conventional
MOSFET mode
2.2 V > VDS > 1.25 V
 Impact Ionization
Mode
 Holes generated by
impact ionization
charge the body.
Dynamic lowering of
VT!
W = 2µm
VDS > 2.2 V
 Bipolar Mode
 Parasitic bipolar
transistor contributes
to ID
Ulrich Abelein
NGC 2007
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Device Concept – Operating Modes
VDS < 1.25 V
 Conventional
MOSFET mode
VDS > 1.25 V
 Beginning of
significant impact
ionziation
W = 2 µm
 Holes generated
by impact
ionization charge
the body
 Dynamic lowering
of VT
 S is reduced
below kT/q
Ulrich Abelein
NGC 2007
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Influence of Doping Profiles
Unintentional changes in doping profiles due to diffusion!
 p+ delta layer doping diffuses into intrinsic zones!
Diffusion   Sharper delta layer, larger barrier, higher eelctric fields!
 Impact Ionization rates  (at const. VDS)
 Lower S due to increased body charge for low VDS
Diffusion   Lower barrier
 Switch on voltage of parasitic bipolar transistor 
 Extremley low S due to current amplification
 Hysteresis in input characteristics
Ulrich Abelein
NGC 2007
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Experimental Results – Doping Profiles
Using 750 °C and
800 °C gate oxide
process:
Decreasing of boron
diffusion for 750 °C
 Maximum doping
level increased by
a factor of 3
 Larger barrier!
Ulrich Abelein
NGC 2007
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Electrical Characerization – Output Characteristics
Low thermal budget
sample
 Impact ionization
mode begins at
lower voltage
 Later transistion
to bipolar mode
W = 2 µm
 VDS = 2.25 V
• LT sample in
Impact Ioniziation
mode
• HT sample in
bipolar mode
Ulrich Abelein
NGC 2007
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Electrical Characerization – Input Characteristics
VDS = 2.25 V
 LT sample in
Impact Ioniziation
mode
 S = 4 mV/dec
W = 2 µm
Ulrich Abelein
NGC 2007
 No hysteresis!
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Electrical Characerization – Input Characteristics
VDS = 2.25 V
 HT sample in
bipolar mode
 S = 1.06 mV/dec!
 Hysteresis visible
W = 2µm
 Gate controlled
switch-off
possible!
Ulrich Abelein
NGC 2007
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Summary and Outlook
Summary:
• Influence of boron diffusion on device performance was shown
• Subthreshold slope of 1.06 mV/dec was shown
• Devcie can be optimized to needs of application
– Very low subthreshold slope with measurable hysteresis
– Low subthreshold slope without any hystersis
Outlook:
• Realization of the p-channel device
• Shrinking device dimensions and reducing supply voltages
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NGC 2007
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