Transcript Diapositiva 1

Comparative Analysis of the RF and Noise
Performance of Bulk and Single-Gate
Ultra-thin SOI MOSFETs by Numerical
Simulation
M.Alessandrini, S.Eminente, S.Spedo, C.Fiegna
Department of Engineering - University of Ferrara, Italy
MOTIVATIONS
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Ultra-thin fully-depleted SOI MOSFETs can provide an
alternative to conventional bulk MOSFETs due to:
Control of short channel effects with low doping levels.
Improvement of mobility at given surface density of
inversion charge [Esseni IEDM 2000].
AIMS OF THIS WORK
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To discuss the issue of modeling of mobility in ultrathin SOI MOSFETs.
To compare ultra-thin SOI MOSFETs and bulk devices
in terms of RF and noise performance.
OUTLINE
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Modeling approach and simulated devices.
Modeling of
MOSFETs.
mobility
in
ultra-thin
SOI
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Comparison of RF performance (FT and FMAX).
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Comparison of noise performance.
Simulation approach
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Hydrodynamic simulations (DESSIS-ISE).
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Density gradient model for quantization.
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Direct tunneling through gate oxide.
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Empirically modified mobility model for SOI.
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Post-processor for distributed noise analysis
(thermal and shot noise).
Simulated devices
Bulk and SOI device structures target at the 100
nm node.
SOI devices with TSI=5.2 nm, NA=1015 cm-3.
Mobility in ultra-thin SOI MOSFETs
Conventional mobility models for bulk MOSFETs does not fit experiments for
ultra-thin SOI MOSFETs [Esseni IEDM 2000].
●Empirical fitting by adjusting parameters of an existing mobility model
[Darwish TED 1997].
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Simulation of RF figures of merit
FMAX and FT are evaluated from the admittance matrix obtained by AC
device simulation.
FMAX in MOSFETs
Unilateral Mason's power gain:
Is limited by losses at the output port [Re(Y22), Re(Y12)]
For negligible substrate losses:
Ultra-thin SOI MOSFETs feature large RS values.
Transition Frequency Lgate=70 nm
FT of SOI is degraded at large gate overdrive, due to the parasitic
source resistance that reduces transconductance.
Maximum Oscillation Frequency Lgate=70 nm
FMAX of Bulk MOSFETs is reduced due to short channel effects
leading to large drain-source conductance
Maximum Oscillation Frequency Lgate=70 nm
For small W values, the parasitic source resistance dominates and the SOI
device presents lower FMAX values at large gate overdrives (lower FT).
RF Performance of Bulk and SOI
MOSFETs - Summary
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The parasitic source resistance that affects the
ultra-thin SOI device degrades the transition
frequency.
For large device width (RG>>RS) FMAX is larger in
the SOI case, due to lower short-channel effects.
For narrow devices (RS>>RG) FMAX of SOI
MOSFETs is degraded at large VGS-VT.
Noise Sources in MOSFETs
The spectral densities of noise sources are evaluated by postprocessing device simulations.
Numerical modeling of NOISE in
MOSFETs
The MOSFET is approximated by a distributed lumped-element non-uniform
transmission line.
Parameters are evaluated starting from the results of 2-D hydrodynamic device
simulation
Numerical modeling
Comparison of Noise sources
(L=0.1 um, TOX=1.5 nm, f=4GHz)
Red simbols: SOI MOSFETs
Black simbols: bulk MOSFETs
Dependence on oxide thickness
(L=0.1 um, VGS-VT=0.27 um, f=4GHz)
The gate shot noise
increases dramatically
as oxide thickness is
scaled down.
SIG-SHOTIG
It is larger in the bulk
devices, due to larger
tunneling gate current
(lower oxide field at
given inversion charge
density).
Red Simbols: SOI MOSFETs
Black Simbols: bulk MOSFETs
Comparison of minimum noise Figure
(L=0.1 um, f=4GHz, VGS-VT=0.27V)
The noise figure of BULK is
degraded as the oxide is
scaled down and shot noise
becomes dominant over
induced-gate noise
Red Simbols: SOI MOSFETs
Black Simbols: bulk MOSFETs
Dependence of FMIN on frequency
(L=0.1 um, VGS-VT=0.27V, TOX=1.2nm)
As frequency is reduced,
shot noise becomes
dominant over induced gate
noise (SIGNf2) and NFMIN
becomes independent of
frequency. At medium
frequency NFMIN is lower in
the SOI case
Red simbols: SOI MOSFETs
Black Simbols: Bulk MOSFETs
Comparison of Noise Performance of
SOI and BULK MOSFETs- Summary
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SOI and BULK MOSFETs present comparable
thermal noise (drain and induced gate current
noise currents).
Due to the lower gate tunneling current (lower
oxide field), the gate shot noise current is lower in
the SOI case, at relatively low frequencies.