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MOTIVATIONS

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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MOTIVATIONS

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  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

  2. MOTIVATIONS • 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 • To discuss the issue of modeling of mobility in ultra-thin SOI MOSFETs. • To compare ultra-thin SOI MOSFETs and bulk devices in terms of RF and noise performance.

  3. OUTLINE • Modeling approach and simulated devices. • Modeling of mobility in ultra-thin SOI MOSFETs. • Comparison of RF performance (FT and FMAX). • Comparison of noise performance.

  4. Simulation approach • Hydrodynamic simulations (DESSIS-ISE). • Density gradient model for quantization. • Direct tunneling through gate oxide. • Empirically modified mobility model for SOI. • 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.

  5. 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].

  6. Simulation of RF figures of merit FMAX and FT are evaluated from the admittance matrix obtained by AC device simulation.

  7. 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.

  8. Transition Frequency Lgate=70 nm FT of SOI is degraded at large gate overdrive, due to the parasitic source resistance that reduces transconductance.

  9. Maximum Oscillation Frequency Lgate=70 nm FMAX of Bulk MOSFETs is reduced due to short channel effects leading to large drain-source conductance

  10. 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).

  11. RF Performance of Bulk and SOI MOSFETs - Summary • 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.

  12. Noise Sources in MOSFETs The spectral densities of noise sources are evaluated by post-processing device simulations.

  13. 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

  14. Numerical modeling

  15. Comparison of Noise sources (L=0.1 um, TOX=1.5 nm, f=4GHz) Red simbols: SOI MOSFETs Black simbols: bulk MOSFETs

  16. 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

  17. 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

  18. 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

  19. Comparison of Noise Performance of SOI and BULK MOSFETs- Summary • 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.

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