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 ● 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.

OUTLINE ● Modeling approach and simulated devices. ● Modeling of mobility in ultra-thin SOI MOSFETs. ● Comparison of RF performance (F T and F MAX ). ● Comparison of noise performance.

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). Bulk and SOI device structures target at the 100 nm node. SOI devices with T SI =5.2 nm, N A =10 15 cm -3. Simulated devices

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].

Simulation of RF figures of merit F MAX and F T are evaluated from the admittance matrix obtained by AC device simulation.

F MAX in MOSFETs For negligible substrate losses: Is limited by losses at the output port [Re(Y 22 ), Re(Y 12 )] Unilateral Mason's power gain: Ultra-thin SOI MOSFETs feature large R S values.

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

Maximum Oscillation Frequency Lgate=70 nm F MAX 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 F MAX values at large gate overdrives (lower F T ).

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 (R G >>R S ) F MAX is larger in the SOI case, due to lower short-channel effects. ● For narrow devices (R S >>R G ) F MAX of SOI MOSFETs is degraded at large V GS -V T.

Noise Sources in MOSFETs The spectral densities of noise sources are evaluated by post- processing 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, T OX =1.5 nm, f=4GHz) Red simbols: SOI MOSFETs Black simbols: bulk MOSFETs

Dependence on oxide thickness (L=0.1 um, V GS -V T =0.27 um, f=4GHz) Red Simbols: SOI MOSFETs Black Simbols: bulk MOSFETs The gate shot noise increases dramatically as oxide thickness is scaled down. S IG-SHOT  I G It is larger in the bulk devices, due to larger tunneling gate current (lower oxide field at given inversion charge density).

Comparison of minimum noise Figure (L=0.1 um, f=4GHz, V GS -V T =0.27V) Red Simbols: SOI MOSFETs Black Simbols: bulk MOSFETs The noise figure of BULK is degraded as the oxide is scaled down and shot noise becomes dominant over induced-gate noise

Dependence of FMIN on frequency (L=0.1 um, V GS -V T =0.27V, T OX =1.2nm) Red simbols: SOI MOSFETs Black Simbols: Bulk MOSFETs As frequency is reduced, shot noise becomes dominant over induced gate noise (S IGN  f 2 ) and N FMIN becomes independent of frequency. At medium frequency N FMIN is lower in the SOI case

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.