Talking about the improved performance of the device :not susceptible to carrier freeze out and increased cross-talk as bulk Si devices
Threshold voltage fluctuation due to body thickness variation at UTB is approximately two times larger than at TBO under same body thickness fluctuation (1.5nm for 3 σ) but due to body thickness of UTB is 4 times thinner than TBO.
Fig. 2 shows the total inversion charge vs. gate voltage for three buried oxide thickness. Implies BOX : also has effect on sub-threshold slope of the deviceThicker he BOX layer, greater is the sub-threshold slope and even the driving current when the channel is scaled down to nano-scale size.Further; as from fig.3 effect of channel inversion charge on the Si body thickness as “Volume inversion”. ….shows that Si film thickness changes only the amount of sub-threshold inversion charge but has no effect on the strong inversion charge density that explains the “volume inversion” effect which can be used to control the sub-threshold leakage current of UTB-SOI MOSFET.
Ultra-thin body SOI MOSFETs: Term Paper_class presentation on Advanced topics in Microelectronics with CAD
Ultra-thin Body SOI MOSFETs<br />Prajon Raj Shakya<br />Master of Science in Electrical Engineering<br />Instructor: Dr. Long Que<br />Electrical Engineering Program<br />Louisiana Tech University<br />Ruston, LA 71272, USA<br />
SOI Technology <br /><ul><li>Scaling requirements- improved performance.
Control over SCE (short-channel effect) and scaling device architecture.
Layered Si-Insulator-Si substrate in place of conventional Si.
SiO2, or SiGe: insulating material as BOX: improved performance and reduced SCE.
New generation SOI as multiple gate, FinFETs, triple gate, GAA (Gate All Around).
For high density, high performance and low power applications.</li></ul>Fig. 1: Schematic diagram of an UTB-SOI MOSFET <br /> <br />
Performance of SOI devices<br />90% lower junction capacitance; near ideal sub-threshold swing; reduce device cross-talk; <br />Lower junction leakage -> low switching energy of the transistor<br /> No latch-up; Increased radiation hardness<br />Do not suffer from substrate reverse bias effects -> low-power devices.<br />Better electrostatic control: reduce S-D leakage and SCE.<br />Full dielectric isolation of the transistor <br />Reduced junction area<br />Impact ionization strongly balanced by thermal recombination.<br />Critical drawback as: floating body effects: body potential shifts –shift in V T , sub-threshold swing, and kink effects: minimized by thinner silicon.<br />
UTB SOI MOSFET<br />SOI: PD-SOI (Partially depleted- SOI) and FD-SOI (Fully depleted-SOI)<br />FD-SOI: small and well-controlled thickness channel; high series resistance<br />Higher trans-conductance and reduced floating body effects compared to PD-SOI.<br />Thin body thickness; reduced parasitic drain-to-body capacitance but drain field fringe increases DIBL (Drain Induced Barrier Lowering) and gate current worst at short-channel.<br />PDSOI: thicker body; high drain-body capacitance and degraded operating speed.<br />UTB concept evolve to control short-channel effect; along with TBO(Thin Buried Oxide)<br />UTB: Typical body thickness: 1/4th of gate length <br />While TBO: equivalent to gate length.<br />Variation of Vt due to variation of body thickness overcomes all other factors in UTB-SOI devices.<br />
Effect on Inversion charge <br />Fig.2: Inversion charge density versus gate voltage for different buried oxide layer thicknesses in undoped UTB-SOI MOSFETs<br />with the mid-gap gates for Vch = 0. <br />Fig. 3: Inversion charge density versus gate voltage for different silicon film thicknesses in undoped UTB-SOI MOSFETs with the mid-gap gates for Vch = 0. <br />
Effect of Body Doping<br /><ul><li>Effect of light channel doping causing shift in Vt w/o any SCE.
S,B and D: doped uniformly: DIBL and low-drain threshold rollofffs are defined with Vds=1V and 50mv, separately.
Substrate bias: 0 V to avoid threshold roll-off.
SCE reduced with high doping in silicon films (4-10nm) but high threshold voltage and low carrier mobility due to impurities.</li></ul>Fig.4: High drain threshold voltage roll-offs for SOI with varying <br />channel doping concentration <br />
Effect of body doping (contd..)<br /><ul><li>Shows the simulated high-drain threshold voltage roll-offs in sub-0.15µm doped FDSOI MOSFETs for varying BOX thickness.
tBOX = 5nm has L min=38nm and increase with tBOX with L min=48nm for t BOX=100nm and 200nm.
S/D lateral field coupling in the BOX does not increase with BOX thickness after the </li></ul>tBOX is 2 times larger than the channel length of the device with 100mv threshold roll-off.<br />Fig.5: High drain threshold voltage rol-loffs for doped FDSOI with different <br />BOX thicknesses <br />
Effect on Mobility<br /><ul><li>Increases with advancement in UTB SOI.
4-point method used to evaluate for µeffthickness from 44.5nm to 0.9nm.
Degradation of mobility below 3nm: due to influence of thickness/ surface roughness.</li></ul>Fig.6: Electron mobility vs. Ninv for body thicknesses from tSi=44.5nm down to 0.9nm and universal mobility after <br />
Effect on Mobility (contd..)<br /><ul><li>Except for tSOI of 2.3nm; mobility – higher than universal curve of conventional (100) pMOSFETs.
Mobility in DG-UTB MOSFET is higher than SG-UTB MOSFET.</li></ul>Fig.7: µeff – Ninvcharactersistics of (110) pMOSFETs at 300K  <br />
Conclusion<br />Different types of SOI and UTB-SOI as improvement of performance.<br />Various effects as of body doping, mobility enhancement and degradation and inversion charge.<br />Presents the concept of UTB-SOI in terms of device scaling with improved performance.<br />Further, various UTB-SOI as DG, SG, Fin-Fet, GAA and others are being developed for the better performance on various perspectives.<br />UTB-SOI with the adoption of high-k/metal technique are also being developed that acts on the performance of limitation of mobility.<br />Applications in high voltage and Smart Power ckt, RF ckt, Si-based optoelectronic devices, MEMS (Micro Electro Mechanical Systems), back-side illuminated image sensors etc.<br />
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