Engineering the Electron-Hole Bilayer Tunneling Field-Effect Transistor

The electron-hole (EH) bilayer tunneling field-effect transistor promises to eliminate heavy-doping band tails enabling a smaller subthreshold swing voltage. Nevertheless, the electrostatics of a thin structure must be optimized for gate efficiency. We analyze the tradeoff between gate efficiency versus ON-state conductance to find the optimal device design. Once the EH bilayer is optimized for a given ON-state conductance, Si, Ge, and InAs all have similar gate efficiency, around 40%-50%. Unlike Si and Ge, only the InAs case allows a manageable work function difference for EH bilayer transistor operation.National Science Foundation (U.S.). Center for Energy Efficient Electronics Science (Award 0939514

SiGe-On-Insulator (SGOI): Two Structures for CMOS Application

Two SiGe-on-insulator (SGOI) structures for CMOS application are presented: surface-channel strained-Si on SGOI (SSOI) and dual-channel SGOI structures. Comparisons between two structures are made from both device performance and CMOS process point of view. We have demonstrated both structures on SGOI, and have fabricated n-MOSFET’s and p-MOSFET’s on those two structures respectively. Device characteristics are presented. The devices show enhancement on both electron and hole mobilities.Singapore-MIT Alliance (SMA

Gate-all-around n-MOSFETs with uniaxial tensile strain-induced performance enhancement scalable to sub-10-nm nanowire diameter

The effects of high-level uniaxial tensile strain on the performance of gate-all-around (GAA) Si n-MOSFETs are investigated for nanowire (NW) diameters down to 8 nm. Suspended strained-Si NWs with ~2-GPa uniaxial tension were realized by nanopatterning-induced unilateral relaxation of ultrathin-body 30% strained-Si-directly-on-insulator substrates. Based on these NWs, GAA strained-Si n-MOSFETs were fabricated with a Si thickness of ~8 nm and NW widths in the range of 50 nm down to 8 nm. The GAA strained-Si MOSFETs show excellent subthreshold swing and cutoff behavior, and approximately two times current drive and intrinsic transconductance enhancement compared to similar unstrained Si devices.Focus Center Research Program. Center on Materials, Structures, and Device

Enhanced Hole Transport in Short-Channel Strained-SiGe p-MOSFETs

Hole mobility and velocity are extracted from scaled strained-Si[subscript 0.4]5Ge[subscript 0.55]channel p-MOSFETs on insulator. Devices have been fabricated with sub-100-nm gate lengths, demonstrating hole mobility and velocity enhancements in strained- Si[subscript 0.4]5Ge[subscript 0.55]channel devices relative to Si. The effective hole mobility is extracted utilizing the dR/dL method. A hole mobility enhancement is observed relative to Si hole universal mobility for short-channel devices with gate lengths ranging from 65 to 150 nm. Hole velocities extracted using several different methods are compared. The hole velocity of strained-SiGe p-MOSFETs is enhanced over comparable Si control devices. The hole velocity enhancements extracted are on the order of 30%. Ballistic velocity simulations suggest that the addition of (110) uniaxial compressive strain to Si[subscript 0.4]5Ge[subscript 0.55] can result in a more substantial increase in velocity relative to relaxed Si

Investigation of hole mobility in gate-all-around Si nanowire p-MOSFETs with high-k/metal-gate: Effects of hydrogen thermal annealing and nanowire shape

A detailed study of hole mobility is presented for gate-all-around Si nanowire p-MOSFETs with conformal high-κ/MG and various high-temperature hydrogen annealing processes. Hole mobility enhancement relative to planar SOI devices and universal (100) is observed for 15 nm-diameter circular Si nanowires, due to an optimized anneal process which smoothes and reshapes the suspended nanowires. Increasing hole mobility is experimentally observed with decreasing nanowire width down to 12 nm. The measured inversion capacitance-voltage characteristics are in excellent agreement with quantum mechanical simulations. In addition, a method to extract areal inversion charge density in Si nanowires is introduced and its impact on the mobility of Si nanowires with various shapes is explored.Semiconductor Research Corporation. Center for Materials, Structures and Device

Strained-Si [subscript 1-x]Ge [subscript x/Si Band-to-Band Tunneling Transistors: Impact of Tunnel-Junction Germanium Composition and Doping Concentration on Switching Behavior

Strained pseudomorphic Si/Si [subscript 1-x]Ge [subscript x]/Si gate-controlled band-to-band tunneling (BTBT) devices have been analyzed with varying Ge composition up to 57% and p+ tunnel-junction (source) doping concentration in the 10 [superscript 19] -10 [superscript 20 cm [superscript -3] range. Measurements show the impact of these parameters on the transfer and output characteristics. Measurements are compared to simulations using a nonlocal BTBT model to analyze the mechanisms of device operation and to understand the impact of these parameters on the device switching behavior. The measured characteristics are consistent with simulation analysis that shows a reduction in energy barrier for tunneling (E [subscript geff]) and a reduction in tunneling distance with increasing Ge composition and source doping concentration. Increases in the pseudomorphic layer Ge content and doping concentration of the tunnel junction produce large improvements in the measured switching-behavior characteristics (I [subscript on], slope, turn-on voltages, and sharpness of turn-on as a function of V [subscript ds]). Simulations are also performed to project the potential performance of more optimized structures that may be suitable for extremely low power applications (V [subscript dd] < 0.4 V)

Enhanced Hole Mobility in High Ge Content Asymmetrically Strained-SiGe p-MOSFETs

The hole mobility characteristics of 〈110〉 /(100)-oriented asymmetrically strained-SiGe p-MOSFETs are studied. Uniaxial mechanical strain is applied to biaxial compressive strained devices and the relative change in effective hole mobility is measured. The channel Ge content varies from 0 to 100%. Up to −2.6% biaxial compressive strain is present in the channel and an additive uniaxial strain component of −0.06% is applied via mechanical bending. The hole mobility in biaxial compressive strained-SiGe is enhanced relative to relaxed Si. It is observed that this mobility enhancement increases further with the application of 〈110〉 longitudinal uniaxial compressive strain. The relative change in mobility with applied stress is larger for biaxial compressive strained-SiGe than for Si and increases with the amount of biaxial compressive strain present in the channel

Reducing optical and resistive losses in graded silicon-germanium buffer layers for silicon based tandem cells using step-cell design

Si solar cells with a SiGe graded buffer on top are fabricated as the initial step in GaAsP/Si tandem cell fabrication. Using this structure, the impact of the SiGe buffer layer on the Si solar cells is characterized. To mitigate the impact of the narrow-bandgap SiGe on the electrical and optical characteristics of the Si sub-cell, a portion of the underlying Si is exposed using a step-cell design. The step-cell design is demonstrated to increase the Jsc of the SiGe/Si stack from 5 to 20 mA/cm2. The layout of the top mesa is shown to have an impact on the device characteristics with the finger design giving better results than the rectangular mesa with respect to fill factor and series resistance. In addition, utilizing the step-cell design increases overall spectral response of the bottom cell, with significant improvements in the short wavelength range