Zhengping Jiang

Efficient and realistic device modeling from atomic detail to the nanoscale

As semiconductor devices scale to new dimensions, the materials and designs become more dependent on atomic details. NEMO5 is a nanoelectronics modeling package designed for comprehending the critical multi-scale, multi-physics phenomena through efficient computational approaches and quantitatively modeling new generations of nanoelectronic devices as well as predicting novel device architectures and phenomena. This article seeks to provide updates on the current status of the tool and new functionality, including advances in quantum transport simulations and with materials such as metals, topological insulators, and piezoelectrics.

Design, fabrication, and analysis of p-channel arsenide/antimonide hetero-junction tunnel transistors

In this paper, we demonstrate InAs/GaSb hetero-junction (hetJ) and GaSb homo-junction (homJ) p-channel tunneling field effect transistors (pTFET) employing a low temperature atomic layer deposited high-κ gate dielectric. HetJ pTFET exhibited drive current of 35 μA/μm in comparison to homJ pTFET, which exhibited drive current of 0.3 μA/μm at VDS = −0.5 V under DC biasing conditions. Additionally, with pulsing of 1 μs gate voltage, hetJ pTFET exhibited enhanced drive current of 85 μA/μm at VDS = −0.5 V, which is the highest reported in the category of III-V pTFET. Detailed device characterization was performed through analysis of the capacitance-voltage characteristics, pulsed current-voltage characteristics, and x-ray diffraction studies.

Million Atom Electronic Structure and Device Calculations on Peta-Scale Computers

Semiconductor devices are scaled down to the level which constituent materials are no longer considered continuous. To account for atomistic randomness, surface effects and quantum mechanical effects, an atomistic modeling approach needs to be pursued. The Nanoelectronic Modeling Tool (NEMO 3-D) has satisfied the requirement by including empirical sp 3 s* and sp 3 d 5 s* tight binding models and considering strain to successfully simulate various semiconductor material systems. Computationally, however, NEMO 3-D needs significant improvements to utilize increasing supply of processors. This paper introduces the new modeling tool, OMEN 3-D, and discusses the major computational improvements, the 3-D domain decomposition and the multi-level parallelism. As a featured application, a full 3-D parallelized Schrodinger-Poisson solver and its application to calculate the bandstructure of delta doped phosphorus(P) layer in silicon is demonstrated. Impurity bands due to the donor ion potentials are computed.

Electron transport in nano-scaled piezoelectronic devices

The Piezoelectronic Transistor (PET) has been proposed as a post-CMOS device for fast, low-power switching. In this device, the piezoresistive channel is metalized via the expansion of a relaxor piezoelectric element to turn the device on. The mixed-valence compound SmSe is a good choice of PET channel material because of its isostructural pressure-induced continuous metal insulator transition, which is well characterized in bulk single crystals. Prediction and optimization of the performance of a realistic, nano-scaled PET based on SmSe requires the understanding of quantum confinement, tunneling, and the effect of metal interface. In this work, a computationally efficient empirical tight binding (ETB) model is developed for SmSe to study quantum transport in these systems and the scaling limit of PET channel lengths. Modulation of the SmSe band gap under pressure is successfully captured by ETB, and ballistic conductance shows orders of magnitude change under hydrostatic strain, supporting operability o...

Tunneling and Short Channel Effects in Ultrascaled InGaAs Double Gate MOSFETs

Full-band quantum transport simulations are performed to study the scaling of InGaAs MOSFETs. Short-channel effects evoke severe performance degradation in InGaAs MOSFETs and the tunneling leakage further deteriorates their performances. Reducing the body width is shown to suppress the short channel effects. Doping densities show big impacts on device performances. With inhomogeneous doping InGaAs could outperform Si at gate lengths below 15 nm with 5-nm body width. The density of state bottleneck does not affect InGaAs in simulated devices at 0.5 V supply voltage. At the ultrascaled dimensions the full band simulations are essential to capture strong nonparabolic dispersion. Comparison with a multivalley effective mass model shows that the population of higher conduction band valleys contributes to the total current at thin body widths.

Effects of Interface Disorder on Valley Splitting in SiGe/Si/SiGe Quantum Wells

A sharp potential barrier at the Si/SiGe interface introduces valley splitting (VS), which lifts the 2-fold valley degeneracy in strained SiGe/Si/SiGe quantum wells (QWs). This work examines in detail the effects of Si/SiGe interface disorder on the VS in an atomistic tight binding approach based on statistical sampling. VS is analyzed as a function of electric field, QW thickness, and simulation domain size. Strong electric fields push the electron wavefunctions into the SiGe buffer and introduce significant VS variations from device to device. A Gedankenexperiment with ordered alloys sheds light on the importance of different bonding configurations on VS. We conclude that a single SiGe band offset and effective mass cannot comprehend the complex Si/SiGe interface interactions that dominate VS.

Performance degradation of superlattice MOSFETs due to scattering in the contacts

Ideal, completely coherent quantum transport calculations had predicted that superlattice MOSFETs (SL-MOSFET) may offer steep subthreshold swing performance below 60 mV/dec to around 39 mV/dec. However, the high carrier density in the superlattice source suggests that scattering may significantly degrade the ideal device performance. Such effects of electron scattering and decoherence in the contacts of SL-MOSFETs are examined through a multi-scale quantum transport model developed in NEMO5. This model couples the NEGF-based quantum ballistic transport in the channel to a quantum mechanical density of states dominated reservoir, which is thermalized through strong scattering with local quasi-Fermi levels determined by drift-diffusion transport. The simulations show that scattering increases the electron transmission in the nominally forbidden minigap, therefore, degrading the subthreshold swing (S.S.) and the ON/OFF DC current ratio. This degradation varies with both the scattering rate and the length of ...

Scaling effect on specific contact resistivity in nano-scale metal-semiconductor contacts

Progressive downscaling has allowed semiconductor industries to continuing improve the performance of integrated circuits (ICs) [1]. The downscaling adversely affects the performance of interconnects. Specifically, the resistivity of metal interconnects and metal-semiconductor contacts increases due to downscaling. The metal-semiconductor contact resistance is becoming a performance limiting factor as it takes larger fraction of the total on-state resistance [2]. Hence, the contact resistance must be reduced to meet ITRS performance requirements of future technology nodes. As metal-semiconductor interface shrinks simultaneously as device shrinks, it becomes questionable if the resistivity still can meet the ITRS requirements when it reaches to the certain scaling limit (sub-10nm). Specific contact resistivity (ρC) is one of important factors affecting total contact resistance, and it is determined by important factors such as metal-semiconductor Schottky barrier height and semiconductor doping. This work investigates the effects of contact geometry, Schottky barrier height, and doping concentration on the specific contact resistivity.

Comprehensive Simulation Study of Direct Source-to-Drain Tunneling in Ultra-Scaled Si, Ge, and III-V DG-FETs

As the scaling of transistors approaches the 7-/5-nm technology nodes, direct source-to-drain tunneling (SDT) is becoming increasingly important with the shrinking gate lengths. In this paper, we present a comprehensive simulation study on the effects of SDT in ultrascaled FETs with various channel materials (Si, Ge, SiGe, InGaAs, and so on), surface/channel orientation configurations, gate lengths, body thicknesses, doping concentrations, stress levels, and temperatures. The nonequilibrium Green’s function formalism with the atomistic tight-binding basis is used to accurately model both the quantum-mechanical tunneling and the bandstructure effects. To quantify the strength of SDT, we propose a current ratio (

Quantum Transport in AlGaSb/InAs TFETs With Gate Field In-Line With Tunneling Direction

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