Quasi-ballistic Devices Research Articles

A virtual-source emission-diffusion I-V model for ultra-thin black phosphorus field-effect transistors

In this paper, we develop an analytic model based on the theory of virtual-source emission-diffusion (VS-ED) to describe ambipolar current conduction in ultrathin black phosphorus (BP) field-effect transistors (FETs). Unlike the VS model which is strictly applicable to quasiballistic devices, the VS-ED model can be applied to long-channel devices with drift-diffusive transport. The model comprehends the in-plane band structure anisotropy in BP, as well as the asymmetry in electron and hole current conduction characteristics. The model also includes the effect of Schottky-type source/drain contact resistances, which are voltage-dependent and can significantly limit current conduction in the on-state in BP FETs. Model parameters are extracted using measured data of back-gated BP transistors with gate lengths of 1000 nm and 300 nm with BP thicknesses of 7.3 nm and 8.1 nm, and for the temperature range 180–298 K. Compared to previous BP models that are validated only for room temperature and near-equilibrium bias conditions (low drain-source voltage), we demonstrate an excellent agreement between the model and data over a broad range of bias and temperature values. The model is also validated against numerical technology computer-aided design data of back- and top-gated BP transistors with a channel length of 300 nm and a thickness of 8.1 nm. The model is implemented in Verilog-A, and the capability of the model to handle both dc and transient circuit simulations is demonstrated using SPECTRE. The model not only provides physical insight into technology-device interaction in BP transistors but can also be used to design and optimize BP-based circuits using a standard hierarchical circuit simulator.

A unified static-dynamic analytic model for ultra-scaled III-nitride high electron mobility transistors

This paper presents an analytic model to calculate nodal charges and their corresponding internodal capacitances in ultra-scaled III-nitride high electron mobility transistors (HEMTs) in which carrier transport is expected to be quasiballistic. The dynamic model is combined with our previously published static model [K. Li and S. Rakheja, J. Appl. Phys. 123, 184501 (2018)] to provide a unified physical description of the HEMT. In the dynamic model, the position-dependent channel charge is obtained using a realistic potential profile in the channel, obtained from the solution of the Poisson equation. The effect of electric field lines penetrating from the ungated access regions into the channel (gated region) is included. Fringing electric field lines from the gate to the channel, which are especially critical in the off-region of the device, are included in the charge description. The channel charges are obtained self-consistently with the transport model and introduce 16 additional input parameters, which are necessary to explain the bias dependence of internodal capacitances in the off-state and in the off-to-on transition region of the device. Using the model, we elucidate the difference in the capacitance-voltage behavior of drift-diffusive and quasiballistic devices. The sensitivity of channel charges to the specific formulation of the potential profile is also examined. The unified model is applied to the experimental capacitance-voltage data of 42-nm and 105-nm gate-length InAlN/GaN HEMTs with an InGaN backbarrier. Additionally, the model is validated against hydrodynamic simulations of a 50-nm gate-length AlGaN/GaN HEMT with significant Joule heating. The model yields an excellent agreement with the measured and simulated data sets over a broad range of bias and temperature conditions. The model is also used to elucidate the role of contact resistance, Joule heating, and nodal capacitances on the RF performance, i.e., cut-off frequency, third-order intermodulation current, and the input third-order intercept point, of scaled III-nitride HEMTs.

A New Interpretation for the Anomalous Channel-Length Dependence of Low-Frequency Noise in Quasi-Ballistic Transistors

The operating regime of MOS transistors shifts from collision-dominated drift-diffusion (DD) to less-collision quasi-ballistic (QB) regime. The impact of transport mechanism on the low-frequency noise (LFN) is not well studied for QB devices. Most of the studies rely on the existing theories that were developed for DD-based devices to study LFN even in nanoscale regime. Validity of these models is questionable. In this letter, we extend the conventional carrier-number and correlated-mobility fluctuation model to the QB regime. The model is in accordance with the “apparent mean free path” $(\lambda _{\textit {app}})$ model, which predicts scattering limited noise mechanism in the QB regime. The model is validated with LFN measurement data for III-V QB devices, and shows good agreement with the expected behavior.

Magnetoconductance signatures of subband structure in semiconductor nanowires

The radial confining potential in a semiconductor nanowire plays a key role in determining its quantum transport properties. Previous reports have shown that an axial magnetic field induces flux-periodic conductance oscillations when the electronic states are confined to a shell. This effect is due to the coupling of orbital angular momentum to the magnetic flux. Here, we perform calculations of the energy level structure, and consequently the conductance, for more general cases ranging from a flat potential to strong surface band bending. The transverse states are not confined to a shell, but are distributed across the nanowire. It is found that, in general, the subband energy spectrum is aperiodic as a function of both gate voltage and magnetic field. In principle, this allows for precise identification of the occupied subbands from the magnetoconductance patterns of quasiballistic devices. The aperiodicity becomes more apparent as the potential flattens. A quantitative method is introduced for matching features in the conductance data to the subband structure resulting from a particular radial potential, where a functional form for the potential is used that depends on two free parameters. Finally, a short-channel InAs nanowire field-effect transistor device is measured at low temperature in search of conductance features that reveal the subband structure. Features are identified and shown to be consistent with three specific subbands. The experiment is analyzed in the context of the weak localization regime; however, we find that the subband effects predicted for ballistic transport should remain visible when backscattering dominates over interband scattering, as is expected for this device.

MONTE CARLO TRANSPORT AND HEAT GENERATION IN SEMICONDUCTORS

Understanding transport and energy use in nanoscale devices is essential for the design of low-power electronics and efficient thermoelectrics. This chapter examines transport physics and the electron-phonon interaction in the context of Monte Carlo simulations, which treat electrons and phonons with comparable attention to detail. The Monte Carlo method is described in depth, including scattering physics, electron energy band and phonon dispersions, Poisson solution, and contacts within realistic devices. This approach uncovers, for instance, that Joule heating in silicon devices is distributed between the (slow) optical and (fast) acoustic phonon modes by a ratio of two to one. In nanoscale transistors, nonequilibrium transport affects heat generation near strongly peaked electric fields, and Joule heating occurs almost entirely in the drain of short, quasi-ballistic devices. Evidence is also uncovered for thermionic cooling at the source terminal of transistors, and the physics of this phenomenon are described. Although the discussion is often with respect to silicon for specificity, key methods can be broadly applied to many semiconductor devices and structures. Such aspects are only expected to increase in importance as nanoscale devices are reduced to dimensions comparable to or smaller than the electron and phonon mean free paths (10−100 nm).

Length and temperature dependent 1/f noise in vertical single-walled carbon nanotube arrays

We report measurements of temperature- and length-dependent 1/f noise in vertical single-walled carbon nanotube (SWCNT) arrays. Carbon nanotubes are synthesized in a porous anodic alumina template with sub-micrometer channel lengths ranging from 100 to 700 nm. A significant difference is observed in the 1/f noise magnitude of quasi-ballistic and diffusive SWCNT devices, with quasi-ballistic devices exhibiting 1/f noise levels that are one to two orders of magnitude less than diffusively conducting devices. Furthermore, 1/f noise was measured from 90 to 400 K, and the noise prefactor decreased significantly at temperatures below 250 K.

Single Source Heterojunction Metal–Oxide–Semiconductor Transistors for Quasi-Ballistic Devices: Optimization of Source Heterostructures and Electron Velocity Characteristics at Low Temperature

We have experimentally studied the electron velocity behavior of single source heterojunction metal–oxide–semiconductor field-effect transistor (SHOTs) without a drain energy spike fabricated by optimizing the source heterostructures. The electron velocity vE of SHOTs with source conduction band offset ΔEC strongly depends on the source heterojunction position, and the enhancement of vE for SHOTs relative to that for single drain-heterojunction transistors (DHOTs) and strained-Si-on-insulator devices (SSOIs) slightly increases with decreasing lattice temperature T. We have also found that vE for SHOTs, DHOTs, and SSOIs has a power law dependence on low-field electron mobility µEFF, that is, the mean free path of electrons λ at various T. Thus, the electron kinetic energy ΔEK converted by the source band offset in SHOTs increases with increasing mean free path of electrons at lower T. Using vE∝λα (α: constant), we can also estimate vE for both SHOTs and DHOTs at the ballistic transport limit.

Single Source Heterojunction Metal–Oxide–Semiconductor Transistors for Quasi-Ballistic Devices: Optimization of Source Heterostructures and Electron Velocity Characteristics at Low Temperature

Single Source Heterojunction Metal–Oxide–Semiconductor Transistors for Quasi-Ballistic Devices: Optimization of Source Heterostructures and Electron Velocity Characteristics at Low Temperature