S. M. Goodnick

Diffusive transport in quasi-2D and quasi-1D electron systems

Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.

Room temperature velocity saturation in intrinsic graphene

We report ensemble Monte Carlo (EMC) studies of velocity saturation in intrinsic graphene at room temperature. The parameters for the phonon scatterings were obtained from fitting experimental data of graphene sheets in a variety of dielectric media, in which mobilities as high as 44,000 cm2/Vs were observed at 300 K. In our work, velocity saturation was clearly observed at low carrier density (≤5×1012 cm-2). Saturation velocity as high as 5.4×107 cm/s was achieved at the lowest density (5×1011 cm-2). Only scatterings due to the acoustic phonon and the K-point intervalley optical phonon were considered in the simulation. As the density increases, the onset value increases and the saturation velocity decreases.

High Field Transport in GaN and AlGaN/GaN Heterojunction Field Effect Transistors

Here we report on high field transport in GaN and GaN field effect devices, based on the rigid-ion model of the electron-phonon interaction within the Cellular Monte Carlo (CMC) approach, including quantum- mechanical effects. The calculated velocity is compared with experimental data from pulsed I–V measurements, where good agreement with experiment is found. We have applied the CMC transport kernel above to the simulation of the DC and high frequency characteristics of GaN MESFETs and AlGaN/GaN HFET devices. Various effects are considered, such as thermal heating and nonequilibrium phonons, in comparing with the dc and high frequency behavior of these devices.

Effects of surface treatment on the velocity-field characteristics of AlGaN/GaN heterostructures

AlGaN/GaN test structures were fabricated with an etched constriction. A nitrogen plasma treatment was used to remove the disordered layer, including natural oxides on the AlGaN surface, before the growth of the silicon nitride passivation film on several of the test structures. A pulsed voltage input, with a 200 ns pulse width, and a four-point measurement were used in a 50 Ω environment to determine the room temperature velocity–field characteristic of the structures. The samples performed similarly over low fields, giving a low-field mobility of 545 cm2 V−1 s−1. The surface treated sample performed slightly better at higher fields than the untreated sample. The highest velocity measured was 1.25 × 107 cm s−1 at a field of 26 kV cm−1.

Electron holographic characterization of nanoscale charge distributions for ultra shallow PN junctions in Si

Abstract This study extends electron holography as a quantitative characterization tool for nanoscale charge distributions associated with ultra shallow PN junctions in Si, which are needed for fabricating nanoscale MOSFETs, Si quantum dots and single electron transistors. The ultra shallow junctions were fabricated using rapid thermal diffusion from a heavily doped n-type surface source onto a heavily doped p-type substrate. Chemical characterization of the dopant profiles was performed using secondary ion mass spectrometry, which were analyzed to derive the metallurgical junction depth. 1-D characterization of the electrical junction depth associated with the electrically activated fraction of the incorporated dopants was performed using off-axis electron holography in a transmission electron microscope. 1-D potential profiles across the p–n junctions derived from electron holographic analysis were used to calculate the electric field and total charge distributions in the space charge region of the p–n junctions using numerical derivatives. Quantitative comparison between calculated electric field and total charge from the measured potential profiles and the simulated distributions using the secondary ion mass spectrometry profiles provide a reasonable estimate of the electrical activation of dopants in the ultra shallow junctions considered for this investigation.

Overshoot velocity in ultra-broadband THz studies in GaAs and InP

We model velocity overshoot in GaAs and InP using a fullband, particle-based device simulator based on the so-called cellular automaton method. This study has been motivated by the recent availability of experimental measures of the THz radiation generated by transient acceleration of photogenerated charge carriers in pin diode structures. The fullband code used in this study includes the full energy-momentum dispersion relation of electrons and holes as well as the full dispersion for the relevant phonons. Here we use this code for the simulation of high-field transient transport in bulk GaAs, InP, and for the experimental pin structure, where favorable comparison is found with the velocities measured from the transient THz radiation after ultrafast excitation.

Energy exchange in single-particle electron-electron scattering

Abstract Electron–electron scattering has been of interest in the treatment of hot carriers for several decades. Recently, the details of this scattering process have become much more important with regard to the degradation of Si MOS transistors, where it is suggested that intercarrier scattering excites electrons well above 3 eV, even with only 2 V bias. On the other hand, it is generally found that the energy exchange found in the electron–electron interaction is quite small. As a consequence, we have examined the detailed energy loss (gain) through the electron–electron interaction for carriers in Si. This is evaluated assuming a thermal Maxwellian hot carrier distribution at 2500 K and the parameters for Si. We find that the energy exchange is highly peaked around small energies (less than 20 meV), and falls off rapidly with energy.

Large-signal mm-wave InAlN/GaN HEMT power amplifier characterization through self-consistent Harmonic Balance / Cellular Monte Carlo device simulation

We report the simulation of the large-signal performance of mm-wave FET power amplifiers obtained for the first time through Full Band Monte Carlo particle-based device simulation self-consistently coupled with a Harmonic Balance (HB) frequency domain circuit solver. Due to the iterative nature of the HB algorithm, this FET simulation approach is possible only due to the computational efficiency of our Cellular Monte Carlo (CMC), which uses pre-computed scattering tables. On the other hand, a frequency domain circuit solver such as HB allows the simulation of the steady-state behavior of an external passive reactive network without the need for simulating long transient time (i.e. RC, L/C time constants) typical of time domain solutions. By exploiting this newly developed self-consistent CMC/HB code, we were able to time-efficiently characterize the mm-wave power performance of a state-of-the-art 30-nm gate-length InAlN/GaN HEMT.

Impact ionization and high-field effects in wide-band-gap semiconductors

Impact ionization is important for electron transport in wide-band-gap semiconductors at high electric fields. We consider a realistic band structure as well as high-field quantum corrections such as the intracollisional field effect in the calculation of the microscopic scattering rate. A pronounced softening of the impact ionization threshold is obtained. This field-dependent impact ionization rate is included within a full-band ensemble Monte Carlo simulation of high-field transport in ZnS. Although the impact ionization rate itself is strongly affected, little effect is observed on measurable quantities such as the impact ionization coefficient.

Large-signal full-band Monte Carlo device simulation of millimeter-wave power GaN HEMTs with the inclusion of parasitic and reliability issues

We report for the first time the simulation of the large-signal dynamic load-line of high-Q matched mm-wave power amplifiers obtained through a Monte Carlo particle-based device simulator. Due to the long transient time of large reactive circuit elements, the time-domain solution of power amplifier high-Q matching networks requires prohibitive simulation time for the already time-consuming Monte Carlo technique. However, by emulating the high-Q matching network and the load impedance through an active load-line, we show that, in combination with our fast Cellular Monte Carlo algorithm, particle-based accurate device simulations of the large signal operations of AlGaN/GaN HEMTS are possible in a time-effective manner. Reliability issues and parasitic elements (such as dislocations and contact resistance) are also taken into account by, respectively, exploiting the accurate carrier dynamics description of the Monte Carlo technique and self-consistently coupling a Finite Difference Time Domain network solver with our device simulator code.