Stephen M. Goodnick

Modeling Thermal Effects in Nanodevices

In order to investigate the role of self-heating effects on the electrical characteristics of nanoscale devices, we implemented a 2D Monte Carlo device simulator that includes the self-consistent solution of the energy balance equations for both acoustic and optical phonons. The acoustic and optical phonon temperatures are fed back into the electron transport solver through temperature-dependent scattering tables. The electrothermal device simulator was used in the study of different generations of nanoscale fully depleted silicon-on-insulator devices that are either already in production or will be fabricated in the next five to ten years. We find less degradation due to self-heating in very short channel device structures due to the increasing role of nonstationary velocity-overshoot effects which are less sensitive to the local temperature.

Hole initiated impact ionization in wide band gap semiconductors

Band-to-band impact ionization by hot electrons and holes is an important process in high-field transport in semiconductors, leading to carrier multiplication and avalanche breakdown. Here we perform first principles calculations for the respective microscopic scattering rates of both electrons and holes in various wide band gap semiconductors. The impact ionization rates themselves are calculated directly from the electronic band structure derived from empirical pseudopotential calculations for cubic GaN, ZnS, and SrS. In comparison with the electron rates, a cutoff in the hole rate is found due to the relatively narrow valence bandwidths in these wide band gap semiconductors, which correspondingly reduces hole initiated carrier multiplication.

Field effect on the impact ionization rate in semiconductors

Impact ionization plays a crucial role for electron transport in semiconductors at high electric fields. We derive appropriate quantum kinetic equations for electron transport in semiconductors within linear response theory. The field-dependent collision integral is evaluated for the process of impact ionization. A known, essentially analytical result is reproduced within the parabolic band approximation [W. Quade et al., Phys. Rev. B 50, 7398 (1994)]. Based on the numerical results for zero field strengths but realistic band structures, a fit formula is proposed for the respective field-dependent impact ionization rate. Explicit results are given for GaAs, Si, GaN, ZnS, and SrS.

Acoustic phonon scattering in silicon quantum dots

Intravalley acoustic phonon scattering of electrons in fully quantized systems based on n-type inversion layers on a (100) surface of p-type Si is studied theoretically. The confining potential normal to the Si/SiO2 interface is modelled by a triangular quantum well. For the confinement in the lateral directions we assume a parabolic potential. The calculations reveal that the anisotropic electron-acoustic-phonon interaction strongly affects the scattering rate and the average scattering angle. The calculated transition rate of electrons from the first excited state to the ground state shows a strong dependence on spatial confinement and lattice temperature, with the longitudinal phonon mode giving the main contribution to the total rate.

A Multiscale Modeling Approach to Study Transport in Silicon Heterojunction Solar Cells

Single junction solar cells based on Silicon continue to be relevant and commercially successful in the market due to their high efficiencies and relatively low cost processing. Heterojunction solar cells based on crystalline (c-Si) and amorphous (a-Si) silicon (HIT Cells) have paved the way for devices with high VOCs (>700 mV) and high efficiencies (>20%) [1]. Panasonic currently holds the world record efficiency of 25.6% for its trademark a-Si/c-Si HIT cell [2]. The novel structure of the device precludes the usage of traditional methods (such as drift diffusion) to accurately understand the nature of transport. Theoretical models used by commercial simulators make a variety of assumptions that simplifies the transport problem (assumes a Maxwellian distribution of carriers) and thus lacks the sophistication to study defect transport. In this work we utilize a combination of Ensemble Monte Carlo (EMC) simulations, Kinetic Monte Carlo (KMC) simulations and traditional drift - diffusion (DD) simulations t...

Importance of the Gate‐Dependent Polarization Charge and the Electron‐Electron Interactions on the Operation of GaN HEMTs

We investigate the influence of the gate‐voltage dependence of the polarization charge on the electron‐sheet density in the channel and how it reflects on the device transfer and output characteristics. We also investigate the role of the electron‐electron interaction on the magnitude of the drain current. We find that 10% increase in the polarization charge is needed to match the experimental data when these two effects are included in the theoretical model.