D. G. Walker

SIMULATION OF NONEQUILIBRIUM THERMAL EFFECTS IN POWER LDMOS TRANSISTORS

Abstract The present work considers electrothermal simulation of LDMOS devices and associated nonequilibrium effects. Simulations have been performed on three kinds of LDMOS: bulk Si, partial SOI and full SOI. Differences between equilibrium and nonequilibrium modeling approaches are examined. The extent and significance of thermal nonequilibrium is determined from phonon temperature distributions obtained using a common electronic solution and three different heating models (Joule heating, electron/lattice scattering, phonon scattering). The results indicate that, under similar operating conditions, nonequilibrium behavior is more significant in the case of full SOI devices, where the extent of nonequilibrium is estimated to be twice that of the partial SOI device and four times that of the bulk device. Time development of acoustic phonon and lattice temperatures in the electrically active region indicates that nonequilibrium effects are significant for times less than 10 ns.

Thermal and Electrical Energy Transport and Conversion in Nanoscale Electron Field Emission Processes

This paper considers the theory of electron field emission from nanoscale emitters with particular focus on thermal and electrical energy transport. The foundational theory of field emission is explored, and a model is presented that accounts explicitly for the energy band curvature produced by nanoscale tip emitters. The results indicate that the inclusion of band curvature strongly influences the energetic distribution of electrons for emitter radii less than 50 nm. The energy exchange process between emitted and replacement electrons is shown to allow high local energy transfer rates that can be exploited in direct thermal-to-electrical energy conversion processes. The dependence of energy conversion rates on material and operational parameters is demonstrated. Throughout the paper, opportunities for further research involving nanoscale heat transfer, materials development, and modeling are highlighted. @DOI: 10.1115/1.1494091#

Temperature-dependent luminescence of Ce3+ in gallium-substituted garnets

The luminescent lifetime of cerium-doped yttrium aluminum garnet has been determined as a function of temperature and as a function of gallium content. We have shown that increasing gallium content decreases the decay lifetime and results in luminescence quenching at lower temperatures. The results are quantitatively explained using a configurational coordinate diagram.

Computational Study of Thermal Rectification From Nanostructured Interfaces

Thermal rectification is a phenomenon in which transport is p referred in one direction over the opposite. Though observations of thermal rectification have been elusive, it could be useful in many applications such as thermal management of electronics and improvement of thermoelectr ic devices. The current work explores the possibility of thermally rectifying devices with the use of nanostru ctured interfaces. Interfaces can theoretically result in thermally rectifying behavior because of the difference in phonon frequency content between two dissimilar materials. The current work shows an effective rectification of g reater than 25% in a device composed of two different materials divided equally by a single planar interface.

High-temperature electron emission from diamond films

This work examines electron field-emission characteristics of polycrystalline diamond films at elevated temperatures. Diamond is an excellent material as a field emitter because of its exceptional mechanical hardness and chemical inertness. The motivation behind this study involves the use of field emitters in applications where high temperatures exist. Nitrogen-doped polycrystalline diamond films were grown by plasma-enhanced chemical-vapor deposition. To investigate the effect of increased temperatures on field emission, current–voltage measurements were taken from the same diamond film at varying temperatures. Results from these measurements indicate a decrease in the turn-on voltage with increasing temperature. Further analysis of the temperature dependence of emission is achieved through parameter estimation of the effective emitting area, field enhancement factor, and work function. These results suggest that thermally excited electrons are responsible for improved emission at high temperature.

Coupled electro-thermal Simulations of single event burnout in power diodes

Power diodes may undergo destructive failures when they are struck by high-energy particles during the off state (high reverse-bias voltage). This paper describes the failure mechanism using a coupled electro-thermal model. The specific case of a 3500-V diode is considered and it is shown that the temperatures reached when high voltages are applied are sufficient to cause damage to the constituent materials of the diode. The voltages at which failure occurs (e.g., 2700 V for a 17-MeV carbon ion) are consistent with previously reported data. The simulation results indicate that the catastrophic failures result from local heating caused by avalanche multiplication of ion-generated carriers.

Quantum modeling of thermoelectric performance of strained Si∕Ge∕Si superlattices using the nonequilibrium Green’s function method

The cross-plane thermoelectric performance of strained Si∕Ge∕Si superlattices is studied from a quantum point of view using the nonequilibrium Green’s function method. Strain causes the germanium well layers to turn into barriers that promote electron tunneling through the barriers. Electron tunneling produces oscillations in the Seebeck coefficient due to shift in subband energies near the Fermi level. Strain-induced energy splitting can increase the power factor by up to four orders of magnitude in germanium-rich substrates. Also, at large doping, strain lowers the subband energies such that thermoelectric performance is independent of layer thickness between 2 and 4nm germanium barrier layers. The results imply that larger barrier layers can be used at high doping without a performance penalty while avoiding problems with interlayer diffusion that are prevalent in films with small thicknesses.

Phonon wave-packet simulations of Ar/Kr interfaces for thermal rectification

The frequency and direction dependence of transmission coefficients at interfaces was investigated theoretically. The interfaces are formed by having two Lennard-Jones materials differing in mass and interatomic potential equally divided at the center of an fcc lattice system. A single frequency wave-packet is generated at one end of the system and allowed to propagate through the system until all interactions with the interface are complete. The transmission coefficient is then calculated by comparing the energy of the packet that is transmitted with the original wave-packet. Results show a difference in transmission when the wave-packet originates from opposite sides.

Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Using a nonequilibrium Greens function approach, quantum simulations are performed to assess the device characteristics for cross-plane transport in Si/Ge/Si-superlattice thin films. The effect of quantum confinement on the Seebeck coefficient and electrical transport and its impact on the power factor of superlattices are studied. In this case, decreasing well width leads to an increased subband spacing causing the Seebeck coefficient of the superlattice to decrease. Electron confinement also causes a drastic reduction in the overall available density of states. Results show that confinement effects in the silicon barrier are responsible for a 40% decrease in the electrical conductivity of superlattices where barrier films are thinner by a factor of 1.5. In the same two devices, there is a negligible change in the Seebeck coefficient, which results in a decrease in the power factor corresponding to the decrease in conductivity. This decrease in the electrical performance for superlattices with thinner layers may offset the previously hypothesized gains of highly scaled superlattice structures resulting from reduced thermal conductivity. Simulations of the present superlattice structure at varying doping levels show a decrease in power factor with a decrease in device size parameters.

Single event burnout in power diodes: Mechanisms and models

Power electronic devices are susceptible to catastrophic failures when they are exposed to energetic particles; the most serious failure mechanism is single event burnout (SEB). SEB is a widely recognized problem for space applications, but it also may affect devices in terrestrial applications. This phenomenon has been studied in detail for power MOSFETs, but much less is known about the mechanisms responsible for SEB in power diodes. This paper reviews the current state-of-knowledge of power-diode vulnerability to SEB, based on both experimental and simulation results. It is shown that present models are limited by the lack of detailed descriptions of thermal processes that lead to physical failure.