Akira Yoshii

Temperature dependence of hot carrier effects in short-channel Si-MOSFETs

Full-band Monte Carlo simulations were carried out to investigate hot carrier effects associated with impact ionization under the lateral electric field profiles typical of submicrometer Si-MOSFETs. It is shown that the temperature dependence of the band-gap energy of Si plays an important role for hot carrier suppression at low temperature in submicrometer devices. On the other hand, as the device size shrinks into the sub-0.1 regime, in which the high-field region is comparable in size to or smaller than the energy relaxation length, the number of electrons with energy below the supply drain voltage becomes less sensitive to temperature. As a result, the suppression of impact ionization at low temperature in sub-0.1 /spl mu/m devices could be ascribed to both quasi-ballistic transport characteristics and temperature-dependent band-gap energy.

Nonstationary carrier dynamics in quarter-micron Si MOSFETs

An application of Monte Carlo particle and relaxation time approximation modeling to quarter-micron Si MOSFETs is presented. Through a comparison between these two nonstatic models and a conventional model, nonstationary carrier transport is shown to dominate in 0.4 mu m or less channel devices, with peak velocities exceeding 1.0*10/sup 7/ cm/s. It is shown that the relaxation time approximation model tends to overestimate nonstationary carrier dynamics, especially the energy distribution. >

A Monte Carlo study for minority‐electron transport in p‐GaAs

Minority‐electron transport in p‐GaAs with a wide range of acceptor doping concentration from 1017 to 1019 cm−3 is studied in detail. Using the Monte Carlo method in which electron‐hole interactions are taken into account, electron transport properties in p‐GaAs, such as drift velocity and electron temperature, are calculated. The calculated results show good agreement with the experimental ones. Furthermore, in order to make the features of minority‐electron transport clear, the electron transport properties in n‐GaAs, where electrons act as majority carriers, are also calculated. In comparison with majority‐electron transport, drift velocity, and electron temperature for the minority electron are greatly reduced. Throughout the study, it is shown that the interaction with holes is essential for minority‐electron transport in p‐GaAs.

Investigation of numerical algorithms in semiconductor device simulation

Abstract The algorithms used in semiconductor device simulation are investigated. Inversion algorithms, such as SOR, SI, generalized ICCG and Crout methods are compared in terms of convergency and required computer resources for various devices and bias conditions. For linearization of the basic equations, a quasi-coupled method is also compared with Gummels conventional decoupled method. Numerical experimentation shows that even the SOR method, which has the slowest convergency among these algorithms, efficiently provides good results when used properly. Moreover, the quasi-coupled method is also effective in linearizing the basic equations for transient analysis or high bias conditions without a significant increase in the required memory. Consequently, a properly used simulator having several algorithms is shown to be necessary for two- and three-dimensional analysis. Furthermore, guidelines for applying these numerical algorithms effectively are described in detail.

Precise ion-implantation analysis including channeling effects

A precise channeling model is proposed for ion-implantation analysis. This channeling model includes defect scattering effects on the impurity profiles. It is restricted to only a major axial channel for simplicity and is introduced to the two-dimensional Boltzmann transport equation method in order to accurately calculate the impurity profiles. The calculations are in good agreement with experimental values at a wide range of conditions (As⁺and B⁺, 5-130 keV, 5 × 10¹³to 1 × 10¹⁵cm^-2). Furthermore, it is concluded that more than 2 × 10¹⁷cm^-3recoiled atoms prevent the ions from channeling in the Si target. Thus, this channeling model is precise enough to use in the design of even shallow-junctioned and fine-structured devices.

Ultrafast energy relaxation phenomena of photoexcited minority electrons in p‐GaAs

Energy relaxation processes for minority electrons in p‐GaAs are investigated by time‐resolved photoluminescence (PL) measurements using an up‐conversion technique with a high time resolution of 130 fs. By measuring the time evolution of PL intensity, the energy relaxation time of electrons is obtained directly. Moreover, electron distribution created by laser excitation which is in a thermally nonequilibrium state is successfully observed. With increasing hole concentration, the time response in PL intensity becomes fast. This implies that electron‐hole interaction plays a key role in energy relaxation in high hole concentration. By detailed analyses of PL intensity, it can be found that the relaxation time by electron‐hole interaction is approximately 500 fs or less, and electrons which are in a nonequilibrium state just after excitation are thermalized rapidly within about 200 fs at the first stage by electron‐hole interaction.

Two-dimensional, static and dynamic device simulation of laser diodes

A two-dimensional device simulator for laser diodes is introduced, and its capability for quantitative device design of InGaAsP lasers is shown. This static and dynamic device simulator is based on the self-consistent analysis of five basic equations (Poissons equation, current continuity equations for electrons and holes, the wave equation, and the rate equation for photons). The simulator has been applied to the optimum design of simplified buried-heterostructure (BH) lasers. The simulator has made it possible to optimize device structure with respect to several requirements, such as low threshold current, low leakage current, fundamental transverse lasing mode, and high-frequency modulation capability.<<ETX>>

Low-field mobility enhancement in AlGaAs/GaAs/AlGaAs double-heterojunction structures

Two-dimensionally-quantized electron transport in modulation-doped double-heterojunction structures is investigated using a Monte Carlo simulation. Enhanced low-field mobility in the quantum well is observed. This results from the reduction of optical phonon scattering rates, which can be attributed to the spread of the two-dimensional electron gas. It is demonstrated that the carrier transport related to this enhancement and carrier confinement effectively contribute to device operation in a double-heterojunction FET.<<ETX>>

Impact Ionization in Submicron and Sub-0.1 Micron Si-MOSFETS

Correct knowledge of impact ionization in semiconductors is an essential ingredient for precise analyses and predictions of device characteristics such as reliability and degradation. In particular, as the device size shrinks into deep-submicron regime, energy dependence of the impact ionization rate in low energy regions (below 3 eV) plays a much more important role than the cases in bulk.1 This is because, in addition to an obvious reason that such ultra-small devices are to be operated at reduced applied voltages, the so-called nonlocal effects are so pronounced inside devices that many impact ionization events are actually induced by the electrons with energy below 3 eV (near threshold energy). Therefore, the ionization rate near threshold is of crucial importance for the investigations of hot carrier effects in ultra-small devices.

Two-dimensional Analysis of Resonant Tunneling Using the Time-dependent Schrödinger Equation

Numerical solution of the time-dependent two-dimensional Schrodinger equation is used here to analyze resonant tunneling in double-barrier structures. One-dimensional simulation is shown to be insufficient even for a system with a perfect barrier, which is regarded as one-dimensional because it does not include the two-dimensional effect attributed to the effective mass difference between the barrier and well regions. The effect of barrier roughness on tunneling characteristics is also analyzed and the transmitted fraction is calculated for various systems with different structural parameters, including the roughness of the barrier. Decomposition of the fraction into transverse momentum clarifies tunneling characteristics in two dimensions, and two-dimensional calculation is shown to be necessary for accurate analysis.