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We present a study of the photo-excited charge carriers relaxation dynamics in polar semiconductors comparing calculations to pump probe experiments. Hot carrier densities in the 1018cm-3 range can easily be photo-generated using moderately intense optical excitations. This can lead to known phenomena, namely, hot phonon populations and the coupling of polar optical phonons with plasmon modes. However, these two phenomena can affect the hot carriers relaxation and have never been examined together. This is a problem for the theoretical study of future Hot Carrier Solar Cells, where the conditions allow both of these phenomena to occur. The charge carriers dynamics and the coupling of polar optical phonons with plasmon modes are treated by a Full Band Ensemble Monte Carlo simulation code featuring a self-consistent dielectric function. To take into consideration hot phonon populations and the subsequent phonon bottleneck for the carriers relaxation, the charge carriers simulation code is coupled to a phonon dedicated Ensemble Monte Carlo code. This enables for the first time an accurate study of both the charge carriers and phonon systems dynamics, the latter being most of the time overly simplified in previous studies. The present work explores to which extent the two aforementioned phenomena affect the photo-generated charge carriers relaxation in GaAs and can be easily adapted to other polar semiconductors. [ABSTRACT FROM AUTHOR]

The Monte Carlo (MC) stochastic method is used to solve the phonon Boltzmann transport equation. A solution scheme taking into account all the different individual types of Normal and Umklapp processes which respect energy and momentum conservation rules is presented. The use of the common relaxation time approximation in the MC solver is thus avoided. A generalized Ridley theoretical scheme is used instead to calculate three-phonon scattering rates, with the Grüneisen constant as the only adjustable parameter. A method for deriving adequate adjustable parameters is presented. Polarization branches with real nonlinear dispersion relations for transverse or longitudinal optical and acoustic phonons are considered. Zone-center longitudinal optical (LO) phonon lifetimes are extracted from the MC simulations for GaAs, InP, InAs, and GaSb. Decay channels contributions to zone-center LO phonon lifetimes are investigated using the calculated scattering rates. Vallée-Bogani's channel is found to have a negligible contribution in all studied materials, notably GaAs. A comparison of phonons behavior between the different materials indicates that the previously reported LO phonon lifetimes in InAs and GaSb were quite underestimated. [ABSTRACT FROM AUTHOR]

A strain Bir–Pikus Hamiltonian H[sub st], based on a 20 band sps[sup *] k·p Hamiltonian H[sub kp], is used to describe the valence band and the first two conduction bands over the entire Brillouin zone. This full-band k·p computation of the carrier dispersion relation is used to calculate electron and hole effective masses in strained silicon. Hole density of states masses are found to be very temperature dependent whereas electron effective masses can be considered temperature independent to first order. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A strain Bir-Pikus Hamiltonian H[sub st], associated with a 20-band sp³s* k·p Hamiltonian H[sub kp], is used to describe the valence band and the first two conduction bands all over the Brillouin zone. Because the local (in k space) deformation potentials ... and ... used in pseudopotential method are unusable in k·p theory, we show that taking into account the Bir-Pikus parameters (a, b) of the Brillouin zone center in the H[sub st] Hamiltonian allows one to describe the dispersion relation in the whole Brillouin zone. The method is applied to strained Si on a relaxed Si[sub 1-x]Ge[sub x] alloy. The values of the energy band gap, and of the Δ[sub 2-4] conduction band splitting between the four equivalent in-plane valleys Δ[sub 4] and the two valleys along the growth direction Δ[sub 2] are in very good agreement with those reported in other publications. The small value of the spin-orbit splitting in silicon is taken explicitly into account. We show that the valence band splitting is consequently not proportional to the stress. [ABSTRACT FROM AUTHOR]

Conductor-backed coplanar waveguide on 30-μm-thick BCB polymer is fabricated. In order to eliminate unwanted propagation modes for these lines designed for frequencies up to 1 THz, electroplated vias interconnect each coplanar ground to the backed conductor. Transmission line properties are extracted here up to 67 GHz, showing a characteristic impedance of about 50 Ω, but with high conduction losses for this frequency range due to the thin metal layer used. Thru-reflect-line correction and a de-embedding with electrical circuit of accesses are applied to extract line properties. The comparison with a coplanar waveguide model leads to a relative permittivity of 2.676 for the BCB in this low-frequency range. [ABSTRACT FROM AUTHOR]

This paper proposes an investigation in the terahertz (THz) frequency range of the dispersion and an individual quantitative treatment of the losses of the most classical microwave waveguides (coplanar, slotline, microstrip and stripline) numerically led in three dimensions (3D). An original strategy has been used to quantify radiation losses associated with leaky modes. A very low THz permittivity polymer (benzocyclobutene (BCB)) was used as a very convenient substrate to be easily grafted as a THz environment of integrated passive or/and active devices. Direct comparisons of the losses and the dispersion have been performed following two criteria: a constant characteristic impedance Zc fixed at 100 Ω and a constant effective width Weff fixed at 30 μm. The best waveguides are microstrip (αT = 2.52 dB/mm for Zc = 100 Ω and for W/H = 35/50 μm (with W the strip width and H the substrate height) and αT = 2.29 dB/mm for Weff = 30 μm at 1 THz with H = 30 μm) and stripline (with quasi-null radiation losses and the best quality factor QT = 63 for Zc = 100 Ω). The large dispersion and radiation losses of the slotline (SL) can be reduced with a thick BCB encapsulation to enhance the THz signal. The coplanar waveguide (CPW) remains in a medium position. Besides the parasitic mode (SL) and low QT problems due to mainly ohmic losses, its major advantage is its planar geometry allowing to an easy circuit integration with THz sources, amplifiers and detectors based on semiconductor. Consequently, these THz studies on BCB microwave standard waveguides open to various perspectives to carry out a broad panel of integrated THz circuits. [ABSTRACT FROM AUTHOR]

The aim of this study is to analyze the losses of coplanar waveguide (CPW) in the Terahertz (THz) frequency range. A numerical analysis of the origin of losses (conductor, dielectric and radiation) for different geometries of the THz CPW on an Indium Phosphide substrate is presented. Two air-bridges have been created over the central metallization and they are linked to the two ground planes in order to select the fundamental CPW mode by strongly reducing the unwanted slotline mode. Three numerical codes (HFSS, CST and an indigenously developed code called MAXTRA3D) are used for CPW modeling. Several CPW shapes are investigated showing that ohmic losses are dramatic for small central metallization-ground spacing while radiation losses are dominating for large spacing. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:213-219, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26448 [ABSTRACT FROM AUTHOR]

Microwave noise performance of SiGe-based heterostructure field effect transistors (HFETs) is presented. Noise parameters for devices with buried channel fabricated on several virtual substrates are discussed. The impact of such strain relieved buffers on device noise performance is estimated by a proper noise dc-embedding technique. Then, the noise properties measured in the 25-18-GHz frequency range are compared with those of other technologies. Noise parameters of SiGe HFETs are simulated using Pospieszalskis and Van Der Ziel's noise models. Some detrimental effects like access resistances and self-heating effects that negatively impact the microwave noise behavior are discussed and some alternatives to overwhelm them are proposed. [ABSTRACT FROM AUTHOR]

This paper describes a 230-GHz self-aligned SiGeC heterojunction bipolar transistor developed for a 90.nm BiCMOS technology. The technical choices such as the selective epitaxial growth of the base and the use of an arsenic-doped monocrystalline emitter are presented and discussed with respect to BiCMOS performance objectives and integration constraints. DC and high-frequency device performances at room and cryogenic temperatures are given. HICUM model agreement with the measurements is also discussed. Finally, building blocks with state-of-the-art performances for a CMOS compatible technology are presented: A ring oscillator with a minimum stage delay of 4.4 ps and a 40-GHz low-noise amplifier with a noise figure of 3.9 dB and an associated gain of 9.2 dB were fabricated. [ABSTRACT FROM AUTHOR]

We present a terahertz photoconductive switch analysis. We explain the mechanism that allows the generation of the electromagnetic pulse and the phenomena that model its shape. We outline the main parameters influencing the output "electric pulse." It is necessary to use a full-wave numerical model when the device signal rapidly varies in time. We use the finite-difference time-domain (FDTD) method to solve the whole equation system (Maxwell's equations and drift-diffusion equations). The three-dimensional variable step mesh allows a realistic space step (25 nm) for the resolution of the carrier transport equations. [ABSTRACT FROM AUTHOR]