Nonequilibrium Carrier Transport Research Articles

Polaronic centres in single crystals

The autolocalization of free electrons at regular lattice sites, namely at complex anions, has been demonstrated by ESR measurements. It has been shown that the centre is a shallow donor; its energy level is situated meV below the bottom of the conduction band. At temperatures of 50-60 K the captured electrons are detrapped and the formation of other paramagnetic centres occurs. In La-doped , an enhanced cross-section for electron trapping by impurity centres with respect to that for undoped samples is found. In contrast to the case for other tungstates, no intrinsic oxygen hole and/or centres were revealed in . The role of the centres in the processes of transport and recombination of nonequilibrium carriers in the lattice is discussed.

Behavior of epitaxial GaAs layers as α particle detectors

The properties of epitaxial GaAs-based p+-n structures used as light-ion (α particle) were studied. A comparison is made with the latest published data on the possibilities of present-day semi-insulating GaAs (SI-GaAs). It is noted that the content of impurities and structural defects forming deep levels in the band gap of the material is two orders of magnitude lower in epitaxial layers. The deep levels determine the conditions of transport of nonequilibrium carriers in the detector, allowing for trapping of the carriers, and they also determine the electric-field profile. The charge-carrier lifetime was found to be ≥ 200 ns. This is two orders of magnitude longer than the values for SI-GaAs, in complete agreement with the lower content of deep centers. It is shown how deep centers influence the field profile, forming a quite large region of low field values.

Ultrashallow p +-n junctions in silicon (100): Electron-beam diagnostics of the surface zone

Ultrashallow p +-n junctions formed in silicon (100) under nonequilibrium impurity diffusion conditions are analyzed by electron-beam diagnostics of the surface zone using a probe of low-to medium-energy electrons. The energy dependence of the radiation conductivity is investigated, along with its distribution over the area of the p +-n junction. This procedure can be used to determine the depth distribution (in the crystal) of the probability of separation of electron-hole pairs by the field of the p-n junction; the experimental results show that this distribution differs according to whether the kick-out mechanism or the dissociative vacancy mechanism of impurity diffusion is predominant as the basis of formation of the ultrashallow p +-n junctions. Also reported here for the first time are the results of investigations of the distribution of secondary point centers formed near the boundary of the ultrashallow diffusion profile, which exert a major influence on the transport of nonequilibrium carriers. The data obtained in the study demonstrate the possibility of improving the efficiency of photodetectors, α-particle detectors, and solar batteries constructed on the basis of ultrashallow p-n junctions.

Quadrilateral Finite Element Monte Carlo Simulation of Complex Shape Compound FETs

The complex recess and gate shape of modem compound FETs greatly affect the device parasitics and therefore impose the need for proper description of the device geometry and surface conditions in any practical device simulations. In this paper we describe a new Monte Carlo (MC) module incorporated in our Heterojunction 2D Finite element FET simulator H2F [1]. The module combines realistic quadrilateral finite‐element description of the device geometry with realistic particle simulation of the non‐equilibrium hot carrier transport in short recess gate compound FETs. A Single Programme Multiple Data (SPMD) parallel approach makes it possible to use our MC simulator for practical design work, generating the necessary I‐V characteristics in parallel. The capabilities of the finite element MC module are illustrated in example simulations of a 200nm pseudomorphic HEMT fabricated in the Nanoelectronics Research Centre of Glasgow University.

Investigation of transient transport effects in semi-insulating GaAs caused by defect state change and their interaction

We investigated the transport of nonequilibrium charge carriers in semi-insulating liquid encapsulated Czochralski grown GaAs. The temporal changes of the Hall mobility were analysed after strong photo-excitation over long time intervals. They ranged up to 3000 cm 2 V −1 s −1 and depended sensitively on the crystal technology, defect nature and concentration. The maximum mobilities were anti-correlated with the dislocation density, while for the stationary values no clear correlation was found. In all crystals a long-lasting saturation of the mobility took place, which in some cases was interrupted by a ‘step-like’ increase or decrease. Different scattering centres appeared to be effective at different time intervals. The changes in the product of their concentration and scattering cross section were up to 2 × 10 6 cm −1. The observed effects confirm the influence of complex scatterers. It could have been defect associations and dislocations with Cottrell spheres, which created drift and recombination barriers of different heights. Screening by carriers changes not only the effective barrier heights, but the effective volume of the inhomogeneities and the drift paths of the carriers as well. The associations of scattering centres especially can be divided into smaller centres.

Spatially ununiform gain in MQW lasers caused by nonequilibrium carrier transport

Spatially ununiform gain in MQW lasers caused by nonequilibrium carrier transport are studied based on the Wigner distribution function. In the simulation of MQW lasers, the three Liouville equations with respect to the Wigner functions for electron, heavy-hole and light-hole, and the Poisson's equation are solved self-consistently. The simulation results are compared with the experimental data and the conventional gain analysis.

Evidence for non-equilibrium base transport in Si and SiGe bipolar transistors at cryogenic temperatures

In this paper we observe discrepancies between measured and simulated values for collector current and transconductance in advanced Si and SiGe bipolar devices at cryogenic temperatures. The deviation of these quantities from values predicted by standard drift-diffusion theory is shown to be consistent with the expected effects of non-equilibrium carrier transport across the neutral base region. A simple model based on non-equilibrium transport theory is used to account for the observed discrepancies in both parameters, resulting in excellent quantitative agreement with measured data over the temperature range 50–300 K.

Photo- and cathodoluminescence of ZnSSe quantum well heterostructures grown by MOVPE

Lateral- and depth-resolved cathodoluminescence as well as photoluminescence of MOVPE-grown single- and multi quantum well ZnSSe-based heterostructures in the temperature range of 14–300 K were investigated. The intensity ratios between different emission peaks and their dependencies on the electron penetration depth together with the temperature dependence of photoluminescence spectra permit concluding that the transport of non-equilibrium carriers or excitons in the structure is hindered by potential fluctuations created by microvariations of the sulphur content. The deep centres are localized mostly in the near-substrate region which is indicated by an increase of the self-activated emission at 2.2 eV with increasing electron beam penetration depth. Further improvement of layer homogeneity as well as a sufficient distance between the quantum well and the substrate together with minimization of the interlayer strain is necessary to improve the radiative properties of ZnSe-based heterostructures.

Nonequilibrium carrier transport in III–V compounds at low lattice temperatures

The effect of finite energy of the deformation potential and piezoelectric acoustic phonons on the electric field dependence of the effective temperature of the nonequilibrium carriers in n-type InSb and InAs has been calculated here by a numerical method for low lattice temperatures taking the nonparabolicity of the conduction band into account. Unlike the traditional method of neglecting the phonon energy in comparison to the carrier energy the calculations are carried out here under the condition that the acoustic phonon energy can neither be neglected nor the phonon distribution be represented by the equipartition law. The results are significantly different from those that follow from the traditional approach and are found to be in good agreement with the experimental data.

NOISE CHARACTERISTICS OF InP-BASED HETEROSTRUCTURE BIPOLAR TRANSISTORS

The noise characteristics of InP -based heterostructure bipolar transistors (HBTs) are studied from low frequency to microwave frequency. The nonequilibrium minority carrier transport in the thin base region is very effective in reducing the 1/f noise current at low frequency as well as reducing the uncorrelated shot noise current at high frequency. Experimentally, a very low 1/f noise corner frequency of 1.55 kHz is obtained in AlInAs/InGaAs HBTs with a 70 nm-thick base. This is the lowest 1/f corner frequency amongst any compound semiconductor devices reported to date. Minimum noise figures of 0.46 dB, 2.0 dB and 3.33 dB are also demonstrated at 2 GHz, 10 GHz and 18 GHz, respectively, with InP/InGaAs HBTs with a 35 nm-thick base and 3.5×3.5 μ m 2 emitter.

Non-equilibrium transport of hot electrons in AlGaAs/GaAs

Monte Carlo simulation is carried out to investigate the transport parameters of the two-dimensional electron gas in AlGaAs/GaAs heterostructures. In addition to diffusion coefficient and drift velocity at high electric fields, we derive the length coefficient defined by Price for an extended drift-diffusion model, which describes non-equilibrium carrier transport in semiconductors. The length coefficient is found to be almost zero at electric fields less than 3 kV cm-1 and shows a maximum of about 550 nm at 4 kV cm-1, decreasing gradually to 100 nm at 10 kV cm-1. The diffusion coefficient is found to be strongly affected by the degeneracy of the distribution function.

Nonequilibrium carrier transport in superconducting niobium-silicon heterostructures.

Nonequilibrium carrier transport in superconducting niobium-silicon heterostructures.

A theory of nonequilibrium carrier transport in multivalley semiconductors

The concept of a macrokinetic distribution (MKD) is used to develop a theory of nonequilibrium carrier transport in multivalley semiconductors. The MKD is a momentum-dependent function with macroscopic space-time scales, whose equation of evolution is macroscopically equivalent to the semiclassical Boltzmann transport equation for the phase-space distribution. A MKD is obtained for each valley and used to close the set of coupled moment equations that describe carrier transport in each valley. The resulting set of equations constitutes the model. A two-valley model is developed to illustrate the theoretical procedure. Results are obtained for the response of electrons to a rapidly varying (in time) electric field. These results are compared to those obtained using Monte Carlo (MC) methods and also an equivalent single-valley model. Excellent agreement is obtained between the macrokinetic model and the MC results. The single-valley model is shown to be inadequate in situations where intervalley carrier transfer is significant.

High-current-gain submicrometer InGaAs/InP heterostructure bipolar transistors

Common-emitter current gains of 115 and 170 are achieved in transistors with emitter dimensions as small as 0.3*3 and 0.8*3 mu m/sup 2/, respectively. These results are comparable with scaling experiments reported for Si bipolar devices and represent a significant improvement over AlGaAs/GaAs heterostructure bipolar transistors. Both the low surface recombination velocity and nonequilibrium carrier transport in the thin (800-AA) InGaAs base enhance the DC performance of these transistors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hot-electron spectroscopy of GaAs.

We report a new technique that enabled us to measure nonequilibrium carrier transport in GaAs. Hot electrons were injected into ${n}^{+}$-GaAs with an excess energy of 0.25 eV above the conduction-band edge. The effect of scattering on the injected electrons was observed by use of a planar doped barrier as a "hot-electron spectrometer." The measured spectra indicated that significant scattering occurred and that mean free paths were on the order of a few hundred angstroms.

Picosecond nonequilibrium carrier transport in GaAs

We report a new picosecond optical technique for observation of nonequilibrium carrier transport. An ’’overshoot’’ electron velocity of 4.4×107 cm/sec is observed at short times in GaAs under appropriate electric field conditions.

WP-A1 experimental observation of nonequilibrium carrier transport in GaAs

WP-A1 experimental observation of nonequilibrium carrier transport in GaAs