Carrier Density Distribution Research Articles

Self‐Supporting Acoustoelectronic Pulses Driven by Optical Excitation of a Semiconductor in the Transparency Region

AbstractIt is shown that the electron‐phonon interaction under certain circumstances can lead to the existence of moving self‐consistent distributions of lattice deformation and carrier density which are supported by an external optical excitation. They are moving towards the source of light with an intensity‐dependent velocity of the order of the sound velocity. The conditions of existence and the stability of such structures are investigated.

Characterization of semiconductors by photoluminescence and electroluminescence measurements in electrolyte solutions

Luminescence measurements of semiconductors in electrolyte solutions are described. These measurements provide useful information on charge and potential distribution, surface recombination velocity, energy levels of surface and bulk trap states, spatial distribution of dopant and carrier density.

Static frequency chirping in lambda /4-phase-shifted distributed-feedback semiconductor lasers: influence of carrier-density nonuniformity due to spatial hole burning

An experimental and theoretical study of the effect of carrier-density nonuniformity on frequency modulation (FM) characteristics of 1.3- μm λ/4-phase-shifted distributed-feedback (DFB) lasers is discussed. The static frequency chirping is measured using a recently developed method that is free from dynamical chirping as well as thermal frequency shift. The amount of chirping shows a strong dependence on the bias current level. This is explained theoretically by considering the nonuniform carrier-density distribution due to the spatial hole burning effect.

Hall effect in gratinglike potential modulated inversion layers

We present a quantitative approach to the potential fluctuation model we have previously developed in disordered semiconductors. MOSFET (metal-oxide-semiconductor field-effect transistors) were irradiated along lines parallel (L structures) or perpendicular (T structures) to the drain-source direction, using a 20-keV scanned e-beam. Thus, in the inversion regime, the surface potential ψs was periodically modulated, thereby strongly modifying the conduction processes. Hall measurements were performed as a function of temperature and gate voltage at various irradiation doses and for different periods. Conduction in L structures was influenced by lateral potential barrier scattering, whereas it was barrier controlled in T structures. The experimental results are interpreted by a simple model of energetic distribution of both carrier density n(E) and mobility μ(E), based on the existence of a conduction threshold. When barriers were wide in T structures, Hall measurements showed that part of the carriers did not participate in conductivity in the weak inversion regime, while mixed conduction processes occurred when barriers were narrow. In contrast, all the carriers conducted in the strong inversion regime, and only the mobility was perturbed. Thus the field effect in irradiated MOSFET was used to correlate the Hall carrier density with the conduction threshold energy.

Carrier transport near the Si/SiO 2 interface of a MOSFET

We present a rigorous investigation of carrier transport near the semiconductor-insulator interface of a MOSFET. Deriving a new current equation we include the quantum mechanical correction on the carrier density distribution near the interface. This equation also contain a new mobility model for the surface field reduced channel mobility, that is derived from Boltzmann's equation. We implemented this new equation into the device simulator MINIMOS and discuss its consequences to real devices.

Theory of the Residual Resistivity Dipole Diffusion of Non‐Interacting Particles Around a Fixed Point Scatterer

AbstractEmploying a superposition representation of the microcanonical density matrix the perturbation of the stationary diffusion process by a fixed point scatterer is treated approximately. The primary dipole distributions of carrier and current density are calculated and interpreted without additional assumptions. The stationarity of the current is shown explicitly. The primary dipole is localized in a region limited by the mean free path. The classical diffusion process which follows removes this localization, the electrostatic screening by the ensemble of all electrons restores it. The ideas of Landauer are confirmed within a coherent theoretical scheme.

Pure frequency modulation of a multielectrode distributed-Bragg-reflector (DBR) laser diode

Intensity-modulation suppressed frequency modulation (FM) is demonstrated over a 600-MHz bandwidth by using a three-electrode distributed-Bragg-reflector (DBR) laser diode (LD). A nonuniform carrier-density distribution in the active region is realized by two separated electrodes, whose push-pull modulation yields only a 0.7% intensity fluctuation per 1-GHz frequency deviation. This corresponds to a 20-dB FM purity improvement. A third electrode on the DBR region makes it possible to tune the wavelength simultaneously. Because of its wider modulation bandwidth, the LD is suitable for the lightwave oscillator in frequency-shift-keyed (FSK) transmissions and for linewidth reduction by electrical feedback. >

Pure frequency modulation or intensity modulation with suppressed frequency chirp using active Bragg reflector integrated laser

The modulation properties of a laser structure which consists of an active Bragg reflector (300µm) integrated with an uncorrugated gain region (600µm) have been measured. The laser exhibited a flat FM response and very low spurious intensity modulation when modulating the current in the Bragg reflector. Furthermore, broadband intensity modulation with suppressed frequency chirp could also be achieved. An inhomogeneous linewidth enhancement factor α caused by the uneven carrier density distribution between the two sections gives a qualitative explanation to our results.

Lateral Distribution of Si-Related Defects and Carrier Density in GaAs Crystals

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Finite-element analysis of magnetotransistor action

Most of the semiconductor device geometries that are being experimented with for improved magnetic field sensing do not lend themselves to accurate analytical modeling. The vectorial interaction taking place between the electrical current and the magnetic field complicates the problem of solving nonlinearly coupled carried density and electrostatic potential equations. In this work, solutions to the carrier density and electrostatic potential distributions are obtained by using a finite-element technique for a structure representing lateral and vertical bipolar transistors with multiple collectors. The results presented show the effect of the magnetic field to be very nonuniformly distributed around the emitter with a heavy-concentration near the emitter edges where the Lorentz-force-driven carriers are stopped at the insulating boundaries and give rise to a magnetoconcentration in the carrier densities. Distribution of currents collected by a multitude of collectors surrounding the emitter and their sensitivity to the magnetic field are given. The vertical magnetosensitivities are found to be an order of magnitude smaller than the lateral magnetosensitivity because of the lack of such a magnetoconcentration of carriers under the emitter. The computer program developed can simulate unipolar as well as bipolar magneto-sensitive devices with any boundary geometry.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new theoretical model for a p- n junction realistic diode

Attempts have been made to give a new theoretical model for semiconductor bipolar realistic diodes. The fact that the realistic devices are not completely free from the effects of traps, defects, band to band radiative and non-radiative (Auger effect) transitions etc., leads to the mathematical solutions which show oscillatory variations (between two positive values) in the carrier density distributions. This directly leads to the deviation of a diode characteristics from those of the ideal diode. An application of the model has been made to a p- n junction. Experimental evidence of this deviation in the Schottky heterostructures found in the literature can act as a direct support of the model.

Numerical study of currents and fields in a photoconductive detector

A numerical study of the current, field, and carrier density distributions within a photoconductive detector is presented. The photodetector, an interdigitated Schottky barrier diode, is made with metallic fingers of alternating voltage bias on a thin semiconductor layer grown on a transparent dielectric substrate. The Poisson and continuity equations for electrons and holes are treated in two dimensions. A modified successive line overrelaxation method, faster than the capacitance matrix method, is developed as the Poisson solver. A simple alternative to the Scharfetter-Gummel treatment of current density is also introduced. We investigate steady-state cases with and without optical illumination, and transient responses to picosecond optical pulses. The steady-state current Shows near saturation with increasing voltage, as observed experimentally. The calculated typical response of a silicon detector to a picosecond optical pulse is a current pulse lasting on the order of 10 ps.

Mode characteristics of phase-locked semiconductor laser arrays at and above threshold

A detailed eigenmode analysis of diode laser phased arrays both at and above threshold condition is presented. This analysis is based on the models developed for stripe-geometry diode lasers, in which the field equation is solved self-consistently with the carrier equation by taking into account the effect of carriers on the effective refractive index and including the stimulated emission in the carrier equation. The effect of a varying transverse confinement factor across the array is included. Numerical simulation results are obtained for threshold currents as well as laser behavior above the threshold, including the carrier density distribution, perturbed index distribution, array-mode patterns, and their associated far-field intensity distribution. Special attention has been paid to the development of the various array modes at power levels at which two array modes are oscillating simultaneously and the third one just reaches threshold.

Optical time-of-flight investigation of the exciton transport in silicon

The transport of carriers created at the surface of a silicon sample by a short laser pulse is investigated by a new optical time-of-flight technique. This method is based on the excitation of charge carriers in a small region of the sample and the subsequent time-resolved detection of the carrier density at a well-defined distance. This provides direct access to the carrier velocity and the carrier density distribution. In the low-temperature regime investigated here the carrier transport is shown to be performed by free excitons. A comparison of the experiment with a hydrodynamic model yields an exciton diffusivity of D=300T−1/2 cm2/s. It is shown that acoustic-phonon scattering via the deformation potential is the dominating scattering process.

Multielectrode distributed feedback laser for pure frequency modulation and chirping suppressed amplitude modulation

A new concept for operation of distributed feedback lasers with multielectrodes is proposed and its capability for amplitude or frequency modulation is demonstrated. The device has electrically separated electrodes so that carrier density distribution along the laser cavity can be controlled artificially by adjusting the current distribution for each electrode. The lasing frequency can be controlled by changing the current ratio applied to the electrodes. Frequency modulation experiments disclose that neither frequency modulation efficiency nor phase delay depend on modulation frequency. It turned out that the frequency shift under modulation showed blue shift or red shift depending on bias current distribution and the position of the modulation applied electrode. Using these characteristics, chirping suppressed amplitude modulation and frequency modulation with constant output power are realized.

A self-consistent model of the lateral behavior of a twin-stripe injection laser

A fully self-consistent computer model of the steady-state behavior of the zero-order lateral optical field of a GaAs twin-stripe injection laser is presented which takes into account current spreading in the p-type confining layer, the effect of lateral diffusion of carriers in the active layer, and bimolecular and stimulated radiative recombination. The results predict the lateral movement of the near field of the optical signal under asymmetric drive conditions, as observed in practice. Also calculated are the corresponding carrier and current density distributions. It is shown that the near-field zero order lateral optical field can be beam steered across the facet by only 2μm, typically. However, the initial position of the beam can be controlled by the two-stripe currents and also the geometry of the device. For the case where I_{s1} \approx I_{s2} the beam movement is seen to be proportional to either I_{s1} or I_{s2} . The results show that beam steering is not accompanied by a negative slope to the I-L characteristics. The effect of geometry and diffusion coefficient on the value of maximum current allowed before modal instability occurs is also given.

Analytical integration of small-signal transport equations in an MIS structure at flat-band condition

The small-signal transport equations are solved analytically in a homogeneously doped MIS structure at flat-band condition. We can establish exact expressions of ac carrier density, electric field, and potential distributions in the MIS structure for any frequency. The plot of these expressions, in the frequency range from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Hz, proves the existence of majority and minority modes. A complete impedance expression is sufficient to determine all equivalent circuit components of the MIS structure at flat-band condition. The proposed method is valid for any bulk lifetime of carriers and for any semiconductor thickness.

Stripe-structure dependence of frequency modulation characteristics of AlGaAs lasers

Frequency modulation characteristics of AlGaAs lasers having three kinds of stripe structures, channelled substrate planar (CSP), buried hetero- (BH) and transverse junction stripe (TJS) structures, are investigated. The phase relation between the frequency modulation and modulation current show two types of dependence on the modulation frequency. It is shown that this result is explained well by considering inhomogeneous carrier density distributions in the lateral direction across active layers.

Numerical solution of lateral current spreading and diffusion in near-threshold DH twin-stripe lasers

A numerical solution is presented to the problem of lateral current spreading in double heterostructure twin-stripe lasers when lateral diffusion of carriers and bimolecular recombination effects are present. The current components flowing into both the p-type confining layer and the active layer are solved simultaneously, subject to the boundary conditions imposed at the heterojunction. This yields the two-dimensional potential and the current density distributions in the p-type confining layer and the lateral carder density distribution in the active region. Diffusion and bimolecular radiative recombination effects are included in the solution of the current distribution in the active layer. The lateral current and carrier density distributions along the active layer are presented for single- and twin-stripe devices for a variety of electrode injection currents. The paper highlights the influence of device geometry and diffusion on the carrier distributions found beneath the stripes of twin-stripe lasers.

ANALYSIS OF A DISCRETIZATION ALGORITHM FOR STATIONARY CONTINUITY EQUATIONS IN SEMICONDUCTOR DEVICE MODELS, II

In a previous paper [COMPEL, Vol. 2, No. 3, pp. 117–139], an analysis was presented of a discretization procedure for a class of elliptic problems, including the Scharfetter‐Gummel method for the continuity equations of stationary semiconductor device models. Here, the previous results are extended in two directions: firstly, to such problems in an arbitrary number of dimensions and, secondly, to include the computation of suitable moments, as opposed to point‐values, of the carrier density distributions.