Carrier Density Fluctuations Research Articles

Flux-flow voltage noise and normal-state resistance fluctuations in epitaxial (Dy, Y)Ba 2Cu 3O 7 thin films

We have studied low-frequency noise in the normal and mixed states of high- T c thin films. In the normal state, resistance fluctuations with a 1/ f power spectrum are observed from 100 K up to 300 K. The normal state noise is largely attributable to hopping or reordering of oxygen vacancies which in turn lead to fluctuations in local carrier density in the copper oxygen planes. Analysis shows a substantial density of oxygen defect states with activation energies below 0.5 eV, followed by a rapid increase at higher energies. In the mixed state, we have measured the current-induced, vortex-motion voltage noise. When the current–voltage characteristics are nonlinear there exist two distinct regimes. The first is a low magnetic field regime where measurements of the variations of voltage noise with magnetic field reveal reproducible, microstructure-dependent noise magneto-fingerprints. Rapid variations of the noise as a function of magnetic field and ac current drive experiments indicated that the noise is associated with metastability of pinned vortex configurations. Above magnetic fields which roughly match vortex separation with the grain size in the films, the magnitude and fine structure of the noise magneto-fingerprints are suppressed, suggesting a crossover to a more weakly pinned vortex state. The second regime is a low bias voltage regime where the observed noise scales linearly with bias voltage and has a 1/ f-like spectrum. The linear voltage scaling is reminiscent of vortex shot noise although no characteristic transit time could be determined. At higher voltage biases, the noise power drops off rapidly. We attribute the sharp drop off to a loss of pinning at higher temperatures and fields which results in the absence of metastability in the vortex pinned states. The temperature and magnetic field dependence of the noise are discussed in the context of various models including vortex shot noise, channel, critical current fluctuation, and vortex-glass models.

Spatially dependent noise model for vertical-cavity surface-emitting lasers

We present a comprehensive noise model for vertical-cavity surface-emitting lasers (VCSELs). The time-domain model accounts for the stochastic fluctuations in the local carrier density in the separate confinement heterostructure and quantum wells, and in the modal intensity and phase of both the internal and the out-coupled optical field. In this work, we consider these fluctuations to be caused by the temporal uncertainty of the processes that supply or consume carriers and photons, such as carrier diffusion and photons escaping the cavity, and the processes that create or annihilate carriers and photons, such as stimulated emission and absorption. The noise model is based on a deterministic quasithree-dimensional dynamic model that treats the carrier transport, heat generation and dissipation, and optical fields in the device. Langevin noise terms are derived and added to the rate equations in the numerical solution. The noise model is applied to simulate the noise characteristics of fundamental-mode stabilized VCSELs with a shallow, mode discriminating, surface relief. The relative intensity noise and frequency noise of the output are calculated. From the latter, the linewidth of the VCSEL can be estimated. The results are compared with those of conventional multimode VCSELs.

Investigations into the source of 1/f noise in HgxCd1−xTe diodes

This work is aimed at elucidating the mechanism of 1/f noise in HgxCd1−xTe diodes in order to reduce 1/f noise in high-performance thermal cameras. Currently, to reach the required signal-to-noise ratio, the operating temperature of thermal cameras is 77 K. If 1/f noise could be reduced, this operating temperature could be raised without reducing the signal-to-noise ratio, allowing cheaper, smaller, and lighter cooling systems to be used. There are still some fundamental aspects of 1/f noise that are not fully understood. One aspect in particular is whether the noise is caused by mobility or number density fluctuations because both could give the conductivity fluctuations observed. This paper explores the noise coupling and current coupling between diodes on an array. It is shown that 1/f noise is present on the photons emitted from a forward-biased diode, suggesting that the rate of radiative recombination fluctuates. The results suggest that these fluctuations are due to carrier number-density fluctuations. From correlation measurements, it is shown that at least 32% of the 1/f noise in the forward-biased diode is correlated with carrier density fluctuations in HgxCd1−xTe diodes.

Simulation of the frequency behavior of external-cavity semiconductor lasers

A theoretical model of semiconductor lasers mounted in external cavities is developed. The modes' frequencies and gains are calculated. We study the frequency of the oscillating mode versus the laser injection current. We explain its hysteresis behavior by introducing in the model the coupling between the semiconductor gain and its index of refraction induced by fluctuations in carrier density. We also determine the influence of the laser diode output facet reflection coefficient on mode hops and frequency tuning. Theoretical simulations coincide well with experimental observations.

Electronic noise due to multiple trap levels in homogeneous solids and in space-charge layers

A near-exact expression is derived for carrier-density fluctuations associated with multiple trap levels, interacting with one band only (say, the conduction band), using the many-variate Master Equation. The canonical constraint is handled with the Darwin–Fowler method. For a homogeneous solid with a discrete number of traps the spectrum consists of a sum of Lorentzians, as expected. For the case of a continuously distributed energy range, the results do not support a Bernamont–Surdin–McWhorter 1/f-like envelope spectrum; on the contrary, no more than two decades of 1/f noise can be expected. The situation is different, however, when band-bending occurs. Employing a Green’s function approach, developed by us previously, we give a formal complete result, which can be evaluated when Ec(r) is known.

Theory of carrier-density-fluctuation-induced transport noise in metal-oxide-semiconductor field-effect transistors

An advanced theoretical approach to understanding the carrier-density fluctuation noise in n-channel metal-oxide-semiconductor field-effect transistors in the nonequilibrium steady state is introduced. Starting with the current continuity condition, a partial differential equation for carrier-density fluctuation is, on the basis of the drift-diffusion current model, derived from the charge conservation equation. The simulated noise spectral density characteristics determined for two different diffusion constant values are compared. It is demonstrated that the transport noise is suppressed compared to the case in which only the drift current component is considered when deriving a theoretical formulation of transport noise. The suppression of carrier-density fluctuation power can be explained by the interference of the forward and backward propagating carrier-density fluctuation power, which is reasonable on the basis of quantum mechanical and statistical physics. Drain current noise power increases with drain voltage as expected. However, it is shown that the power is suppressed compared to the result of a past simulation that considered only the drift current component. It is also shown that the fluctuation power of drain current decreases with gate voltage; it is clearly demonstrated that this characteristic is strongly influenced by carrier mobility.

Hooge parameter in buried-channel metal-oxide-semiconductor field-effect transistors

This article describes the low-frequency noise characteristics of various buried-channel metal-oxide-semiconductor field-effect transistors. Experimental results show that their are peculiar behaviors in Hooge parameter. However, the original mobility-based model for the Hooge parameter is shown to provide partial understanding of the experimental results on the basis of a phenomenological consideration. It is also observed that, in contrast to previous theories, the interpretation of some aspects of the Hooge parameter strongly depends on how the two fluctuation modes, the carrier-density fluctuation and the mobility fluctuation, correlate. The proposed model gives a fundamental physical basis that allows aspects of the Hooge parameter to be interpreted. The gate voltage dependence of the Hooge parameter can be explained reasonably well by introducing the correlation of the mobility fluctuation and the carrier density fluctuation. Regarding the drain voltage dependence of the Hooge parameter, however, the present model for the Hooge parameter needs further modification.

Impact of transport noise enhancement in scaled-down MOSFET

This paper describes a theoretical approach to understanding the impact of carrier-density-fluctuation-induced high-frequency transport noise in scaled-down MOSFETs. A theoretical expression is formulated on the basis of the charge-density conservation equation and current continuity condition. The relation between drain current noise and carrier-density fluctuation is also discussed. As scaling level is increased, the high-frequency component of normalized carrier-density fluctuation power is enhanced because the averaging effect of fluctuation power is suppressed. It is shown that the high-frequency component of drain current noise in a 0.1-μm-channel device is more significant than that in a 1-μm-channel device.

Evolution of elementary excitations at a doped polar semiconductor surface in a depletion-layer formation process

We investigate the evolution of elementary excitations at a doped polar semiconductor surface in a depletion-layer formation process. The elementary excitations analyzed are two coupled plasmon-phonon modes and a surface optical-phonon mode involving screening charges. Surface excitations of free carriers are calculated by taking account of the Coulomb interaction between carriers, the dynamical exchange-correlation effect, and the coupling with surface polar phonons, on the basis of thermal-equilibrium states calculated self-consistently by the local-density approximation. We focus our attention on the coupling character and the spatial structure of each surface-excitation mode, as well as the energy dispersion and the energy-loss intensity involved in the excitation. The induced charge-density distribution in the excitation mode is composed of a carrier component due to carrier density fluctuation, a phonon component arising from longitudinal polarphonon polarization, and two on-surface components originating from the termination of the polar phonon and background polarization at the surface. The coupling character is elucidated by the phase relation and the amplitude ratio among these induced charge components. The spatial structure is visualized by the contour map of the induced charge-density distribution. We follow the variation in the coupling character and the spatial structure in the depletion-layer formation process. Our analysis gives a clear explanation of the variation in each of the three loss peaks in the electron energy-loss spectrum.

Carrier-Density Fluctuation Noise and the Interface Trap Density in GaN/AlGaN HFETs

ABSTRACTThe interface trap density NT is extracted from the experimental low-frequency noise data for GaN/AlGaN heterostructure field-effect transistors (HFETs). The trap density is determined based on the carrier-density fluctuation formalism. We show that the value of NT approximately defines the noise response of different GaN/AlGaN HFETs fabricated on the same wafer and it weakly depends on the gate bias. The dependence is due to the non-uniformity of the trap distribution. A model for computer simulation of the low-frequency noise in GaN devices is proposed.

Low-frequency noise in low temperature unhydrogenated polysilicon thin film transistors

Low-frequency noise measurements are performed in two types of low temperature polysilicon thin film transistors (TFTs). For the first TFT process, the polysilicon two layer structure induces large values of the channel access resistances, whose contribution to noise is dominant for large gate bias. For the second TFT process, the polysilicon single layer structure induces small access resistances and the measured noise is mainly due to channel sources. For small voltages, the channel noise spectral density evolution with gate bias agrees with the mobility fluctuation model and is identical for both processes. For large voltages (>2 V), the channel noise spectral density evolution, observed only in the case of the single layer structure, seems to agree with the fluctuations of carrier density. However, this interpretation is discussed. The results of static characterization show that the quality of the channel active layer is quite different from the two layer structure to the single layer structure. In agreement with these observations, the observed evolution of the relative noise with increasing gate bias in TFTs can be interpreted from intergrain potential lowering.

Monte Carlo particle-based simulations of deep-submicron n-MOSFETs with real-space treatment of electron–electron and electron–impurity interactions

In modern deep-submicron devices, for achieving optimum device performance, the doping densities must be quite high. This necessitates a careful treatment of the short- and long-range electron–electron and electron–impurity interactions. We have shown before that by using a corrected Coulomb force, in conjunction with a proper cutoff range, one can properly account for the short-range portion of the force. Our approach naturally incorporates multi-ion contributions, local distortions in the scattering potential due to the movement of the free charges, and carrier-density fluctuations. The doping dependence of the low-field electron mobility obtained from 3D resistor simulations closely followed the experimental results, thus proving the correctness of our approach. Here, we discuss how discrete impurity effects affect the threshold voltage of ultra-small n-channel MOSFETs with gate lengths ranging from 50 to 100 nm. We find that the fluctuations in the threshold voltage increase with increasing the oxide thickness and substrate doping. The averaging effect over the width of the device leads to significantly smaller fluctuations in the threshold voltage for devices with larger gate width. The observed trends are in agreement with the experimental findings.

3D Simulations of Ultra-small MOSFETs with Real-space Treatment of the Electron – Electron and Electron-ion Interactions

We present a 3D Ensemble Monte Carlo particle-based simulator with a novel realspace treatment of the short-range electron – electron and electron-ion interactions. By using a corrected Coulomb force in conjunction with a proper cutoff range, the shortrange portion of the force is properly accounted for, and the ‘double counting’ of the long-range interaction is eliminated. The proposed method naturally incorporates the multi-ion contributions, local distortions in the scattering potential due to the movement of the free charges, and carrier-density fluctuations. The doping dependence of the low-field mobility obtained from 3D resistor simulations closely follows experimental results, thus supporting the appropriateness of the proposed scheme. Simulations of ultra-small MOSFETs demonstrate that the short-range electron – electron and electronion interactions are responsible for the fast thermalization of the carriers at the drain end of the device, which occurs over distances that are on the order of few nanometers. The omission of the short-range portions of these two interaction terms leads to significant overestimation of the distance over which carriers thermalize.

Cross modulation of two lasers in a semiconductor

A high power amplitude modulated laser of carrier frequency ω1 and modulation frequency Ω propagating through a semiconductor exerts a ponderomotive force on electrons at Ω. When Ω∼ωp/εL (ωp being the plasma frequency and εL the lattice permittivity), the ponderomotive force produces a large amplitude plasma wave. The free carrier density fluctuations associated with the plasma wave are also large, particularly when the laser has forward and backward propagating components. Another laser of frequency ω2 propagating through this region couples with the free carrier density oscillations to produce sidebands at ω2±Ω. Thus the information contained in the first laser is transferred to the second laser. The scheme is effective when Ω is larger than the collision frequency.

A novel approach for introducing the electron-electron and electron-impurity interactions in particle-based simulations

A new method for employed electron-electron (e-e) and electron-ion (e-i) interactions in Monte Carlo particle based simulators is presented. By using a corrected Coulomb force in conjunction with a proper cutoff range, the "double" counting of the long range interaction is eliminated while reducing the simulation time for molecular dynamics by a factor of 1000. The proposed method naturally incorporated the multi-ion contributions, local distortions in the scattering potential due to the movement of the free charges, and carrier-density fluctuations. The doping dependence of the low-field mobility obtained from three-dimensional (3-D) resistor simulation closely follows experimental results, thus proving the correctness of the proposed approach.

Low frequency noise in intrinsic low pressure chemical vapour deposited polysilicon resistors

Low-frequency noise measurements were performed on intrinsic low pressure chemical vapor deposited polycrystalline silicon resistors. The current noise exhibits a transition from 1/f to 1/f 0.6 behavior with the resistors biased in the linear region. The origin of the noise is related to carrier density fluctuation between conduction band and gap states consisting of deep states lying close to midgap with uniform energy distribution and exponential band tails. From analysis of the experimental data, the quality of the material is characterized with respect to the deposition pressure. When the resistors are biased in the non-linear regime, an additional noise is observed which is attributed to the temperature rise due to Joule-induced heating within the samples.

Excess noise caused by transverse inhomogeneity of conductive channels

A type of excess noise which emerges in transversely inhomogeneous conductive channels is described. Its origin is traced back to the diffusion of the electric-field and carrier-density fluctuations across the channel. By making use of a WKB analysis, explicit analytical formulas for the current-noise spectrum are obtained. It is argued, that this type of noise should be manifested in a variety of currently used semiconductor devices.

Resistance fluctuations inGaAs/AlxGa1−xAsquantum point contact and Hall bar structures

We study low-frequency noise in selectively doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum point contact and Hall bar structures. Experiments are carried out at 4.2 K and constant magnetic fields where noise in both diagonal and Hall voltages is measured using a four-terminal lock-in technique. In all cases, for low currents and low frequencies, we find the size of excess voltage noise power to be quadratic in current and, furthermore, Hall voltage noise power to be quadratic in magnetic field. Resistance fluctuations of the quantum point contact samples are mostly dominated by a single two-level or multilevel switching event that leads to a Lorentzian noise spectrum, whereas resistance fluctuations of the Hall bar structures exhibit $1/f$ noise, which results from the superposition of many independent switching events. Three possible sources of $1/f$ noise, i.e., fluctuations in carrier density, mobility, and quantum interference corrections, are considered in the analysis of the Hall bar data. From the size of $1/f$ noise in the Hall voltage, we deduce an upper bound for the size of carrier density fluctuations that is so small that we rule out electron trapping as a main source of resistance fluctuations in the quantum point contact structures. Instead, we explain observed low-frequency noise in both mesoscopic and macroscopic structures by fluctuations in the remote impurity configuration.

Subpicosecond carrier dynamics in GaAs studied with optical heterodyne detection

The transient reflectivity change in GaAs has been studied using an optical heterodyne detection technique with femtosecond laser pulses (FWHM = 65 fs) at 800 nm. An oscillation due to carrier density and/or spin density fluctuations has been observed on the transient reflectivity change and its decay time was estimated to be 250 ± 40 fs.

Structure of coupled carrier plasmon-polar phonon modes at semiconductor surfaces

By combining the dielectric-response theory of a semi-infinite electron gas with the Lorentzian oscillator model, we investigate the structure of coupled carrier plasmon—polar phonon modes at semiconductor surfaces. The coupling character is clarified by decomposing the induced charge-density distribution into a carrier component due to carrier-density fluctuation, a phonon component originating from longitudinal polar—phonon polarization, and an on-surface component resulting from termination of the phonon and background polarization at the surface. The spatial structure of each mode is visualized in contour maps of the induced charge-density distribution. We follow the variation of the coupling character and the spatial structure of each coupled surface mode with change of carrier concentration through a strong-coupling range.