Combined breakdown in bismuth—Antimony semiconductor alloys

S-shaped current-voltage characteristics for Bi_1?xSb_x alloys are studied theoretically and experimentally. The phenomenon is shown to occur due to the combined interband breakdown, the impact ionization being caused both by the external electric field and the Hall field. The latter is governed by the proper magnetic field of the plasma current. The negative differential resistance (NDR) occurs for the range of currents where the impact ionization is growing intensively but the pinch effect is not yet developed to full extent. The phenomenon is enhanced if the impact ionization rate in the Hall field is greater than in the applied one.     

Instabilities of electron-hole plasma under impact ionization and microwave emission

The nonlinear stage of the evolution of electron—hole plasma instabilities in semiconductors under impact ionization (static differential conductivity σ_d > 0) is considered for the case of spatially uniform pertubations of density and electric field. If the differential mobility of carriers μ _d > 0, the instability arises only with allowance for the retardation of the process of impact ionization (linear theory of this effect was developed by M. Toda [4]). When μ _d < 0, the instability may appear in the absence of the retardation. Both these instabilities exhibit oscillating form and are due to h.f. negative dynamic differential conductivity. We determine time sweeps of the electric field under condition of fixed current for the case n-InSb at T = 77 K. The amplitude and the frequency (f>10¹⁰Hz) of the oscillations are evaluated and the conditions, when the shape of E(t)-oscillations is essentially non-harmonic, are determined. Microwave emission observed in semiconductors under impact ionization may have resulted from the instabilities at hand.     

Electron lucky-drift impact ionization coefficients of ZnS : Mn

Fit of the experimental data of ZnS : Mn by a modified lucky-drift formula has been performed using the least square algorithm. The fit agrees well with the experimental data only at high field. The best fitting parameters at high field are the mean free path of order 102.74 Å and Keldysh factor,p ₀ = 0.0138. A generalized Keldysh formula has been used, due to introduction of a soft threshold factor. The soft lucky-drift theory can also be used to calculate the impact ionization coefficients of high electron energy of ZnS : Mn without losing its physical significance compared to full band-structure Monte Carlo calculation with a remarkably reduced amount of computer resources. The curvature on semi-log plot of experimental impact ionization coefficient against the inverse of electric field is different from what is observed for other materials at low electric fields due to impact ionization of deep level impurities.     

Coercivity and induced magnetic anisotropy by stress and/or field annealing in Fe- and Co- based (Finemet-type) amorphous alloys

Uniaxial magnetic anisotropy has been induced in amorphous Fe_73.5Cu₁Nb₃Si_15.5B₇ (Fe-rich) and (Co₇₇Si_13.5B_9.5)₉₀Fe₇Nb₃ (Co-rich) ferromagnetic alloys by annealing under stress and/or magnetic field. Such anisotropy plays a crucial role on the magnetization process and, consequently, determine the future applications of these materials. The mechanisms involved on the origin of such induced magnetic anisotropy showed significant differences between Fe-rich and Co-rich amorphous alloys. This work provides a comparative study of the coercive field and induced magnetic anisotropy in Fe-rich and Co-rich (Finemet) amorphous alloys treated by stress and/or field.     

Calculation of I–V curves in narrow-gap semiconductors with symmetric electron and hole energy dispersion laws

The hot electron phenomena are considered in the narrow-gap semiconductors with Lax band structure. Methods for calculation of I–V curves are developed in the case when only the quasielastic scattering processes occur. The following factors are taken into consideration: Bloch structure of wave functions; electron-electron interaction; change in concentration of carriers due to the warming of electrons in the electric field; anisotropy and many-valley character of band spectrum. Results obtained here may be useful for the interpretation of experimental data for alloys Bi_1-xSb_x, lead and tin chalcogenides.     

Inducing anisotropy in bulk Nd-Fe-Co-Al-B nanocrystalline alloys by quenching in magnetic field

We have observed magnetic anisotropy in bulk Nd_55−xCo_xFe₃₀Al₁₀B₅ (x=10, 15 and 20) alloys prepared by copper mold suction casting method with a presence of external magnetic field (quenching field) μ₀H=0.25 T. By changing direction of the measuring field from perpendicular to parallel one in comparison with that of the quenching field, coercive force of the alloys slightly decreases while remanent magnetization and squareness of hysteresis loop increase more clearly. It is also found that the higher Co-concentration in the alloys the larger magnetic anisotropy is induced. The structure analyses manifest nanocrystalline particles embedded in residual amorphous matrix of the alloys. The size of the particles is in range of 10-30 nm and their crystalline phases consist of Nd₂(Fe,Co)₁₄B, Nd₃Co, Nd₃Al, NdAl₂ and Nd.     

Magnetic resonance in amorphous FexNi80-xP14B6: II. Spin glass alloys

We report EPR measurements on amorphous Fe_xNi_80-xP₁₄B₆ alloys for concentrations less than that required for ferromagnetism. Measurements were made at microwave frequencies between 10 and 35 GHz and at temperatures between 2 and 150 K. We find i) at high temperatures the alloys are simple paramagnets with ? ≈ 2.14 and a relaxation rate (linewidth) increasing linearly with temperature, ii) the linewidth increases at low T and follows an empirical form proposed earlier, iii) as the temperature is decreased, the susceptibility (measured by reference to the line intensities) increases, iv) in order to account for the frequency dependence of the resonance field we must introduce an anisotropy energy with uniaxial symmetry; the “hard axis” being normal to the sample plane. The associated anisotropy constant K' appears at several times the spin glass transition temperature T_SG, v) at still lower temperatures ( < 2T_SG) another type of anisotropy field appears. The corresponding anisotropy energy is similar to that introduced to explain data on the reentrant alloys of this system and the archetypal spin glasses CuMn and AgMn and the anisotropy constant K has the same type of temperature dependence. However, the frequency dependence of K is different.     

Field-induced uniaxial anisotropy in partially ordered Ni3Mn at low temperatures

A uniaxial anisotropy is induced in partially ordered MnNi alloys in the composition range near Ni₃Mn, by applying a magnetic field during cooling from room temperature to 77 K. The uniaxial anisotropy constant K_u depends on both the magnitude of the torque measuring field and that of the cooling field. The easy axis direction tends to be between the 〈100〉 and the direction of the cooling field. The value of K_u shows a maximum when the cooling field is applied along 〈100〉, amounting to 2 × 10⁴ crg/cm³. On the other hand, the crystalline anisotropy is not affected by field cooling. When the temperature is raised from 77 K to 300 K, neither K_u nor the rotational hysterisis loss is observed to have any critical temperature, for a torque measuring field of 20 kOe. As atomic ordering proceeds, these effects become smaller and appear to vanish in the perfectly ordered state.     

Effect of ionization of impurity centres by electric field on the conductivity of superlattice

The electric fieldE₀effect on ionization on impurity centres and on the conductivity of superlattices (SLs) has been studied theoretically. It is observed that as the fieldE₀increases the current rises, reaches a maximum, then falls off, i.e. show a negative differential conductivity (NDC). Further increase inE₀leads to an exponential rise of the current. This occurs aroundE₀=3×10⁴ V cm⁻¹. Hence the current density field shows a ‘N’ shape characteristics as against the ‘n’ shape characteristics in the absence of impurity.     

Nonmonotonic magnetic-field dependence of transport coefficients for n-Bi-Sb semiconducting alloys

The transport coefficients of tellurium-doped n-Bi_{1 ? x} Sb _x semiconducting alloys (0.07 ≤ x ≤ 0.15) are studied for single-crystal samples in the temperature range 1.5 ≤ T ≤ 40 K and in magnetic fields 0 ≤ H < 20 kOe. The theory developed in this study attributes the specific features in the behavior of the transport coefficients observed in a magnetic field to a strong anisotropy of the electron spectrum and anisotropy in the electron relaxation time. It is found that the dependences of the transport coefficients on the magnetic field for H ∥ C ₃ can be theoretically expressed through one anisotropy parameter δ, and those for H ∥ C ₂, by means of several anisotropy parameters, namely, γ, η, ζ, and m ₃/m ₁. It is established that the anisotropy parameter δ in the n-Bi-Sb semiconducting alloys can be estimated from measurements of the electrical resistivity ρ₂₂(∞)/ρ₂₂(0) ? δ and the Hall coefficient R _12.3(∞)/R _12.3(H → 0) ? δ in a magnetic field H ∥ C ₃. It is shown that the observed increase in the thermoelectric efficiency by a factor of 1.5–2.0 in the transverse magnetic fields H ∥ C ₃ and H ∥ C ₂ originates from the nonmonotonic dependence of the diffusion component of the thermopower Δα₂₂(H)(?T ∥ C ₁) on the magnetic field. The nonmonotonic dependence of the diffusion thermopower in n-Bi-Sb semiconducting alloys is associated with the strong anisotropy of the electron spectrum, the anisotropy in the electron relaxation time, and the many-valley pattern of the spectrum.     

Influence of electric current and magnetic field on terahertz photoconductivity in Pb1 − x Sn x Te(In)

Photoconductivity of Pb_{1 ? x} Sn _x Te(In) solid solutions in the terahertz spectral range is defined by a new type of local electron states linked to the quasi-Fermi level. The paper deals with investigation of the influence of electric current and magnetic field on the amplitude of the terahertz photoconductivity in Pb_{1 ? x} Sn _x Te(In) alloys of different composition. It is shown that the density of local electron states responsible for the positive persistent photoconductivity decreases with increasing electric current via a sample, as well as with transition to the hole conductivity in samples with a high content of tin telluride (x > 0.26). It is found that the magnetic field dependence of the positive photoconductivity is non-monotonous and has a maximum. The maximum position in magnetic field is proportional to the terahertz radiation quantum energy. Mechanisms responsible for the effects observed are discussed.     

Field injection of low-energy electrons into the ZnSe/CdSe/ZnSe heterostructure with the use of an ultra-high-vacuum tunneling microscope

The field emission injection of low-energy electrons (E _e ?? 10 eV) into the ZnSe/CdSe/ZnSe heterostructure has been considered. The probe of the ultra-high-vacuum tunneling microscope has been used as a field emitter. It has been shown that the energy of injected electrons is sufficient for impact ionization in ZnSe. The impact ionization creates a high concentration of nonequilibrium carriers in the near-surface ZnSe layer. The transport of nonequilibrium carriers in the heterostructure under study has been simulated. The electric field of the near-surface space charge and surface recombination have been taken into account. The calculation has demonstrated that filling the active region of CdSe with nonequilibrium carriers is highly efficient.     

Impact ionization in silicon dioxide at fields in the breakdown range

A new method is described for the evaluation of impact ionization coefficients, α, in insulators by the measurement of current transients which grow due to impact ionization. Impact ionization coefficients for SiO₂ evaluated by this method obey roughly the relation α = α₀ exp (?H/F) where F is the field. For p type substrates α₀ = 6.5 × 10¹¹cm^?1 and H = 1.8 × 10⁸V/cm.     

On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

Quantum well engineering of InAlAs/InGaAs HEMTs for low impact ionization applications

The performance of InAlAs/InGaAs quantum well field effect transistors are subject to high impact ionization and band-to-band tunneling (BTBT) due to its narrow bandgap feature. In this work, the energy gap is engineered using strain and quantization techniques to increase the effective energy gap leading to low impact ionization and BTBT leakage current. It is shown that the impact ionization is reduced in 5 nm channel device as compared to 13 nm device with onset at approximately E_geff/q. Also the band-to-band-tunneling current is reduced due to the increase in effective energy gap. We have also investigated the effects of quantum well engineering on the dc performance of InGaAs HEMTs.     

A negative differential conductivity due to recombination and impact ionization in semiconductors at low temperatures

The probability of impact ionization and the recombination time are known to increase monotonically with the electric fieldE. I show that at low temperatures both functions achieve a maximum and decrease in the electric field range where the emission of optical phonons with subsequent impurity scattering dominate. This nonmonotonicity results in three different types of N-shaped negative differential conductivity (n-ndc). The carrier concentration and the current decrease whenE increases due to decreasing of the impact ionization probability for weakly compensated samples and of the recombination time for highly compensated samples. At the antithreshold electric-field impact ionization dies out, which results in a dramatic decrease of the current for intermediately compensated samples. This huge n-ndc could be used in a novel type of the Gunn diode. The essential increase of threshold electric field of impact ionization is also predicted, and the effect could enhance the efficiency of photodetectors.The research was supported by the Alexandervon-Humboldt-Foundation     

Formation of the space charge region in diffusion p-n junctions under high-density current interruption

The recovery of diodes with diffusion p-n junctions in the case of high reverse current density j is analyzed. A condition for quasi-neutrality breaking in the diffusion layers with allowance for the dependence of charge carrier mobility μ on electric field strength E is obtained that is valid for a wide range of j. The problem of formation of the space charge region in a circuit with inductance L and resistance R is reduced to a system of two ordinary differential equations. Approximation of a numerical solution to this system makes it possible to derive crude analytical relationships between interrupted current density {ie88-1}, circuit parameters, diode parameters, and parameters of a forming voltage pulse (with amplitude V _m and pulse rise time t _p). The limiting parameters of a pulser with an inductive energy storage and current interrupter based on diffusion diodes are studied. The critical density of interrupted current {ie88-2} is determined at which the field in the space charge region near the anode reaches breakdown value E _b and intense impact ionization by holes begins. The impact ionization decreases the rates of current decay and voltage increase in the space charge region. As a result, at {ie88-3}, t _p starts increasing and the overvoltage factor of the pulser decreases. The value of V _m corresponding to {ie88-4} is roughly given by {ie88-5}, where m is the number of diodes in the interrupter, ? is the permittivity of the semiconductor, {ie88-6} is the saturated drift velocity of holes, and l _p is the depth of the p-n junction (diffusion depth). Theoretical predictions are confirmed by exact numerical simulation of the recovery process and qualitatively agree with the available experimental data.     

Untersuchung der Ionenenergieverteilung in einer starken stoßfreien Kompressionswelle

In a strong collisionfree implosion wave generated by a fast theta pinch discharge the density profile and the distribution of ion energies are investigated by spectroscopic methods. At low initial pressures (20 mTorrD ₂) an overturning of the ion sheath leads to an extended density profile and an effective thermalization of the ions in the plane perpendicular to the magnetic field with energies up to 500 eV, which are comparable with the energies of the directed ion motion. However, the thermal ion energies parallel to the direction of the magnetic field are lower by more than a factor of 2. This anisotropy may not be destroyed by ion two-stream instabilities, but possibly by anisotropy instabilities.     

Effect of electric current on the magnetic and magnetoelastic properties of amorphous ferromagnetic alloys

The magnetic and magnetoelastic parameters of Fe₆₄Co₂₁B₁₅ and Fe_81.5B_13.5Si₃C₂ amorphous ferromagnetic alloys treated by direct electric current in air are studied as functions of the applied magnetic field and current density. The samples of the alloy have the form of narrow strips with different lengths. It is found that the magnetoelastic parameters of the dc-treated alloys depend on the magnetic field in a qualitatively different way. From the behavior of the magnetic and magnetoelastic parameters of the alloys in the magnetic field a model of magnetization nonuniform distribution in amorphous ferromagnetic alloys subjected to dc treatment is proposed.     

高压双扩散漏端MOS晶体管双峰衬底电流的形成机理及其影响

利用0.15μm标准CMOS工艺制造出了工作电压为30V的双扩散漏端MOS晶体管(double diffused drain MOS, DDDMOS).观察到DDDMOS的衬底电流-栅压曲线(I_b-V_g曲线)有两个峰.通过实验和TCAD模拟揭示了DDDMOS衬底电流的形成机理,发现衬底电流第一个峰的成因与传统MOS器件相同;第二个峰来自于发生在漂移区远离沟道一侧高场区的碰撞离化电流.通过求解泊松方程和电流连续性方程,分析了器件的物理和几何参数对导致衬底电流重新上升的漂移区电场的影响.在分析了DDDMOS衬底电流的第二个峰形成机理的基础上,考察了其对器件的可靠性的影响. 关键词： 高压器件 衬底电流 可靠性