Region Of Negative Differential Mobility Research Articles

Power-frequency performance of step-graded double Gunn diodes

Simulations of conventional Gunn diodes containing the double Gunn effect have shown a significant increase in oscillation frequency and frequency range. However, conventional Gunn diodes suffer from higher phase noise, lower efficiency, and reduced temperature stability due to the lack of a hot electron injector. A study on the performance of a Double Gunn diode with a graded gap injector is presented. The second region of negative differential mobility is gradually introduced in several temperature-dependent mobility models, which are implemented in an energy balance simulation. Devices with transit lengths of 0.7μm, 1.1μm, and 1.65μm are simulated. All devices experience an increase in oscillation frequency with increasing intensities of the double Gunn effect. The frequency range doubles at significant intensities of the effect in the devices with transit lengths of 0.7μm and 1.65μm.

The wideband tunability of double Gunn diodes

Gunn diodes use the Ridley–Watkins–Hilsum effect to generate low noise millimeter wave signals for communication and sensing applications. However, Monte Carlo calculations indicate that the velocity–field characteristics of compensated GaAs contain a second peak due to phonon scattering. A novel device that leverages this double Ridley–Watkins–Hilsum effect to achieve ultra-wideband tunability of low-noise millimeter waves is proposed in this article. The second region of negative differential mobility is gradually introduced in seven mobility profiles, which are each simulated in eight device lengths. It is found that a 2.5 μm Gunn diode with a compensation ratio of 0.6 operating at 160 K has a 700% increase in frequency tunability. An analysis of the domain formation indicates that the increase in tunability is due to significant distortions in domain shape. Gunn diodes with transit lengths of 2 μm and smaller show a 100% increase in tunability due to modulations in the effective transit length and absorption rate. These simulations indicate that compensated GaAs could provide linear, ultra-wideband frequency tunability.

Steady-state and transient electronic transport properties of β-(Al x Ga1–x )2O3/Ga2O3 heterostructures: An ensemble Monte Carlo simulation

The steady-state and transient electron transport properties of β-(Al x Ga1−x )2O3/Ga2O3 heterostructures were investigated by Monte Carlo simulation with the classic three-valley model. In particular, the electronic band structures were acquired by first-principles calculations, which could provide precise parameters for calculating the transport properties of the two-dimensional electron gas (2DEG), and the quantization effect was considered in the Γ valley with the five lowest subbands. Wave functions and energy eigenvalues were obtained by iteration of the Schrödinger–Poisson equations to calculate the 2DEG scattering rates with five main scattering mechanisms considered. The simulated low-field electron mobilities agree well with the experimental results, thus confirming the effectiveness of our models. The results show that the room temperature electron mobility of the β-(Al0.188Ga0.812)2O3/Ga2O3 heterostructure at 10 kV⋅cm−1 is approximately 153.669 cm2⋅V−1⋅s−1, and polar optical phonon scattering would have a significant impact on the mobility properties at this time. The region of negative differential mobility, overshoot of the transient electron velocity and negative diffusion coefficients are also observed when the electric field increases to the corresponding threshold value or even exceeds it. This work offers significant parameters for the β-(Al x Ga1−x )2O3/Ga2O3 heterostructure that may benefit the design of high-performance β-(Al x Ga1−x )2O3/Ga2O3 heterostructure-based devices.

Empirical model for the velocity-field characteristics of semiconductors exhibiting negative differential mobility

An empirical model for the velocity-field characteristic of a semiconductor exhibiting a peak and a region of negative differential mobility is proposed. The adjustable parameters in this model are shown to be directly determinable from the form of the velocity-field characteristic itself, rather than being resolvable through the use of a trial-and-error approach. The model is then applied to a classical compound semiconductor of interest, namely gallium arsenide. Through a direct comparison with results acquired through the use of Monte Carlo electron transport simulations, the strengths and weaknesses of this model are critically assessed.

Research on Time Jitter of GaAs Photoconductive Semiconductor Switches in the Negative Differential Mobility Region

Time jitter of GaAs photoconductive semiconductor switches (PCSS) is investigated at an optical excitation of 1053-nm wavelength and 500-ps pulse duration. The experimental results indicate that the time jitter of the GaAs PCSS exhibits a nonmonotonic variation in the negative differential mobility (NDM) region. All of these time jitters are lower than 4% of the rise time of the switching waveform. The optimum time jitter of ~15 ps is achieved at the onset of the NDM region. The theoretical relationship between the optical excitation parameters, the bias electric field, and the time jitter of the GaAs PCSS is built up. The nonmonotonic behavior of the time jitter with electric field is attributed to the instability of the relative fluctuation of drift velocity caused by inter-valley transition of carriers in GaAs.

Scalability of the drain-current drive of AlGaN/GaN HFETs with gate-length

The scaling-trend of the current-drive of AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) with gate-length is studied with the application of a realistic steady-state drift transport characteristics and an approximate purely-saturating drift transport characteristics. Findings show that due to an overwhelming presence of a region of negative differential mobility in the transport characteristics of GaN, a scaling-trend different from the one observed in mainstream silicon MOSFETs should be expected for AlGaN/GaN HFETs. The role of improvement in Ohmic contact technology on this scaling trend is also investigated.

Experimental investigation of limit space charge accumulation mode operation in a semi-insulating GaAs photoconductive semiconductor switch

Experiments with the limited space-charge accumulation (LSA) mode of oscillation in a large gap semi-insulating (SI) GaAs photoconductive semiconductor switch (PCSS) are discussed. It has been observed that growth and drift of a photo-activated charge domain (PACD) are quenched only when the bias voltage is more than twice the threshold voltage. The original negative resistance characteristics are directly utilized in the LSA mode; during LSA operation the spatial average of the electric field varies over a large portion of the negative differential mobility region of the velocity—electric field characteristic. The work efficiency of an SI GaAs PCSS is remarkably enhanced by electric field excursions into the positive resistance region when the total electric field is only below the threshold part of the time. The LSA mode can only operate in the certain conditions that satisfy the quenching of the accumulation layer and the smaller initial domain voltage.

Steady-state and transient electron transport within wurtzite and zinc-blende indium nitride

We study the steady-state and transient electron transport that occurs within wurtzite and zinc-blende indium nitride using a three-valley Monte Carlo simulation approach. For our steady-state results, we find that, for both cases, initially the electron drift velocity monotonically increases with the applied electric field strength, reaching a peak value followed by a region of negative differential mobility, and then a region of saturation. The peak fields are found to be around 30 kV/cm for the case of wurtzite indium nitride and about 50 kV/cm for the case of zinc-blende indium nitride, the corresponding peak and saturation electron drift velocities being around 5.6×107 and 1.2×107 cm/s for the case of wurtzite indium nitride and about 3.3×107 and 1.0×107 cm/s for the case of zinc-blende indium nitride. For the purposes of our transient electron transport analysis, we follow the approach of O'Leary et al. [Appl. Phys. Lett. 87, 222103 (2005)], and examine how an ensemble of electrons responds to the sudden application of a constant electric field. We find that the electrons within wurtzite indium nitride exhibit higher electron drift velocities and longer relaxation times than those within zinc-blende indium nitride. The device implications of these results are then discussed.

Dynamics of field-driven domain-wall propagation in ferromagnetic nanowires

Ferromagnetic nanowires are likely to play an important role in future spintronic devices. Magnetic domain walls, which separate regions of opposing magnetization in a nanowire, can be manipulated and used to encode information for storage or to perform logic operations. Owing to their reduced size and dimensionality, the characterization of domain-wall motion is an important problem. To compete with other technologies, high-speed operation, and hence fast wall propagation, is essential. However, the domain-wall dynamics in nanowires has only been investigated in the last five years and some results indicate a drastic slowing down of wall motion in higher magnetic fields. Here we show that the velocity-field characteristic of a domain wall in a nanowire shows two linear regimes, with the wall mobility at high fields reduced tenfold from that at low fields. The transition is marked by a region of negative differential mobility and highly irregular wall motion. These results are in accord with theoretical predictions that, above a threshold field, uniform wall movement gives way to turbulent wall motion, leading to a substantial drop in wall mobility. Our results help resolve contradictory reports of wall propagation velocities in laterally confined geometries, and underscore the importance of understanding and enhancing the breakdown field for practical applications.

Inhomogeneous electric fields in oscillatory photoconductivity

Monochromatic photoexcitation of a semiconductor can produce a photoconductive response which is a periodic function of the optical excitation energy. This 'oscillatory photoconductivity' effect is caused by the interaction of the photoexcited carriers with optical phonons. Under conditions in which this effect exists, it is theoretically possible for a spatially uniform negative drift velocity state to occur for certain ranges of excitation energy. This type of state has the interesting property that a fraction of the optical excitation energy is continuously transferred directly to the electric field. For this type of excitation energy, the drift velocity as a function of electric field exhibits, in addition to the region of negative velocity, a region of negative differential mobility which is intrinsically unstable against space-charge formation. The results of a simple numerical model are presented here in order to classify the conditions under which a stable negative resistance or periodic instabilities can occur in the bulk of such a sample connected in a resistive circuit. The nature of these propagating instabilities is described.

Influence of one-roton emission processes on the velocity-field characteristics of negative ions in liquidHe4at very low temperatures

The effect of the simultaneous presence of one- and two-roton emission processes on the motion of negative ions in liquid $^{4}\mathrm{He}$ at very low temperatures is investigated theoretically. For a sufficiently small ratio of the emission rates, a dramatic structure in the velocity-field characteristics is predicted with the possibility of a region of negative differential mobility. It is pointed out that experimental determination of the emission-rate ratio would provide a direct measure of the relative linear and nonlinear responses of liquid $^{4}\mathrm{He}$ in a specific region of the excitation spectrum. The feasibility of experimental detection of the elusive one-roton emission processes is discussed.

The influence of boundary conditions and the bias on amplification from negative differential mobility elements

We demonstrate from a study of the small signal impedance properties of supercritical negative differential mobility (NDM) element that the various modes of device amplification are determined primarily by cathode conditions and the bias. These conclusions are drawn from 1) a detailed analytical investigation where cathode conditions are represented by prespecified values of electric field, and 2) by synthesizing previously published studies. Briefly, it is shown that multiple (bias induced stable-unstable-stable) amplifying states will arise from NDM elements with cathode fields E c within the NDM region. (Here, the different states occur when the device dc current density is, respectively, less than, approximately equal to, or greater than a critical value of current J c . J c is determined by E c .) Single amplifying states will arise from elements with values of E c that are either less than the NDM threshold field value, or within the saturated drift velocity region. (In the latter case, time-dependent values for E c are required.) The study proposes that general features of the space-charge profiles and the cathode conditions may be ascertained from measurements of 1) the small signal device impedance as a function of bias level, and 2) the dc current versus voltage relation.

On the form and stability of electric-field profiles within a negative differential mobility semiconductor

Using a fixed cathode boundary field model we examine analytically the form and stability of inhomogeneous prethreshold electric-field profiles in long bulk negative differential mobility (NDM) semiconductors. We show that the electric-field profiles and their associated current-voltage relationships depend in a detailed way on the characteristics of the bulk material (e.g., the velocity-electric-field relation) and on the boundary conditions imposed on the bulk. We investigate situations leading to the nucleation of a high-field domain at the cathode and find that for its occurrence: 1) the electric field at the cathode boundary must lie within the NDM region; and 2) space-charge neutrality must be approximated within the vicinity of the cathode. When these conditions are satisfied the current instability at threshold is accompanied by bulk electric-field values that are below the threshold field for NDM. The stability conclusions are drawn from a small-signal analysis that avoids many of the approximations of earlier studies. Supplementary numerical computations are included to demonstrate that the subsequent time development of the current instability depends in a detailed way on: 1) conditions at the cathode boundary; and 2) the external circuit. It is shown that the recycling Gunn effect cathode-to-anode transit-time mode in a resistive circuit is not a necessary consequence of the bulk having a region of NDM.

Wall contraction in Bloch wall films

The phenomenon of wall contraction characterized by a peak in the velocity-field relationship and a region of negative differential mobility is observed in uniaxial magnetic thin films of various magnetic properties by careful interrupted-pulse experiments. The observed results agree quite well with the theory for bulk samples when the extensive flux closure of thin film walls is accounted for by a suitable empirical scaling factor.

Note Concerning the Velocity of Gunn Domains

Using computer solutions of the relevant differential equations, many authors have determined the velocity for which traveling domains exist in a semiconductor with a region of negative differential mobility. Recently, Gunn presented a topological argument leading to a closed form equation for the velocity of a finite amplitude domain. For the case of a domain shape controlled by diffusion we show that Gunn's argument is based on a necessary, but not sufficient, condition that a certain singular point be a center. This leads to an incorrect expression for the domain velocity for all but infinitesimal amplitude domains. The topological character of each solution as the velocity parameter is varied is also described.

A NEW MEASUREMENT OF THE VELOCITY-FIELD CHARACTERISTIC OF GaAs

The velocity-field characteristic for electrons in GaAs has been measured in the negative differential mobility region between 4.5 and 11 kV/cm by a new direct method involving probe measurements on the surface of a bulk GaAs two-port amplifier. The measurements were taken on GaAs of resistivity in the 500–1000 Ω-cm range.

Determination of the Hall-mobility of hot electrons in gallium-arsenide using 8 mm-microwaves

The Hall mobility of n-type gallium arsenide has been determined up into the region of negative differential mobility using a microwave heating technique. At higher fields the transfer of electrons from the lower conduction band towards a higher band of lower mobility is apparent from our measurements.

Effect of nonparabolicity on drift velocity in GaAs

Incorporation of nonparabolicity of the (100) valley into our calculations of drift velocity versus field in GaAs has raised the beginning of the negative differential mobility region from 2.1 to 2.9 kV/cm and the valley field from 10 to 12 kV/cm for the same model and parameters as used earlier.

DIRECT OBSERVATION OF THE DRIFT VELOCITY AS A FUNCTION OF THE ELECTRIC FIELD IN GALLIUM ARSENIDE

The drift velocity as a function of the electric field in gallium arsenide was measured directly by studying the response of a reverse-biased Schottky-barrier i-n+ GaAs photodetector to a step input of light. The photoresponse was dominated by trapping of electrons in the i region. Analysis based on a simple model yielded the drift velocity—electric field characteristic in GaAs up to fields of 9000 V/cm. A region of negative differential mobility was observed to exist between a peak field of 2200 V/cm and an apparent valley field of 8000 V/cm.