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The small-signal equivalent circuit of GaN-based self-switching diodes has been obtained, which apart from the intrinsic R ‖ C branch, generally used to describe the diode performance, needs new elements to describe the low-frequency dispersion of the impedance originated by the presence of surface and bulk traps. The proposed model allows us to reproduce not only the high-frequency results (extracted from S-parameter measurements in the 40 MHz–43.5 GHz range) at room temperature, but also the low-frequency impedance measurements (75 kHz–30 MHz) at cryogenic temperatures down to 70 K. These new elements are a self-inductance associated to the effect of surface states (typical of a device with a high surface-to-volume ratio) and an extra series R – C branch modeling the influence of the bulk traps. [ABSTRACT FROM AUTHOR]

In this paper, we report an analysis of reverse current mechanisms observed in GaN Schottky barrier diodes leading to hysteretic behavior of the I–V curves at low temperature. By means of DC measurements from 33 to 475 K, we demonstrate the presence of two leakage mechanisms when comparing the experiments with the results obtained using a unified model to predict the ideal reverse current of the diode. Poole–Frenkel emission is the dominant mechanism for temperatures above 200 K, while trap-assisted tunneling prevails for lower temperatures, where also, hysteresis cycles are revealed by means of DC dual-sweep voltage measurements. The energy of the corresponding traps has also been determined, being around 0.2 and 0.45 eV, respectively. The hysteresis phenomenon is attributed to the bias-induced occupancy of the energy states originating the leakage-current processes, which leads to the reduction of the reverse current after a high negative voltage is applied to the diode. [ABSTRACT FROM AUTHOR]

We report on the measurements of the two main figures of merit for microwave detection, namely, responsivity and noise equivalent power (NEP), in HEMTs based on two of the most broadly used material systems AlGaN/GaN and AlInAs/InGaAs. A comparison between their performances as RF detectors in a wide temperature range is provided by means of experiments made under probes with direct connection to the drain contact. InGaAs HEMTs exhibit much better responsivity and NEP, which are further improved when lowering the working temperature. Moreover, we analyze the possibility of optimizing the current-mode detection of the transistors by improving the impedance matching conditions through an adequate choice of the device width W. [ABSTRACT FROM AUTHOR]

A model to predict the ideal reverse leakage currents in Schottky barrier diodes, namely, thermionic emission and tunneling components, has been developed and tested by means of current–voltage–temperature measurements in GaN-on-SiC devices. The model addresses both current components and both forward and reverse polarities in a unified way and with the same set of parameters. The values of the main parameters (barrier height, series resistance, and ideality factor) are extracted from the fitting of the forward-bias I–V curves and then used to predict the reverse-bias behavior without any further adjustment. An excellent agreement with the I–V curves measured in the forward bias in the GaN diode under analysis has been achieved in a wide range of temperatures (275–475 K). In reverse bias, at temperatures higher than 425 K, a quasi-ideal behavior is found, but additional mechanisms (most likely trap-assisted tunneling) lead to an excess of leakage current at lower temperatures. We demonstrate the importance of the inclusion of image-charge effects in the model in order to correctly predict the values of the reverse leakage current. Relevant physical information, like the energy range at which most of the tunnel injection takes place or the distance from the interface at which tunneled electrons emerge, is also provided by the model. [ABSTRACT FROM AUTHOR]

In this paper, the occupancy of sidewall surface states having a clear signature in the performance of AlGaN/GaN-based self-switching diodes (SSDs) is analyzed using a semi-classical Monte Carlo (MC) simulator in a wide temperature (T) range, from 100 to 300 K. Experimental I – V curves show an unusual current decrease at low temperature attributed to surface trapping. The dependence on T of the negative surface charge density σ at the etched sidewalls of the SSDs is essential to explain the measurements. Two devices with different widths (80 and 150 nm) have been characterized and simulated in detail paying especial attention to the modeling of the surface states. At room temperature, MC simulations with a position-independent value of σ are able to qualitatively reproduce the I – V curves. However, a more complex approach is required to correctly replicate the values and shape of the DC experimental curves at low temperature, below 220 K. An algorithm where σ depends not only on T but also on the applied bias V is proposed to successfully fit the current values at every bias point. The model is able to explain the physics of the unexpected dependence of the resistance with the channel width and the sign change in the bowing coefficient, the parameters that govern the detection capabilities of the diodes. [ABSTRACT FROM AUTHOR]

III-V Impact-ionization (II) metal-oxide-semiconductor FETs (I-MOSFETs) and tunnel FETs (TFETs) are being explored as promising devices for low-power digital applications. To assist the development of these devices from the physical point of view, a Monte Carlo (MC) model which includes impact ionization processes and band-to-band tunneling is presented. The MC simulator reproduces the I-V characteristics of experimental ungated In0.53Ga0.47As 100 nm PIN diodes, in which tunneling emerges for lower applied voltages than impact ionization events, thus being appropriate for TFETs. When the structure is enlarged up to 200 nm, the ON-state is achieved by means of impact ionization processes; however, the necessary applied voltage is higher, with the consequent drawback for low-power applications. In InAs PIN ungated structures, the onset of both impact ionization processes and band-to-band tunneling takes place for similar applied voltages, lower than 1V; thus they are suitable for the design of low-power I-MOSFETs. [ABSTRACT FROM AUTHOR]

We investigate damped nonharmonic oscillations at terahertz frequencies observed in the current response of Schottky diodes simulated with the Monte Carlo method under applied signals of a few hundred GHz. From Monte Carlo simulations of different diode structures, two kinds of nonharmonic oscillations have been identified. The first kind of oscillations is due to the coupling of the nonlinear performance of the Schottky junction with the inertial motion of the carriers in the non-depleted region of the epilayer. The second kind of oscillations is due to the modulation of the n+-n junction when high electric fields are induced in the non-depleted region of the epilayer. These oscillations constitute a promising mechanism for THz signal generation. [ABSTRACT FROM AUTHOR]

We present a Monte Carlo time-domain study of nanostructured ballistic three-branch junctions (TBJs) excited by both step-function and Gaussian picosecond transients. Our TBJs were based on InGaAs 2-dimensional electron gas heterostructures and their geometry followed exactly the earlier experimental studies. Time-resolved, picosecond transients of both the central branch potential and the between-the-arms current demonstrate that the bandwidth of the intrinsic TBJ response reaches the THz frequency range, being mainly limited by the large-signal, intervalley scattering, when the carrier transport regime changes from ballistic to diffusive. [ABSTRACT FROM AUTHOR]

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical analysis of the kink effect in InAs/AlSb high electron mobility transistors (HEMTs). Kink effect, this is, an anomalous increase in the drain current ID when increasing the drain-to-source voltage VDS, leads to a reduction in the gain and a rise in the level of noise, thus limiting the utility of these devices for microwave applications. Due to the small band gap of InAs, InAs/AlSb HEMTs are very susceptible to suffer from impact ionization processes, with the subsequent hole transport through the structure, both implicated in the kink effect. The results indicate that, when VDS is high enough for the onset of impact ionization, holes thus generated tend to pile up in the buffer (at the gate-drain side) due to the valence-band energy barrier between the buffer and the channel. Due to this accumulation of positive charge the channel is further opened and ID increases, leading to the kink effect in the I-V characteristics and eventually to the device electrical breakdown. The understanding of this phenomenon provides useful information for the development of kink-effect-free InAs/AlSb HEMTs. [ABSTRACT FROM AUTHOR]

Under certain conditions, plasma instabilities associated with streaming motion of carriers taking place in n+nn+ diodes can lead to current oscillations. The origin of the phenomenon, known as optical phonon transit time resonance, is characterized by a frequency related to the transit time between consecutive optical phonon emissions by electrons along the active region of the diode. By means of Monte Carlo simulations, the possibility to obtaining current oscillations in GaN n+nn+ diodes is analyzed. The optimum conditions for the onset of such mechanism are investigated: applied bias, temperature, doping, and length of the active n region. Simulations show that current oscillations at frequencies in the terahertz range can be obtained at very low temperatures. Moreover, by choosing the appropriate applied voltage and length of the n region, some degree of tunability can be achieved for frequencies close to the plasma frequency of the n region of the n+nn+ diode. [ABSTRACT FROM AUTHOR]

In this work, by means of Monte Carlo simulations, two different narrow band gap semiconductors, InAs and InSb, and their associated heterostructures, AlSb/InAs and AlInSb/InSb, have been studied. The parameters for the bulk simulations have been optimized in order to correctly reproduce the experimental mobility values. For the correct simulation of the heterostructures, roughness scattering has been included in the model, and its strength has been adjusted to achieve a good agreement with the experimentally measured mobility. [ABSTRACT FROM AUTHOR]

In this work, the influence of the geometry of the different branches on the output characteristics of InAlAs/InGaAs three-branch junctions is analyzed. At room temperature experimental measurements show that when increasing the width of the horizontal branches, the nonlinear behavior persists, even if less pronounced. This implies a reduction in the (typically high) impedance of these nanodevices, which is quite interesting in order to minimize the influence of parasitic capacitances on their cutoff frequency and to decrease the reflected power due to the mismatch with the 50 Ω standard equipment. The width of the vertical branch is also a relevant parameter, nonlinear effects being more important for narrow branches. In both cases surface charges at the etched boundaries of the branches play a key role. Monte Carlo simulations (performed with a self-consistent surface charge model recently proposed by the authors) are used to explain the physical origin of the observed behavior and to improve the device performance. We focus on the enhancement of their efficiency as a way to develop their promising functionalities in various analog and digital circuits. [ABSTRACT FROM AUTHOR]

A microscopic analysis of self-generated terahertz current oscillations that take place in planar InAlAs/InGaAs slot diodes operating under dc bias is presented. An ensemble Monte Carlo simulation is used for the calculations. The onset of the oscillations is thresholdlike, for drain-source voltages surpassing 0.6 V. The Gunn-like mechanisms and the modulation of the injection of electrons into the recess-to-drain region, which alternatively takes place in the Γ or L valley, are found at the origin of the phenomenon. Terahertz frequencies are reached because of the presence of ultrafast Γ electrons in the region of interest. Extremely high velocities are achieved by (i) the effect of the recess, which focuses the electric field and launches very fast electrons into the drain region, and (ii) the influence of degeneracy, which significantly reduces the rate of scattering mechanisms and enhances the electron mobility in the channel. [ABSTRACT FROM AUTHOR]

By means of a semiclassical two-dimensional Monte Carlo technique, we analyze current noise spectra of InAlAs/InGaAs-based submicron self-switching diodes. Shot noise (at low bias) and diffusion noise in the series resistance (at high bias) are found to dominate the current noise at low frequency (in the plateau beyond the 1/f range). Two peaks of different origins, which may limit the device performance, are found in the noise spectra at higher frequencies. The dependence of the amplitude and frequency of these peaks on the topology of the diodes is analyzed and discussed. Design indications to improve the noise performance of the devices are provided. [ABSTRACT FROM AUTHOR]

Room temperature electron mobility (μ) in nanometer Si metal-oxide-semiconductor field-effect transistors (MOSFETs) with gate length (LG) down to 30 nm was determined by the magnetoresistance method. A decrease of μ with the decrease of LG was observed. Monte Carlo simulations of electron transport in nanometer MOSFETs were carried out for realistic devices as a function of LG. The dependence with LG and electron concentration of simulated mobility and transmission coefficient agree with experimental data. An analysis of scattering events and time of flight gives evidence of the presence of ballistic motion in the investigated structures and proves its influence on mobility degradation in short transistors. The results give arguments that interpretation of the magnetoresistance coefficient as the square of the mobility is valid also in the case of quasiballistic electron transport. [ABSTRACT FROM AUTHOR]

We present a microscopic analysis of current fluctuations in a GaAs n+-n-metal Schottky barrier diode containing electron traps in the active layer. An ensemble Monte Carlo simulation is used for the calculations. We analyze the influence of generation-recombination mechanisms of electrons with traps on the current–voltage characteristics and noise spectra of the diode. The presence of traps reduces both the flatband voltage and the current level in the series-resistance regime. With respect to the noise, significant modifications are observed in the current noise spectra. In the barrier-limited regime, while at low-frequency shot noise is not found to change, the returning-carriers peak is strongly modulated by the influence of the traps. Beyond flatband conditions generation-recombination noise becomes evident at low frequency, exhibiting a quadratic dependence on the current. [ABSTRACT FROM AUTHOR]

We perform a microscopic analysis of the noise associated with the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs) by using a semiclassical two-dimensional (2-D) ensemble Monte Carlo simulator. The kink effect in HEMTs has its origin in the pile up of holes (generated by impact ionization) taking place under the source side of the gate, that leads to a reduction of the gate-induced channel depletion and results in a drain current enhancement. Our results indicate that the generation of holes by impact ionization and their further recombination lead to fluctuations in the charge of the hole pile up which provoke an important increase of the drain-current noise, even when the kink effect is hardly perceptible in the output characteristics. In addition, shot noise related to the hole gate leakage current is found at the gate terminal, which can further degrade the global noise performance of HEMTs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the use of a semiclassical two-dimensional ensemble Monte Carlo simulator to perform a physical microscopic analysis of the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors. Susceptibility of the devices to suffer impact ionization processes; Lowering of the potential barrier controlling the current through the channel due to the pile up of positive charge.

Presents a study which analyzed the current fluctuations in homogeneous N-type GaAs structures of different lengths and doping concentrations using Poisson solver and Monte Carlo simulation. Calculation of autocorrelation functions and spectral density of current fluctuations; Physical model used for GaAs; Static characteristics of the structures; Conclusions.