Searching for THz Gunn oscillations in GaN planar nanodiodes

A detailed study of GaN-based planar asymmetric nanodiodes, promising devices for the fabrication of room temperature THz Gunn oscillators, is reported. By using Monte Carlo simulations, an analysis of the static I-V curves and the time-domain evolution of the current obtained when varying some simulation parameters in the diodes has been made. Oscillation frequencies of hundreds of GHz are predicted by the simulations in diodes with micrometric channel lengths. Following simulation guidelines, a first batch of diodes was fabricated. It was found that surface charge depletion effects are stronger than expected and inhibit the onset of the oscillations. Indeed, a simple standard constant surface charge model is not able to reproduce experimental measurements and a self-consistent model must be included in the simulations. Using a self-consistent model, it was found that to achieve oscillations, wider channels and improved geometries are necessary.ROOTHz (FP7-243845

Bias-dependence of surface charge at low temperature in GaN Self-Switching Diodes

[EN]In this work, with the help of a semi-classical twodimensional Monte Carlo (MC) simulator, we study the DC current-voltage curves of Self-Switching Diodes (SSDs) fabricated on an AlGaN/GaN heterostructure from 100 K up to room temperature. Due to the very narrow channel of the SSDs, the presence of surface effects plays a key role not only on their DC behavior but also on their RF detection performance. The evolution with temperature of the negative surface charge density σ at the etched sidewalls of the SSD is the key quantity to explain the measurements. At 300 K, MC simulations with a constant value of σ are able to replicate very satisfactorily the experiments. However, to reproduce the shape of the I-V curve at low temperatures, a more realistic approach, where σ depends not only on T, but also on the applied bias V, is necessary.Spanish MINECO and FEDER through project TEC2017-83910-R and Junta de Castilla y León and FEDER through project SA254P1

Monte Carlo analysis of the influence of surface charges on GaN asymmetric nanochannels: Bias and temperature dependence

[EN]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.Spanish MINECO and FEDER through Project No. TEC2017-83910-R and Junta de Castilla y León and FEDER through Project No. SA254P1

Time-domain Monte Carlo simulation of GaN planar Gunn nanodiodes in resonant circuits

In this work we present a theoretical study based on time-domain Monte Carlo (MC) simulations of GaN-based Self-Switching Diodes (SSDs) oriented to the experimental achievement and control of the sub-THz Gunn-oscillations potentially provided by these devices. With this aim, an analysis of the frequency performance of SSDs connected to a resonant RLC parallel circuit, is reported here. V-shaped SSDs have been found to be more efficient, in terms of the DC to AC conversion efficiency η, than similar square-shape ones. Indeed, a value of η of at least 0.80%, can be achieved with appropriate RLC elements, even when considering heating effects. When the influence of parasitic elements such as the crosstalk capacitance Ctalk is evaluated, MC simulations have shown that the resonant circuit must contain a capacitance C higher than Ctalk in order to obtain experimentally useful values of η. This condition can be reached by integrating a sufficiently high number N of parallel SSDs in the fabricated devices. MC simulations have also shown that when several diodes are fabricated in parallel the oscillations of all the SSDs are not synchronized, but this problem is solved by the attachment of a resonant RLC tank

Acción tutorial on-line mediante el uso de tabletas gráficas en el ámbito de la teoría de circuitos electrónicos

Memoria ID-034. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2018-2019

Experimental demonstration of direct terahertz detection at room-temperature in AlGaN/GaN asymmetric nanochannels

The potentialities of AlGaN/GaN nanodevices as THz detectors are analyzed. Nanochannels with broken symmetry (so called Self Switching Diodes) have been fabricated for the first time in this material system using both recess-etching and ion implantation technologies. The responsivities of both types of devices have been measured and explained using Monte Carlo simulations and non linear analysis. Sensitivities up to 100 V/W is obtained at 0.3 THz with a 280 pW/sqrt(Hz) Noise Equivalent Power.ROOTHz (FP7-243845

Temperature Behavior of Gunn Oscillations in Planar InGaAs Diodes

[EN]Planar Gunn diodes on In0.53Ga0.47 As with lengths between 2 and 5 μm have been fabricated and characterized in a temperature range of 10 to 300 K. Two different oscillation regimes are observed depending on temperature. At the higher values, the frequency of the oscillations decreases as the bias increases, as expected for a well-established transit-time domain mode. But below approximately 75 K, the behavior is the opposite, the frequency of the Gunn oscillations increases with the bias. This fact, together with a much lower amplitude of the oscillations, indicate the possible switch to a different oscillation mode in which the domains are not able to attain their complete maturation before reaching the anode.Spanish MINECO under Project TEC2017-83910-R and JCyL and FEDER under Project SA254P1

Exploring Digital Logic Design Using Ballistic Deflection Transistors Through Monte Carlo Simulations

We present exploratory studies of digital circuit design using the recently proposed ballistic deflection transistor (BDT) devices. We demonstrate a variety of possible logic functions through simple reconfiguration of two drain-connected BDTs. We further propose the creation of a three-BDT logic cell to yield differential versions of each logic function, improving overall flexibility of BDT circuit design. Each of the proposed gate configurations has been verified through extensive numerical calculations using an in-house Monte Carlo simulator. Simulation results show that the proposed gate arrangements are capable of achieving 400-GHz operating frequencies at room temperature. A compact fit-based analytical model to aid circuit design using BDTs is also introduced.ROOTHz (FP7-243845

Actividades de Producción Digital destinadas a la difusión y promoción del Máster Universitario en Física

Memoria ID-176. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2013-2014

Analysis of trap states in AlGaN/GaN self-switching diodes via impedance measurements

The presence of trap states in self-switching diodes (SSD) based on an AlGaN/GaN heterojunction has been identified by means of their AC characterization between 75 kHz to 30 MHz in a wide temperature range, from 80 K to 300 K. Measurements allow us to determine two different characteristic energies of the traps, 12 meV and 61 meV, being associated to a distribution of surface states and one discrete bulk trap, respectively. The impact of the trapping effects on microwave detection at zero-bias has also been analyzed in the same temperature range, the measured responsivity showing an unusual enhancement and a low-frequency roll-off at low temperatures.Spanish MINECO and FEDER through project TEC2017-83910-R and Junta de Castilla y León and FEDER through project SA254P1