Optimized V-shape design of GaN nanodiodes for the generation of Gunn oscillations

In this work, recent advances in the design of GaN planar Gunn diodes with asymmetric shape, socalled self-switching diodes, are presented. A particular geometry for the nanodiode is proposed, referred as V-shape, where the width of the channel is intentionally increased as approaching the anode. This design, which reduces the effect of the surface-charges at the anode side, is the most favourable one for the onset of Gunn oscillations, which emerge at lower current levels and with lower threshold voltages as compared to the standard square geometry, thus enhancing the power efficiency of the self-switching diode as sub-millimeter wave emitters

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

Creación de hardware y software propios para la extensión de las capacidades de laboratorios portátiles unipersonales

Memoria ID2022-217 Ayudas de la Universidad de Salamanca para la innovación docente, curso 2022-2023

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

Comparative Monte Carlo analysis of InP- and GaN-based Gunn diodes

In this work, we report on Monte Carlo simulations to study the capability to generate Gunn oscillations of diodes based on InP and GaN with around 1 lm active region length. We compare the power spectral density of current sequences in diodes with and without notch for different lengths and two doping profiles. It is found that InP structures provide 400 GHz current oscillations for the fundamental harmonic in structures without notch and around 140 GHz in notched diodes. On the other hand, GaN diodes can operate up to 300 GHz for the fundamental harmonic, and when the notch is effective, a larger number of harmonics, reaching the Terahertz range, with higher spectral purity than in InP diodes are generated. Therefore, GaN-based diodes offer a high power alternative for sub-millimeter wave Gunn oscillations.European Commision (EC). Funding FP7/SP1/ICT. Project Code: 24384

Monte Carlo studies of the intrinsic time-domain response of nanoscale three-branch junctions

[EN] 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.ROOTHz (FP7-243845

Evidence of surface charge effects in T-branch nanojunctions using microsecond-pulse testing

The understanding of the influence of surface charge effects on the electrical properties of nanostructures is a key aspect for the forthcoming generations of electronic devices. In this work, by using an ultrafast electrical pulse characterization technique, we report on the room-temperature time response of a T-branch nanojunction which allows identifying the signature of surface states. Different pulse widths from 500 ns to 100 µs were applied to the device. For a given pulse width, the stem voltage is measured and compared with the DC result. The output value in the stem is found to depend on the pulse width and to be related to the characteristic charging time of the interface states. As expected, the results show that the well-know nonlinear response of T-branch junctions is more pronounced for long pulses, beyond such a characteristic time.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