Double-barrier Diodes Research Articles

Large-signal dynamics of resonant-tunneling diodes

A model for analyzing dynamic large-signal characteristics of double-barrier resonant-tunneling diodes (RTDs) is presented. The model is based on the analysis of dynamical trajectories in phase space, defined by the RTD bias and electron density in the RTD quantum well. We show that an accurate dynamic model can be reformulated in an approximate way, relying only on a directly measurable DC I–V curve and on few other RTD parameters, which could be easily estimated with simple DC calculations. We further demonstrate that a simple equivalent circuit, composed of a capacitor, inductor, and two resistors (RLRC), accurately describes the large-signal admittance of RTDs. The circuit elements can be described in terms of relaxation time, geometrical RTD capacitance, and low- and high-frequency resistors. The circuit has the very same structure as that previously derived for small-signal RTD admittance, although with deviating parameters, which are now dependent on the AC-signal amplitude. We show that the large-signal RTD relaxation time can be shorter and longer than the small-signal one. In the context of RTD oscillators, a shorter RTD relaxation time allows one to get higher output power at high frequencies. The availability of an accurate, general, but rather simple, physics-based model for analyzing large-signal RTD dynamics removes one of the major hindrances to the further development of sub-THz and THz RTD oscillators.

Numerical analysis of electronic state of CNT/BNNT heterojunction

The I-V characteristics of CNT/BNNT heterojunctions, which comprise quantum double barrier systems (DBS), were numerically investigated using density functional theory (DFT) calculations and the non-equilibrium Green’s function (NEGF) method. First, we performed a structural optimisation of the DBS of CNT/BNNT. Next, the I-V characteristics of the heterojunctions were calculated using the SIESTA package. The I-V characteristics remain flat up to a certain bias voltage, and then the current increases with increasing voltage. Our CNT/BNNT double-barrier heterojunctions have resonant states inside the barriers as semiconductor-based double-barrier diodes; these states determine the transmission function. An energy gap was centred at the Fermi energy and sharp peaks in the gap were observed. It was also found that the energy interval of the peaks near the Fermi energy was dependent on the separation length of the two BNNT barriers. Using projected density of states (PDOS) analysis, we also investigated the local behaviour of electrons in the devices.

Electroluminescence in Unipolar-Doped In0.53Ga0.47As/AlAs Resonant-Tunneling Diodes: A Competition between Interband Tunneling and Impact Ionization

We measure and analyze the light emission from a room-temperature n-type unipolar-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{Al}\mathrm{As}$ double-barrier resonant-tunneling diode (RTD) that occurs just above the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ band edge and peaks around 1631 nm. The emission is attributed to electron-hole recombination emission made possible by holes generated in the high-field region on the collector side of the device by interband tunneling and impact ionization, which contribute comparable hole densities, according to our analysis. Although the external quantum efficiency (EQE) in our experimental configuration is rather low (\ensuremath{\approx}2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ at 3.0-V bias), limited by suboptimal output coupling, the internal quantum efficiency (IQE) is much higher (\ensuremath{\approx}6% at 3.0-V bias), as derived from the experimental EQE and a radiometric analysis. To check this value and better understand the transport physics, we also carry out an independent estimate of the IQE using a combined interband-tunneling impact-ionization transport model, which yields an IQE of 10% at 3.0-V bias. The satisfactory agreement of theory with experimental data suggests that a RTD designed for better hole transport and superior optical coupling could become a useful light-emitting device, while retaining the intrinsic functionality of high-speed negative differential resistance, and all without the need for resistive p-type doping.

An alternating-direction hybrid implicit-explicit finite-difference time-domain method for the Schrödinger equation

This paper proposes a novel hybrid FDTD method for solving the time-dependent Schrödinger equation, which is fundamental for modeling materials and designing nanoscale devices. The wave function is propagated on nonuniform grids by applying explicit updates in part of the grid and implicit updates elsewhere. The latter are based on the Alternating-Direction Implicit (ADI) scheme while the former are constructed with a central difference for the time derivative. A rigorous stability analysis proves that spatial steps can be selectively removed from the stability criterion thus combining the unconditional stability of the ADI scheme with fast explicit calculations. The scheme excels in its flexibility by efficiently discretizing and balancing explicit with implicit updates, as such expediting the computations. Moreover, it retains the linear complexity of explicit schemes with respect to space and time, making it especially scalable to numerically large problems. Several numerical experiments, including a laterally tunnel-coupled quantum wire and a nanowire double-barrier resonant-tunneling diode, show the validity of the scheme by demonstrating its high accuracy and decreased CPU time compared to traditional methods.

Double-Barrier β-Ga2O3 Schottky Barrier Diode With Low Turn-on Voltage and Leakage Current

This work reports a $\beta $ -Ga2O3 double-barrier Schottky barrier diode (DBSBD) with both low turn-on voltage and low reverse leakage current by using Ni and PtOx as the anode electrode. The barrier height of PtOx-based diode can be effectively modulated from 1.26 to 1.62 eV by adjusting oxygen pressure during sputtering processes. Combining the maximum work function of PtOx electrode with the optimization of the electrode ratio of Ni and PtOx, the DBSBD with an electrode diameter ratio of $\text{D}_{\text {Ni}} /\text{D}_{\text {PtOx}}= {75}/{150} \mu \text{m}$ not only exhibits a high forward current of 470.9 A/cm2 (at 3.5 V), a low on-resistance of 4.1 $\text{m}\Omega ~\cdot $ cm2 and a low turn-on voltage of 1.13 V, but also possesses a relatively low reverse leakage current of ${1.2}\times {10}^{-{7}}$ A/cm2 (at −100 V), which is more than one order of magnitude lower than that of the Ni-SBD. Silvaco TCAD simulation reveals that such optimization can be attributed to the suppression of edge leakage current due to the double-barrier contact. Therefore, the strategy of double-barrier design can balance the forward and reverse characteristics in SBD, providing a new device structure for advanced power electronics.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

A Nonlinear Circuit Simulation of Switching Process in Resonant-Tunneling Diodes

A large-signal circuit model is used to compute the switching time for double-barrier resonant-tunneling diodes. The model consists of linear circuit elements plus a nonlinear $I$ – $V$ characteristic. The linear elements include a series resistor, a capacitor, and an inductor. The capacitance considers the charge accumulation, depletion in spacer layers, as well as charging-discharging of the quantum-well (QW) region. The inductance accounts for the delay of the current with respect to the voltage across the QW during the abrupt switching transition through the negative differential resistance region. A second-order Runge–Kutta method is used to solve for the switching transient, and then fit to experimental data for a high-quality InGaAs/AlAs resonant tunneling diode (RTD) using the QW inductance as a fitting parameter. Excellent agreement is found for the 10%–90% switching time with a calculated capacitance of 98 fF and a fitted inductance of 1300 pH. This large-signal inductance is approximately $7\times $ greater than the small-signal inductance that has been successfully used to predict the fmax of RTDs such as the one tested here.

Analysis of single- and double-barrier tunneling diode structures using ultrathin CaF2/CdF2/Si multilayered heterostructures grown on Si

The current–voltage (I–V) characteristics of single-barrier and double-barrier tunneling diode structures using CaF2/CdF2/Si ultrathin multilayered heterostructures grown on Si substrates have been theoretically analyzed and their material parameters, such as the conduction band discontinuity (ΔEC) at the heterointerface and effective mass (m*), have been evaluated by fitting simulation with the measured I–V characteristics. ΔEC between the ultrathin (1–3 nm) CaF2 and Si layers and m* for CaF2 were found to be 1.5–2.3 eV and 0.3–1.0m0, respectively. A clear thickness dependence of these parameters was observed, and the deviations of m* and ΔEC were approximately 30–50%, which probably originated from the thin layer thicknesses in atomic order. Using the estimated values derived from the single-barrier tunneling diodes, m* for CdF2 was also estimated to be 0.36m0 by fitting simulation of double-barrier diodes. These results will contribute to clarifying the design principle of tunneling devices with CaF2 and enhancing quantitative studies on electron transport in atomically thin multilayered heterostructures.

GaAs/AlGaAs resonant tunneling diodes with a GaInNAs absorption layer for telecommunication light sensing

Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the AlGaAs tunnel barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 μm wavelength leads to a pronounced photo-effect with a sensitivities of around 103 A/W.

Electron transport simulation in resonant-tunneling GaN/AlGaN heterostructures

A numerical method for electron transport calculations in resonant-tunneling GaN/AlGaN heterostructures has been developed on the basis of a self-consistent solution of the Schrodinger and Poisson equations. Dependences of the system’s transmission coefficient on the external field and of the peak current on the ratio between the well and barrier widths have been studied for a double-barrier resonant-tunneling diode. For technical applications, the optimal values of the structure’s parameters have been found.

DESIGN & OPTIMIZATION OF AN I-RTD HYBRID THz OSCILLATOR BASED UPON In1−xGaxAs/GaSbyAs1−y HETEROSTRUCTURE SYSTEMS

An In 1− x Ga x As / GaSb y As 1− y hetero-system with staggered band-lineups as solid-state platform for design of an interband resonant double-barrier tunneling diode (I-RTD) based optically-pulsed (OT) hybrid device for generating THz oscillations is theoretically investigated. It is demonstrated that this optical I-RTD hybrid is compatible with the robust state-of-art 1.55 micron laser technology Multi-band wave equations in the framework of six-band Kane's model are applied for understanding the carrier dynamics when strain-induced effects are present. Simulation results for practical circuit implementations clearly show the superiority of this new oscillator concept.

A Study of Inhomogeneous Schottky Diodes on n-Type 4H-SiC

In this study, we performed a statistical analysis of 500 Ni Schottky diodes distributed across a 2-inch, n-type 4H-SiC wafer with an epilayer grown by chemical vapor deposition. A majority of the diodes displayed ideal thermionic emission when under forward bias, whereas some diodes showed ‘double-barrier’ characteristics with a ‘knee’ in the low-voltage log I vs. V plot. X-ray topography (XRT) and polarized light microscopy (PLM) revealed no correlations between screw dislocations and micropipes and the presence of double-barrier diodes. Depth resolved cathodoluminescence (DRCLS) indicated that certain deep-level states are associated with the observed electrical variations.

Influence of minority carrier transport in the optical properties of double barrier diodes

The aim of this work is to study the importance of minority carrier transport in double barrier diodes (DBD). We propose a theoretical model capable to describe the photoluminescence properties observed in GaAs-Al0.35Ga0.65As double barrier diodes with the increase of temperature. In this model, we considered that the minority carries (holes) photocreated at the contact of the structure diffuse, drift and then tunnel to the well. To study the influence of previous transport mechanisms, we solved the continuity equation under the influence of an applied bias and an excitation laser light. The theoretical photocurrent and photoluminescence calculations agree quite well with the experimental observations. Within the approaches of our model, we are able to simulate the minority holes mobility from the photocurrent measurements suggesting the use of the experimental technique as a probe of the carrier mobility.

Negative differential resistance in dislocation-free GaN∕AlGaN double-barrier diodes grown on bulk GaN

Double-barrier GaN resonant tunneling diodes with AlGaN barriers were fabricated on bulk (0001) single-crystal GaN substrates. Layers were grown using molecular-beam epitaxy with a rf plasma nitrogen source. Single diodes of 6μm diameter were prepared by inductively coupled plasma reactive ion etching. For many diodes clear negative differential resistance is observed around 2V with peak currents around 10kA∕cm2 and a peak-to-valley ratio of about 2 at room temperature. Its observation does not depend on specific conditions of measurement; however, it slowly decays after each measurement. The mechanism behind this decay is investigated since it is obviously prohibiting the usage of GaN resonant tunneling diodes so far. It is shown not to be caused by catastrophic breakdown of the devices.

Comparative analysis of sequential and coherent tunneling models in resonant diodes

We compare the sequential and coherent tunneling models for the current voltage $(I\text{\ensuremath{-}}V)$ characteristic and shot noise suppression in double-barrier resonant diodes. The results confirm that the $I\text{\ensuremath{-}}V$ characteristic remains the same for the two models, while at increasing voltage the shot noise power at zero frequency shows significant differences. In sequential tunneling, shot noise exhibits in general two regions of suppression due to Pauli and Coulomb correlations, respectively, and with the associated Fano factor never dropping below the value of 0.5. By contrast, in coherent tunneling shot noise exhibits a single region of suppression with the associated Fano factor having the possibility of dropping below the value of 0.5 at voltages near to the current peak. We also investigate the current spectral density and the Fano factor as a function of frequency up to values below that corresponding to the energy difference between the first and the second resonant electron states. At all frequencies the Fano factor for sequential tunneling never drops below the value of 0.5. By contrast, for coherent tunneling, at high frequencies the Fano factor is found to become suppressed systematically below the value of 0.5 to a minimum value of 0.25 which is independent of frequency. These results confirm that coherent tunneling can lead to shot noise suppression below the value of 0.5 contrary to the case of sequential tunneling.

Voltage-controlled hole spin injection in nonmagneticGaAs∕AlAsresonant tunneling structures

We have investigated polarization-resolved photoluminescence under applied voltage in $p\text{\ensuremath{-}}i\text{\ensuremath{-}}p$ $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ double-barrier diodes. We have observed oscillations in the degree of polarization up to 36% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ with sign reversals occurring near to the hole subband resonances. At high voltages a polarization saturation up to 25% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is observed. The data are interpreted by using simulations based on a simple theoretical model that considers spin conservation for tunneling and the relaxation processes for carriers at Zeeman states in the quantum well. Our work offers the prospect for the development of voltage-controlled spin filtering systems using standard nonmagnetic semiconductor heterostructures.

A theoretical study of resonant tunneling characteristics in triangular double-barrier diodes

Resonant tunneling characteristics of triangular double-barrier diodes have been investigated systematically in this Letter, using Airy function approach to solve time-independent Schrödinger function in triangular double-barrier structures. Originally, the exact analytic expressions of quasi-bound levels and quasi-level lifetime in symmetrical triangular double-barrier structures have been derived within the effective-mass approximation as a function of structure parameters including well width, slope width and barrier height. Based on our derived analytic expressions, numerical results show that quasi-bound levels and quasi-level lifetime vary nearly linearly with the structure parameters except that the second quasi-level lifetime changes parabolically with slope width. Furthermore, according to our improved transmission coefficient of triangular double-barrier structures under external electric field, the current densities of triangular double-barrier diodes with different slope width at 0 K have been calculated numerically. The results show that the N-shaped negative differential resistance behaviors have been observed in current–voltage characteristics and current–voltage characteristics depend on the slope width.

Negative charged excitons in double barrier diodes

We have studied the effects of excitonic complexes formation, such as excitons and trions, on the optical and on transport properties of GaAs–GaAlAs n–i–n double barrier diodes, by measuring the current–voltage characteristics and the photoluminescence emission, as function of bias. The observation of a pre-resonance shoulder in the I(V) curves, under high laser intensities, and a negative charged excitons in the photoluminescence spectra, under the same bias conditions, were associated to the dissociation of these complexes either by thermal excitation or by scattering with ‘free’ carriers in the quantum well layer. A simple rate equation model allows us to explain the kinetics of the excitonic complexes in double barrier devices.

Shot-noise characteristics of triple-barrier resonant-tunneling diodes

We have found experimentally that the shot noise in InAlAs-InGaAs-InAlAs Triple-Barrier Resonant-Tunneling Diodes (TBRTD) is reduced over the 2eI Poissonian value whenever their differential conductance is positive, and is enhanced over 2eI when the differential conductance is negative. This behavior, although qualitatively similar to that found in double-barrier diodes, differs from it in important details. In TBRTDs the noise reduction is considerably larger than predicted by a semi-classical model, and the enhancement does not correlate with the strength of the negative differential conductance. These results suggest an incomplete understanding of the noise properties of multiple-barrier heterostructures.

Controlling the spin polarization of the electron current in a semimagnetic resonant-tunneling diode

The spin filtering effect of the electron current in a double-barrier resonant-tunneling diode (RTD) consisting of ZnMnSe semimagnetic layers has been studied theoretically. The influence of the distribution of the magnesium ions on the coefficient of the spin polarization of the electron current has been investigated. The dependence of the spin filtering degree of the electron current on the external magnetic field and the bias voltage has been obtained. The effect of the total spin polarization of the electron current has been predicted. This effect is characterized by total suppression of the spin-up component of electron current, that takes place when the Fermi level coincides with the lowest Landau level for spin-up electrons in the RTD semimagnetic emitter.