Valley Current Density Research Articles

Simulation and Comparative Study of Resonant Tunneling Diode

This paper studies and investigates the effect of physical and electrical parameters on double, triple and six barrier resonant tunneling diodes (RTD). The materials used for quantum well and barriers are Gallium arsenide (GaAs) and Aluminium gallium arsenide (AlGaAs), respectively. The parameters that were reasoned and studied include conduction band, current density, transmission coefficient and resonance energy. The above parameters were studied by changing bias voltage, temperature, barrier width and doping concentration. From the simulations performed it is observed that for double barrier RTD the peak current density is observed at 0.2 V and the valley current density is observed at 0.3 V, whereas for a triple barrier RTD the peak current density is observed at 0.015 V and the valley current density is observed at 0.06 V. The value of transmission coefficient for double barrier RTD decreases especially after bias applied is more than resonant bias (0.2 V). The effect of increasing bias leads to a decrease in the resonance level in the conduction band. The width of resonance energy decreases with the increase in barrier width. With increase in number of barrier the number of resonance level increases which leads to an increasing peaks in the transmission coefficient curve. The effect of increasing temperature leads to higher current and more resonance energy. With the thickening of barrier width, less transmission of electrons occurs leading to a reduced current density. When the barriers are increased the negative differential region (NDR) is achieved at low voltages.
 HIGHLIGHTS
 
 This paper studies and investigates the effect of physical and electrical parameters on double, triple and six barrier resonant tunneling diodes (RTD)
 The parameters include conduction band, current density, transmission coefficient, and resonance energy
 The parameters were studied by changing bias voltage, temperature, barrier width, and doping concentration
 It is observed that for double barrier RTD the peak current density is observed at 0.2 V and the valley current density is observed at 0.3 V
 For a triple barrier RTD the peak current density is observed at 0.015 V and the valley current density is observed at 0.06 V
 
 GRAPHICAL ABSTRACT

Demonstration of highly repeatable room temperature negative differential resistance in large area AlN/GaN double-barrier resonant tunneling diodes

Here, we present a systematical investigation of AlN/GaN double-barrier resonant tunneling diodes (RTDs) grown by plasma-assisted molecular beam epitaxy on metal-organic chemical vapor deposition GaN-on-sapphire templates. The processed devices featured an active region composed of 2.5 nm GaN quantum well sandwiched by two 1.5 nm AlN barriers and RTD mesa diameter ranging from 1 to 20 μm. Room temperature current–voltage characteristics exhibited a repeatable negative differential resistance (NDR) free of degradation and hysteresis after 1000 times subsequently up-to-down voltage sweeps across different sizes. High peak-to-valley current ratios of 1.93 and 1.58 were obtained at room temperature for 1 and 12 μm diameter devices, respectively, along with peak current densities of 48 and 36 kA/cm2 corresponding to peak voltages of 4.65 and 5.9 V. The peak current density decreased quickly initially and then was less susceptible to this averaging effect with increasing the device diameter. Temperature-dependent measurements revealed that the valley current density displayed a positive relationship to the temperature, and an abruptly increasement was observed for the devices with a diameter of 20 μm when the temperature rose over 230 K. We attributed this abnormal phenomenon to the increased contribution from acoustic and longitudinal optical (LO) phonon scattering, especially for the LO phonon scattering. The area dependence of electrical performance suggested that the leakage pathway through dislocations played a vital role for charge transport and there existed a threshold of dislocation density for NDR characteristics. These results promote further study for future implementation of III-nitride-based RTD oscillators into high-frequency and high-power terahertz radiation.

Superior growth, yield, repeatability, and switching performance in GaN-based resonant tunneling diodes

We report the direct measurement of record fast switching speeds in GaN/AlN resonant tunneling diodes (RTDs). The devices, grown by plasma-assisted molecular-beam epitaxy, displayed three repeatable negative differential resistance (NDR) regions below a bias of +6 V. A room temperature peak-to-valley current ratio (PVCR) > 2 was observed, which represents a marked improvement over recent reports. Measurements carried out on hundreds of devices, of varying sizes, revealed a yield of ∼90%. Repeatability measurements consisting of 3000 sweeps resulted in a standard deviation, relative to the mean, of < 0.1%. Temperature dependent measurements combined with non-equilibrium Green's function based quantum transport simulations suggest the presence of both three-dimensional (3D) and two-dimensional (2D) emitters, giving rise to three NDR regions. Finally, a valley current density vs perimeter-to-area-ratio study indicates the presence of a surface leakage current mechanism, which reduces the PVCR.

Giant peak to valley ratio in a GaN based resonant tunnel diode with barrier width modulation

A barrier width modulated GaN based resonant tunnel diode is theoretically proposed which exhibits a giant peak to valley current ratio as high as 60 and a high negative differential conductance (NDC) of 1.77 × 106 S/cm2 with very low valley current density of 3 mA/cm2. This is achieved by the unique characteristic of the device current which monotonically decreases for applied voltages greater than the valley voltage in our simulation window. This is in contrast to all the other negative differential conductance based devices which experience an immediate exponential increase in current after the NDC region. The proposed device is also the first bidirectional tunneling diode which shows negative differential conductance for both polarity of the applied bias which is normally not observed with the conventional GaN/AlGaN double barrier structures due to the strong asymmetry arising from the internal electric fields due to polarization. The unique characteristics of the device can be attributed to the use of a modulated barrier width which is made possible by a polarization modulating InGaN layer and efficient utilization of internal electric fields in III-nitrides.

Optimum current‐voltage characteristics of GaAs/AlAs intraband microwave devices

In this reported work, both high peak current density and high peak-to-valley current ratio (PVCR) in AlAs/GaAs triple-barrier quantum-well intraband resonant tunnelling diodes (TBQW IRTDs) were accomplished. The valley current density (JV) of TBQW IRTDs can be degraded as the barrier thickness reaches 4 nm thickness. This result suggests narrow barrier thickness is due to the slight band bending effect in the conduction band of i -AlAs barrier layers. The peak current density (JP) of the TBQW IRTDs will be <2.43 KA/cm2 as the i -AlAs barriers thickness of the TBQW IRTD reaches 5 nm thickness. The optimal thickness of barrier layers should be selected at 4 nm thickness to obtain the highest PVCR value (about 4270), in which the sample with barrier layers of 4 nm thickness possesses the lowest valley current density.

Mapping Defect Density in MBE Grown &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;${\rm In}_{0.53}{\rm Ga}_{0.47}{\rm As}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Epitaxial Layers on Si Substrate Using Esaki Diode Valley Characteristics

Growing good quality III-V epitaxial layers on Si substrate is of utmost importance to produce next generation high-performance devices in a cost effective way. In this paper, using physical analysis and electrical measurements of Esaki diodes, fabricated using molecular beam epitaxy grown In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As layers on Si substrate, we show that the valley current density is strongly correlated with the underlying epi defect density. Such a strong correlation indicates that the valley characteristics can be used to monitor the epi quality. A model is proposed to explain the experimental observations and is validated using multiple temperature diode I-V data. An excess defect density is introduced within the device using electrical and mechanical stress, both of which are found to have a direct impact on the valley current with a negligible change in the peak current characteristics, qualitatively supporting the model predictions.

Tunnel-Junction-Limited Multijunction Solar Cell Performance Over Concentration

The simulation of tunnel junctions is performed by using nonlocal band-to-band and trap assisted tunneling models that are capable of reproducing the experimental current-voltage characteristics of p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> AlGaAs/ n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> AlGaAs and p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> AlGaAs/ n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> GaAs based devices. These simulated characteristics are then implemented within a lattice matched InGaP/(In)GaAs/Ge multijunction solar cell (MJSC) to assess the performance as a function of tunnel junction layer doping in the regime where the TJ limits the performance of the MJSC. At 500 suns, a 4.6% absolute drop in simulated efficiency is observed for an AlGaAs/GaAs bottom TJ corresponding to a degenerately p-doped layer of 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> compared to a TJ with a doping of 4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . A minimum p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> doping level of 3.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> is required in order to avoid bottom TJ limitation up to 1000 suns concentration for an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> doping of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> based on the calibrated models. Furthermore, the effects of the peak and valley current densities are shown to have a strong influence on the efficiency over concentration within the TJ limiting regime.

CURRENT–VOLTAGE MEASUREMENTS WITHIN THE NEGATIVE DIFFERENTIAL RESISTANCE REGION OF AlGaAs/AlGaAs TUNNEL JUNCTIONS FOR HIGH CONCENTRATION PHOTOVOLTAICS

The current–voltage characteristics of AlGaAs/AlGaAs tunnel junctions for use in multi-junction solar cells are studied experimentally, where tunneling current peaks of 1100 A/cm2 and specific contact resistivities of 0.3 × 10-4Ω⋅cm2 at 7 A/cm2 (typical concentrated photovoltaic operating current) are measured. This represents an ideal tunnel junction design, with a very low resistance and one of the highest tunneling peak currents reported for solar cells. Normally, solar cell current–voltage characteristics are measured using time-averaged methods, which, in this study, reveal a tunneling peak current density of ~950 A/cm2. Due to nonlinear oscillations within the measurement circuit, the precise locations and magnitudes of the tunneling peak and valley current densities are obscured when using time-average measurement methods. Here we present an alternative method to determine the tunneling peak current density, in which the nonlinear oscillations in the current and voltage are recorded over time and a current density–voltage curve is reconstructed. This time-dependent method results in a measured tunneling peak current density of ~ 1100 A/cm2. The nonlinear oscillations of the experimental circuit are reproduced by modeling an equivalent circuit, resulting in qualitative agreement with the observed oscillations. This model predicts the capacitance and inductance of the equivalent circuit to be approximately 3 nF and 3.5 μH, respectively. This numerical model can be used to determine the inductance and the capacitance of any circuit having a negative differential resistance region.

Detemining the Thickness of Barriers and Well of Resonance Tunneling Diodes by Specified I-V Characteristic

In this paper, a method of determining physical dimension of Double Barrier Quantum Well (DBQW) of Resonance Tunneling Diodes (RTDs) is presented by using I-V characteristic governing on them. In this procedure, first we have used performance metrics related to RTDs I-V characteristic such as Peak to Valley Current Ratio (PVCR), peak current density (JP), valley current density (JV) and Voltage Swing (VS), and by some other arbitrary points, we have fitted a curve to the RTD current-voltage equation by MATLAB software. Then we have obtained the physical parameter of I-V equation and adjusted some of them with modification coefficients. Next, by choosing the material of barriers and the well and amount of doping, we have calculated the thicknesses of both. To review the mentioned method, the experimental result of I-V characteristic of the sample structure DBQW is considered and we have come to this idea that the dimensions gained out of this method are highly correlated with those of the experimental sample.

Dependence of current–voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes on quantum well widths

This paper studies the dependence of I – V characteristics on quantum well widths in AlAs/In0.5<3Ga0.47As and AlAs/In0.53Ga0.47As/InAs resonant tunnelling structures grown on InP substrates. It shows that the peak and the valley current density in the negative differential resistance region are closely related with quantum well width. The measured peak current density, valley current densities and peak-to-valley current ratio of resonant tunnelling diodes are continually decreasing with increasing well width.

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47 As/In As resonant tunnelling diodes

This paper investigates the dependence of current-voltage characteristics of AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes (RTDs) on spacer layer thickness. It finds that the peak and the valley current density J in the negative differential resistance (NDR) region depends strongly on the thickness of the spacer layer. The measured peak to valley current ratio of RTDs studied here is shown to improve while the current density through RTDs decreases with increasing spacer layer thickness below a critical value.

Well-defined electrical properties of high-strain resonant interband tunneling structure

The GaAs/InAs high-strain resonant interband tunneling diodes (HSRITDs) have been implemented by metal organic chemical vapor deposition (MOCVD). The current–voltage characteristics of variable quantum well and barrier thickness grown on (111) GaAs substrates are investigated. Experimental results reveal that the quantum barrier and well layer will influence current–voltage properties such as the peak current density, valley current density, and peak-to-valley current ratio (PVCR). Both peak current and valley current density decrease with increasing layers width. This result also exhibits the variation of PVCR with layers width.

Low sidewall damage plasma etching using ICP-RIE with HBr chemistry of Si∕SiGe resonant interband tunnel diodes

The effect and influence of dry plasma etching processes of Si/SiGe using HBr for the formation of diode mesa structures has been investigated to minimise sidewall leakage current. To characterise sidewall damage electrically, Si-based resonant interband tunnel diodes (RITD) were processed and the completed RITDs compared by their peak-to-valley current ratio (PVCR) and valley current density (VCD), which are sensitive to defect related currents. Dry processed RITDs were compared to reference RITDs fabricated by wet chemical etching (HNO3:HF:H2O=100:1:100). The combination of HBr process gas and very low substrate bias power (10 W) for inductively coupled plasma reactive ion etching (ICP-RIE) yielded the better results. The resulting RITDs processed by ICP-RIE using HBr chemistry show high PVCR of 4.02 with VCD of 32 A/cm2 while wet etched RITDs show a PVCR of only 2.81 with VCD of 40 A/cm2. Hydrogen passivation during the HBr plasma process may play a role that overcomes the slightly higher surface roughness compared to wet etching.

High performance multiple stepped quantum well resonant microwave devices

Implemented by low-pressure metal organic chemical vapour deposition and fabricated by simple wet chemical etching, GaAs/In0.59Ga0.41As/InAs multiple stepped quantum well resonant interband tunnelling diodes (MSQW RITDs) are demonstrated for the first time. The multiple InAs well layers reduce the thermionic current. In particular, it is shown that the viability of the GaAs/In0.59Ga0.41As/InAs MSQW RITD with peak current density reaches over 1.11 kA/cm2, valley current density reaches less than 0.46 A/cm2, and therefore peak-to-valley ratio (PVCR) up to 2.4×103 at room temperature. The excellent negative differential resistance characteristics are explained by good confined effect of quantum well and interband tunnelling structure. The largest PVCR achieved is believed to the highest reported value to date for all two-terminal devices.

Monolithic AlAs-InGaAs-InGaP-GaAs HRT-FETS with PVCR of 960 at 300 K

This letter reports the significant N-shaped negative-differential resistance characteristics with three-terminal controllability of a monolithic heterostructure resonant tunneling field-effect transistor, realized by integrating the AlAs-In/sub 0.25/Ga/sub 0.75/As-AlAs double-barrier single-well resonant tunneling (RT) structure into the drain regime of the In/sub 0.49/Ga/sub 0.51/P-In/sub 0.25/Ga/sub 0.75/As-GaAs /spl delta/-high-electron mobility transistor. A peak-to-valley current ratio in excess of 960 at room temperature has been demonstrated, with a peak current density (J/sub p/) of 50.6 mA/mm, and a valley current density (J/sub v/) of 52.7 μA/mm, respectively, with a transistor gate length of 1.0 μm. The maximum current drive density was observed to be 478 A/mm-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Performance of single-junction and dual-junction InGaP∕GaAs solar cells under low concentration ratios

A study of the performance of single-junction InGaP∕GaAs and dual-junction InGaP∕GaAs tandem cells under low concentration ratios (up to 15 suns), before and after 1 MeV electron irradiation is presented. Analysis of the tunnel junction parameters under different concentrated light illuminations reveals that the peak current (JP) and valley current (JV) densities should be greater than the short-circuit current density (Jsc) for better performance. The tunnel junction behavior against light intensity improved after irradiation. This led to the suggestion that the peak current density (JP) and valley current density (JV) of the tunnel junction were enhanced after irradiation or the peak current was shifted to higher concentration. The recovery of the radiation damage under concentrated light illumination conditions suggests that the performance of the InGaP∕GaAs tandem solar cell can be enhanced even under low concentration ratios.

Experimental Investigation of the Temperature Dependence of InAs–AlSb–GaSb Resonant Interband Tunnel Diodes

The temperature dependence of the current-voltage (I-V) characteristics of InAs-AlSb-GaSb resonant interband tunnel diodes (RITDs) has been investigated from 223 to 423 K. Several device structures were examined, with tunnel barrier thicknesses from 0.6 to 2.0 nm. For all barrier thicknesses, the peak current density (J/sub p/) decreases slightly as the temperature is increased, while the valley current density (J/sub v/) increases with temperature. We found that the temperature rate of change for the peak current density with temperature (/spl part/J/sub p///spl part/T) is nearly independent of barrier thickness, while J/sub v/ increases more rapidly for devices with thicker barriers. In addition, the peak voltage (V/sub p/) was found to be independent of temperature, regardless of barrier thickness. However, the valley voltage (V/sub v/) was observed to decrease with increasing temperature, with more rapid changes observed for RITDs with thicker barriers. Comparison of the temperature performance of RITDs with different barrier thicknesses shows that devices with thinner barriers have I-V characteristics that are less sensitive to temperature, as well as having larger peak and valley current density and voltage for the temperature range from 223 to 423 K. To our knowledge, this is the first report of the temperature dependence of the device characteristics of Sb-based RITDs as a function of barrier width.

Annealing of defect density and excess currents in Si-based tunnel diodes grown by low-temperature molecular-beam epitaxy

Deep-level transient spectroscopy (DLTS) measurements were performed in order to investigate the effects of post-growth heat treatment on deep level defects in Si layers grown by low-temperature molecular-beam epitaxy (LT-MBE) at 320 °C. In the LT-MBE as-grown samples, two dominant divacancy-related complex defects, of which the possible origins are suggested as P–V (E center)+V–V (0/−) and V–V (−2/−) and others, were observed in P-doped n layers. When the as-grown samples were annealed at 700, 800, and 900 °C for 60 s by rapid thermal annealing, the total density of defects were decreased without generating other defects and most defects were annihilated at 900 °C. This study also compared the DLTS trends with performance of Si-based resonant interband tunnel diodes (RITDs) in terms of peak current density, valley current density, and peak-to-valley current ratio, which are closely related to the deep-level defects. The active regions of the RITDs were grown at the same substrate growth temperature and annealed at similar temperatures used in this DLTS study.

High field nonlinearities and current density profiles in off-axis silicon

An off-axis device simulator is developed and used to model the distribution of electrons in a thin off-axis sample of p-type silicon. The results from the simulator are validated by matching with the results from an existing analytical model in the low field limit. At high fields, accumulation of electrons can be observed at both sample surfaces causing surface inversion layers there. The individual electron valley current densities are also calculated. It is found that at low values of applied field, the diffusion components govern the current density trends, whereas at high fields the drift components dominate.

Modelling of the current-voltage characteristics of InAs/AlGaSb/GaSb double barrier diodes with interband resonant tunnelling

Previously we developed a model for InAs/AlSb/GaSb resonant tunnelling structures (RTSs) and investigated the mechanisms of peak and valley current in these diodes, which exhibit high values of peak-to-valley (P/V) current ratio. Good quantitative agreement with the experiment was achieved both for the values of peak and valley current density for the first time. Here we show that the P/V current ratio can be essentially enlarged by employing the AlGaSb material instead of AlSb. The calculated current-voltage (I-V) characteristic of the InAs/AlGaSb/GaSb RTSs with 30 and 20 Å barriers and 65 Å quantum well showed a P/V current ratio of about 800 at lattice temperature T = 77 K. This value is much greater than all known experimental values of P/V current ratio for RTSs until now.