Development of a simple two-step lithography fabrication process for resonant tunneling diode using air-bridge technology

This article reports on the development of a simple two-step lithography process for double barrier quantum well(DBQW)InGaAs/AlAs resonant tunneling diode(RTD)on a semi-insulating indium phosphide(InP)substrate using an air-bridge technology.This approach minimizes processing steps,and therefore the processing time as well as the required resources.It is particularly suited for material qualification of new epitaxial layer designs.A DC performance comparison between the proposed process and the conventional process shows approximately the same results.We expect that this novel technique will aid in the recent and continuing rapid advances in RTD technology.

Fabrication of an AlAs/In_(0.53)Ga_ ( 0.47)As/InAs Resonant Tunneling Diode on InP Substrate for High-Speed Circuit Applications

A high performance AlAs/In0.53 Ga0.47 As/InAs resonant tunneling diode （RTD） on InP substrate is fabricated by inductively coupled plasma etching. This RTD has a peak-to-valley current ratio （PVCR） of 7. 57 and a peak current density Jp = 39.08kA/cm^2 under forward bias at room temperature. Under reverse bias, the corresponding values are 7.93 and 34.56kA/cm^2 . A resistive cutoff frequency of 18.75GHz is obtained with the effect of a parasitic probe pad and wire. The slightly asymmetrical current-voltage characteristics with a nominally symmetrical structure are also discussed.

Design and Realization of Resonant Tunneling Diodes with New Material Structure

A new material structure with Al 0.22Ga 0.78As/In 0.15Ga 0.85As/GaAs emitter spacer layer and GaAs/In 0.15- Ga 0.85As/GaAs well for resonant tunneling diodes is designed and the corresponding device is fabricated.RTDs DC characteristics are measured at room temperature. Peak-to-valley current ratio and the available current density for RTDs at room temperature are computed.Analysis on these results suggests that adjusting material structure and optimizing fabrication processes will be an effective means to improve the quality of RTDs.

Comparison and Analysis of Two Microwave Equivalent-Circuit Models for Resonant Tunneling Diode

The distinction between two microwave equivalent-circuit models,quasi Esaki tunneling model (QETM) and quantum well injection transit model (QWITM),for the resonant tunneling diode (RTD) is discussed in details,and two groups of circuit parameters are extracted from experiment data by the least square fit method.Both theory analysis and the comparison of fit results demonstrate that QWITM is much more precise than QETM.In addition,the rationality of QWITM circuit's parameters confirms it too.On this basis,the resistive frequency is calculated,whose influence factors and improvement method are simply discussed as well.

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

AlN/GaN resonant tunneling diodes(RTDs)were grown separately on freestanding Ga N(FS-GaN)substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy(PA-MBE).Room temperature negative differential resistance(NDR)was obtained under forward bias for the RTDs grown on FS-GaN substrates,with the peak current densities(Jp)of 175-700 kA/cm^(2)and peak-to-valley current ratios(PVCRs)of 1.01-1.21.Two resonant peaks were also observed for some RTDs at room temperature.The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically,showing that lower dislocation densities,flatter surface morphology,and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.

SWITCHING CHARACTERISTICS AND ANALYSIS OF RESONANT TUNNELING DIODES

Resonant tunneling diode （RTD） of AlAs/InGaAs/AlAs double barrier-single well structure was designed and fabricated. The devices showed current-voltage characteristics with peak-valley current ratio of 4 ： 1 at room temperature. The scattering parameter of RTD was measured by using an HP8510（C） network analyzer. Equivalent circuit parameters were obtained by curve fitting and optimized. The RTD switching time was estimated using the measured capacitance and average negative differential resistance. The minimum rise time of the sample was estimated to be 21 ps.

Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes

In this work,the electronic properties of resonant tunneling diodes（RTDs） based on GaN-AlxGa（1-x）N double barriers are investigated by using the non-equilibrium Green functions formalism（NEG）.These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field,which greatly affect the electronic transport properties.The electronic density,the transmission coefficient,and the current–voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations.The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness,AlxGa1-xN width,and the aluminum concentration xAl.The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier;it exhibits a series of resonant peaks and valleys as the quantum well width increases.In addition,it is found that the negative differential resistance（NDR） in the current–voltage（I–V） characteristic strongly depends on aluminum concentration xAl.It is shown that the peak-to-valley ratio（PVR） increases with xAlvalue decreasing.These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors.

Spin injection in a ferromagnet/resonant tunneling diode heterostructure

The spin transport property of a ferromagnet （FM）/insulator （Ⅰ）/resonant tunneling diode （RTD） heterostructure was studied. The transmission coefficient and spin polarization in a multilayered heterostructure was calculated by a Schrdinger wave equation. An Airy function formalism approach was used to solve this equation. Based on the transfer matrix approach,the transmittivity of the structure was determined as a function of the Feimi energy and other parameters. The result shows that the spin polarization induced by the structure oscillates with the increasing Fermi energy of the FM layer. While the thickness of the RTD is reduced,the resonant peaks become broad. In the heterostructure,the spin polarization reaches as high as 40% and can be easily controlled by the external bias voltage.

Monolithic Integration of GaAs-Based Resonant Tunneling Diode and High Electron Mobility Transistor

The resonant tunneling diode (RTD) is a kind of novel ultra-high speed and ultra-high frequency negative differential resistance nanoelectronic device. Integration of RTD and other three-terminal compound semiconductor devices is one important direction of high speed integrated circuit development. In this paper, monolithic integration technology of RTD and high electron mobility transistor (HEMT) based on GaAs substrate was discussed. A top-RTD and bottom-HEMT material structure was proposed and epitaxyed. Based on wet chemical etching, electron beam lithography, metal lift-off and air bridge technology, RTD and HEMT were fabricated on the same wafer. The peak-to-valley current ratio of RTD is 4 and the peak voltage is 0.5 V. The maximal transconductance is 120 mS/mm for a 0.25 μm gate length depletion mode HEMT. Current levels of two devices are basically suited. The results validate the feasibility of the designed integration process.

Generation and Shaping of Soliton-Like Pulses along Resonant Tunneling Diodes NMOS Varactors Lattice

The characteristics of N-type accumulation-mode MOS (NMOS) varactors line periodically loaded with resonant tunneling diodes (RTDs) are used for soliton-like pulses generation and shaping. The problem of wide pulse breaking up into multiple pulses rather than a single is solved. Applying perturbative analysis, we show that the dynamics of the nonlinear transmission line (NLTL) is reduced to expanded Korteweg-de Vries (KdV) equation. Moreover, numerical integration of nonlinear differential and difference equations that result from the mathematical analysis of the line is discussed. As results, NLTL can simultaneously sharpen both leading and trailing of pulse edges and one could obtain a rising and sharpening step pulse.

Design of ternary D flip-flop with pre-set and pre-reset functions based on resonant tunneling diode literal circuit

The problems existing in the binary logic system and the advantages of multiple-valued logic (MVL) are introduced. A literal circuit with three-track-output structure is created based on resonant tunneling diodes (RTDs) and it has the most basic memory function. A ternary RTD D flip-flop with pre-set and pre-reset functions is also designed, the key module of which is the RTD literal circuit. Two types of output structure of the ternary RTD D flip-flop are optional: one is three-track and the other is single-track; these two structures can be transformed conveniently by merely adding tri-valued RTD NAND, NOR, and inverter units after the three-track output. The design is verified by simulation. Ternary flip-flop consists of an RTD literal circuit and it not only is easy to understand and implement but also provides a solution for the algebraic interface between the multiple-valued logic and the binary logic. The method can also be used for design of other types of multiple-valued RTD flip-flop circuits.

Improving the peak current density of resonant tunneling diode based on InP substrate

Resonant tunneling diodes（RTD）have the potential for compact and coherent terahertz（THz）sources operating at room temperature,but their low output power severely restricts their application in THz frequency range.In this paper,two methods are adopted to increase the peak current of RTD for enhancing its output power.First,different metal contact systems（including Pt/Ti/Pt/Au and Au Ge/Ni/Au）for RTD contact are introduced,and a higher current of RTD with Pt/Ti/Pt/Au contact demonstrates the superior contact characteristic of Pt/Ti/Pt/Au contact system.Second,the double barrier structure（DBS）of RTD is well designed to further improve the characteristic of RTD,and a high peak current of 154 kA/cm^2 is achieved at room temperature.The improved peak current is very beneficial for increasing the output power of RTD oscillator.

A novel ternary JK flip-flop using the resonant tunneling diode literal circuit

A literal circuit with a three-track-output structure is presented based on resonant tunneling diodes(RTDs).It can be transformed conveniently into a single-track-output structure according to the definition and properties of the literal operation.A ternary resonant tunneling JK flip-flop is created based on the RTD literal circuit and the module-3 operation,and the JK flip-flop also has two optional types of output structure.The design of the ternary RTD JK flip-flop is verified by simulation.The RTD literal circuit is the key design component for achieving various types of multi-valued logic(MVL) flip-flops.It can be converted into ternary D and JK flip-flops,and the ternary JK flip-flop can also be converted simply and conveniently into ternary D and ternary T flip-flops when the input signals satisfy certain logical relationships.All these types of flip-flops can be realized using the traditional Karnaugh maps combined with the literal and module-3 operations.This approach offers a novel design method for MVL resonant tunneling flip-flop circuits.

High peak-to-valley current ratio In_(0.53)Ga_(0.47)As/AlAs resonant tunneling diode with a high doping emitter

An In0.53Ga0.47As/AlAs resonant tunneling diode （RTD） with a high doping emitter is designed and fabricated using air bridge technology. The RTD exhibits a high peak-to-valley current ratio （PVCR） of more than 40 at room temperature, with a peak current density of 24 kA/cm2. The extraction of device parameters from DC and microwave measurements is presented together with an RTD equivalent circuit. The high PVCR RTD with small intrinsic capacitance is favorable for microwave/THz applications.

A novel micro-accelerometer with adjustable sensitivity based on resonant tunnelling diodes

Resonant tunnelling diodes （RTDs） have negative differential resistance effect, and the current-voltage characteristics change as a function of external stress, which is regarded as mesc-piezoresistance effect of RTDs. In this paper, a novel micro-accelerometer based on AlAs/GaAs/In0.1Ga0.9As/GaAs/AlAs RTDs is designed and fabricated to be a four-beam-mass structure, and an RTD-Wheatstone bridge measurement system is established to test the basic properties of this novel accelerometer. According to the experimental results, the sensitivity of the RTD based micro-accelerometer is adjustable within a range of 3 orders when the bias voltage of the sensor changes. The largest sensitivity of this RTD based miero-accelerometer is 560.2025 mV/g which is about 10 times larger than that of silicon based micro piezoresistive accelerometer, while the smallest one is 1.49135 mV/g.

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In_(0.53)Ga_(0.47)As/InAs 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.

Nanoelectronic devices resonant tunnelling diodes grown on InP substrates by molecular beam epitaxy with peak to valley current ratio of 17 at room temperature

This paper reports that InAs/In0.53Ga0.47As/AlAs resonant tunnelling diodes have been grown on InP substrates by molecular beam epitaxy. Peak to valley current ratio of these devices is 17 at 300K. A peak current density of 3kA/cm^2 has been obtained for diodes with AlAs barriers of ten monolayers, and an Ino.53Ga0.47As well of eight monolayers with four monolayers of InAs insert layer. The effects of growth interruption for smoothing potential barrier interfaces have been investigated by high resolution transmission electron microscope.

A bistable, self-latching inverter by the monolithic integration of resonant tunnelling diode and high electron mobility transistor

This paper reports that the structures of AlGaAs/InGaAs high electron mobility transistor （HEMT） and AlAs/GaAs resonant tunnelling diode （RTD） are epitaxially grown by molecular beam epitaxy （MBE） in turn on a GaAs substrate. An Alo.24Gao.76As chair barrier layer, which is grown adjacent to the top AlAs barrier, helps to reduce the valley current of RTD. The peak-to-valley current ratio of fabricated RTD is 4.8 and the transconductance for the 1-μm gate HEMT is 125mS/mm. A static inverter which consists of two RTDs and a HEMT is designed and fabricated. Unlike a conventional CMOS inverter, the novel inverter exhibits self-latching property.

Dependence of current-voltage characteristics of pseudomorphic AlAs/In_(0.53)Ga_(0.47)As/InAs resonant tunnelling diodes on quantum well widths

This paper studies the dependence of I - V characteristics on quantum well widths in AIAs/In0.53Ga0.47As and AIAs/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.

A switchable high-sensitivity strain sensor based on piezotronic resonant tunneling junctions

Developing emerging technologies in Internet of Things and artificial intelligence requires high-speed, low-power, high-sensitivity, and switchable-functionality strain sensors capable of sensing subtle mechanical stimuli in complex ambience. Resonant tunneling diodes (RTDs) are the good candidate for such sensing applications due to the ultrafast transport process, lower tunneling current, and negative differential resistance. However, notably enhancing sensing sensitivity remains one of the greatest challenges for RTD-related strain sensors. Here, we use piezotronic effect to improve sensing performance of strain sensors in double-barrier ZnO nanowire RTDs. This strain sensor not only possesses an ultrahigh gauge factor (GF) 390 GPa^(−1), two orders of magnitude higher than these reported RTD-based strain sensors, but also can switch the sensitivity with a GF ratio of 160 by adjusting bias voltage in a small range of 0.2 V. By employing Landauer–Büttiker quantum transport theory, we uncover two primary factors governing piezotronic modulation of resonant tunneling transport, i.e., the strain-mediated polarization field for manipulation of quantized subband levels, and the interfacial polarization charges for adjustment of space charge region. These two mechanisms enable strain to induce the negative differential resistance, amplify the peak-valley current ratio, and diminish the resonant bias voltage. These performances can be engineered by the regulation of bias voltage, temperature, and device architectures. Moreover, a strain sensor capable of electrically switching sensing performance within sensitive and insensitive regimes is proposed. This study not only offers a deep insight into piezotronic modulation of resonant tunneling physics, but also advances the RTD towards highly sensitive and multifunctional sensor applications.