Asymmetric Spacer Layer Research Articles

Low-Cost Compact Integrated Rectenna for Implantable Medical Receivers

This work describes a novel fully integrated rectenna circuit using tunnelling-based devices for implanted medical devices. An ASPAT (Asymmetric Spacer Layer Tunnel Diode) was used as the active rectifier due to its high non-linearity and temperature insensitivity features. A miniaturized geometry rectenna ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1 \times 5$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) with improved matching characteristics was demonstrated, by integrating a Cockcroft-Walton rectifier with an L-shaped planar folded antenna structure operating at ISM frequency bands. The circuit performance was experimentally explored at various separation distance between transmitter and receiver units. For a 5cm transmission set-up, the rectenna with a single-stage rectifier delivered 0.8V output at 20dBm transmit power. An extended doubler configuration exhibited enhanced performance when multiple stages are used, is predicted to reach 0.24mW output power at 23dBm transmit power and yielding ~1.6V output voltage with an efficiency of 0.12%. These findings can assist in compensating for the degraded antenna gain attributed to the extremely small effective-radiating area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.04\lambda $ </tex-math></inline-formula> . Furthermore, the ability of controlling the antenna input impedance helps in circumventing the requirement for a matching circuitry thereby offering further reduction in chip size.

Design and analysis of GaAs/AlAs asymmetric spacer layer tunnel diodes for high-frequency detection

Abstract In the present work, we have systematically investigated the design of Asymmetric SPAcer-layer Tunnel (ASPAT) diodes using numerical modeling employing SILVACO Atlas software. After reproducing and validating physical models using experimental data obtained from fabricated ASPAT diodes, a developed SILVACO model was used to evaluate the effect of different structure designs on two key parameters, namely the junction resistance and curvature coefficient at zero bias. The impact of the variation in the spacer layers thicknesses, emitter, collector, their doping profile, and the barrier width was thoroughly explored. It was found that the spacers and the barrier thickness are the most critical design parameters affecting the junction resistance and curvature coefficient. The insertion of an InxGa1-xAs QW layer adjacent to the AlAs barrier, combined with the appropriate choice of spacers and AlAs barrier thicknesses, contributed to an increase in the curvature coefficient by a factor of three while maintaining a low junction resistance. The cut-off frequency of a 4 × 4 μm2 ASPAT device is predicted to be > 500 GHz at zero bias. A comprehensive study is presented to guide researchers working on ASPAT devices, providing valuable prediction for appropriate design parameters toward maximizing the microwave/mm-wave device performance.

Novel barrier-well heterostructure diodes for microwave and mm-wave applications

We present two novel barrier-well heterostructure diodes, for use as zero bias detectors in microwave and mm-wave applications. One based on the GaAs platform and one based on In0.53Ga0.47As lattice matched to InP. This is achieved by adding quantum wells to Asymmetric Spacer Layer Tunnel (ASPAT) diodes next to the barrier. The DC characteristics of these new diodes were simulated in SILVACO Atlas TCAD software using experimentally validated physical models based upon conventional ASPAT diodes. The highest extracted curvature coefficients of these diodes at zero bias were 33 V−1 and 35 V−1 for GaAs and In0.53Ga0.47As based structures respectively. A C-V analysis was performed, and it was found that the addition of quantum wells reduced the zero-bias capacitance of these devices when compared with the standard ASPAT diode. The new diodes exhibited estimated cut-off frequencies of 532 GHz and 800 GHz for the GaAs and In0.53Ga0.47As based structures respectively. The lower cut off frequencies for the In0.18Ga0.82As/AlAs/GaAs devices is largely due to their inherently higher series resistance

InGaAs/AlAs/GaAs metamorphic asymmetric spacer layer tunnel (mASPAT) diodes for microwaves and millimeter-waves detection

We present work on a novel In0.53Ga0.47As/AlAs metamorphic asymmetric spacer layer tunnel (mASPAT) diode structure, which was grown on GaAs by solid source molecular beam epitaxy. mASPAT diodes with different mesa sizes were fabricated and tested following growth under optimal conditions. The measured I–V characteristics of these tunneling devices showed rectifying behavior resulting from the asymmetric design of the epitaxial spacer layers. The extracted curvature coefficient, junction resistance, and leakage currents at −1 V resulted in an estimated theoretical cut-off frequency ft at zero bias exceeding 180 GHz for 4 × 4 μm2 mesa devices. The obtained results demonstrate the potential use of mASPAT devices on GaAs as a low-cost alternative to devices fabricated on InP substrates for high-volume zero bias microwave and millimeter-wave detectors.

Microwave tunnel diodes are not yet manufacturable

We show that heterojunction tunnel structures, as exemplified by the asymmetric spacer layer tunnel (ASPAT) diode, and grown by modern epitaxial techniques, are not yet manufacturable. The growth tolerances required to produced devices to a specified I–V characteristic are too tight. The degree of uniformity is too demanding for maintaining the characteristics within an acceptable spread for viable commercial manufacture. The methods of routine characterisation of as-grown epilayers are inadequate.

Effects of spacer layers on the Wigner function simulation of resonant tunneling diodes

The effects of spacer layer width and asymmetry on the simulation of quantum transport in resonant tunneling diodes are studied. The results show that these layers significantly influence the I–V characteristic, which presents important differences under direct or reverse bias polarity in devices with asymmetric spacer layers. These differences are interpreted in terms of potential profile comparisons of the simulated structures and are in qualitative agreement with experimental data.

Scattering parameter measurements of resonant tunneling diodes up to 40 GHz

The scattering parameters (S parameters) of double barrier quantum well resonant tunneling diodes have been measured at various biases with on-wafer probing techniques. Impedances up to 40 GHz for AlGaAs/GaAs diodes with asymmetric spacer layers were obtained. It was found that the impedances could be accurately described by the lumped equivalent circuit representation. With the conductance-voltage characteristic derived from high frequency S parameter measurement, the portions of current-voltage curve that were distorted by oscillation in the dc measurement are recovered. Peaks corresponding to the process of electrons discharging from the quantum well are found in the capacitance-voltage (C-V) characteristic.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Capacitance and hysteresis study of AlAs/GaAs resonant tunneling diode with asymmetric spacer layers

By measuring scattering parameters, the impedance of an AlAs/GaAs double barrier resonant tunneling diode with asymmetric spacer layers has been obtained up to 40 GHz. It is found that the impedance at various biases can be described well by the lumped equivalent circuit representation. A hysteresis is observed in the impedance measurement in the negative differential resistance region. This hysteresis, unlike the one observed in the dc current-voltage measurement, is not distorted by oscillations and it may be attributed to the load-line effect due to the internal series resistance. Self-consistent calculations have been used to predict the capacitance-voltage curve derived from the impedance measurements. Peaks in capacitance found in the negative differential resistance region are accompanied by a smaller peak. Calculation indicates that the smaller peak is due to electrons discharging from the accumulation region in the undoped region adjacent to the emitter barrier.

Capacitance of resonant tunneling diodes with spacer layers

Self-consistent calculations have been performed on a double barrier resonant tunneling diode (RTD) with asymmetric spacer layers under various biases. The quasi-static state charge distribution in the GaAsAl 0.3Ga 0.7As RTD was studied and the capacitance derived. The theoretical capacitance-voltage ( C- V) characteristic agrees qualitatively with experimental results. In contrast to the general belief that charge accumulation in the quantum well results in the observed peak in capacitance meaurements, our results reveal that the peak is induced by the quantum well discharging process. Comparison between the theoretical and experimental results also supports the suggestion that scattering and localization effects are important in the device. Conditions for optimizing the C- V characteristic for high frequency operations are discussed.

Microwave impedance measurements on resonant tunnelin diodes with asymmetric spacer layers

Scattering parameters ( S parameters) have been measured up to 40GHz on GaAs/AlAs resonant tunneling diodes containing asymmetric spacer layers. On-wafer microwave probing techniques were used. A hysteresis was observed in the vicinity of the negative differential resistance (NDR) region. Unlike the one observed in dc current-voltage measurements, this hysteresis was not affected by oscillations and believed to be intrinsic. The impedance data has been fitted to a lumped equivalent circuit model and capacitance-voltage ( C-V) characteristic extracted. In addition to the already reported capacitance peak in the NDR region, a smaller peak at a lower voltage was found. In comparison to the self-consistent calculation, the smaller peak is due to the electrons discharging from accumulation region between the emitter barrier and the cathode spacer layer. The C-V result agrees qualitatively with the theoretical calculation. It supports the theoretical argument that the negative conductance can be increased by increasing the cathode spacer layer, however, there is also an increase in capacitance and the cutoff frequency may be reduced.

Transport in Al xGa 1−xAs/In yGa 1−yAs resonant tunnelling diodes with asymmetric layers

Al xGa 1−xAs/In yGa 1−yAs resonant tunnelling diodes (RTDs) grown by molecular beam epitaxy with symmetric and asymmetric spacer layers have been fabricated and studied by electric and magnetic field measurements. Pseudomorphic (pm) Al 0.35 Ga 0.65As/In 0.1Ga 0.9As RTDs with asymmetric spacer layers exhibit novel tunneling phenomena depending upon the bias direction. Tunneling is through the ground state energy of the In 0.1Ga 0.9As well when the thick spacer layer is in the leading edge of the device while it is through the first excited state of the In 0.1Ga 0.9As well when the thick spacer layer is in the trailing edge of the diode. Shubnikov-DeHaas oscillations obtained from the pm-RTDs of asymmetric spacer layers also show different features depending upon the bias direction. Improved device performance is observed when a thick spacer layer is on the substrate side rather than the top contact side, which is evidence of silicon dopant atom outdiffusion during molecular beam epitaxial growth.

Designing resonant tunneling structures for increased peak current density

We report a simple method to increase the peak current density in a double-barrier resonant tunneling structure by using a small band-gap emitter spacer layer. We have fabricated AlAs/GaAs/AlAs resonant tunneling structures using a Ga1−xInxAs spacer layer in the emitter. The peak current density was increased systematically with the increasing indium content by a factor ranging from 2.5 at x=0.05 to 5 at x=0.2 from the value at x=0. The increased peak current density was accompanied by increases in the peak voltage and peak-to-valley ratio. The lowering of the bottom of electron energy distribution in the GaInAs emitter spacer layer is shown to explain both the peak current and voltage increases. In the sequential tunneling picture, the peak current increases because of the increase in the number of resonant electrons which occurs if the bottom of energy distribution is lowered. We also report for the first time a strong bistability in the current-voltage characteristics at T≤240 K in the asymmetric spacer layer structure (GaInAs emitter, GaAs collector) in an otherwise symmetric structure.

Tunnel diode with asymmetric spacer layers for use as microwave detector

A novel single-barrier, intraband tunnel diodes with asymmetric spacer layers has been designed and fabricated. The structural asymmetry leads to asymmetric current-voltage characteristics and makes the diodes suitable for use as zero-bias microwave detectors. The voltage sensitivity as 9.375 GHz shows a weak temperature dependence (typically less than 1 dB variation over − 40° –+ 80°C, compared with 3 dB for a zero-bias Schottky diode). The maximum microwave power handling is also superior to a germanium back diode (1 dB rolloff typically at + 10 dBm compared with –10 dBm for the Ge back diode) and similar to a zero-bias Schottky diode.