Fabio Quaranta

Capacitive RF MEMS Switches With Tantalum-Based Materials

In this paper, shunt capacitive RF microelectromechanical systems (MEMS) switches are developed in III-V technology using tantalum nitride (TaN) and tantalum pentoxide (Ta2O5) for the actuation lines and the dielectric layers, respectively. A compositional, structural, and electrical characterization of the TaN and Ta2O5 films is preliminarily performed, demonstrating that they are valid alternatives to the conventional materials used in III-V technology for RF MEMS switches. Specifically, it is found that the TaN film resistivity can be tuned from 0.01 to 30 Ω · cm by changing the deposition parameters. On the other hand, dielectric Ta2O5 films show a low leakage current density of few nanoamperes per square centimeter for E ~ 1 MV/cm, a high breakdown field of 4 MV/cm, and a high dielectric constant of 32. The realized switches show good actuation voltages, in the range of 15-20 V, an insertion loss better than -0.8 dB up to 30 GHz, and an isolation of ~-40 dB at the resonant frequency, which is, according to bridge length, between 15 and 30 GHz. A comparison between the measured S-parameter values and the results of a circuit simulation is also presented and discussed, providing useful information on the operation of the fabricated switches.

Reliability Enhancement by Suitable Actuation Waveforms for Capacitive RF MEMS Switches in III–V Technology

In this paper, the reliability of shunt capacitive radio frequency microelectromechanical systems switches developed on GaAs substrate using a III-V technology fabrication process, which is fully compatible with standard monolithic microwave integrated circuit fabrication, is investigated. A comprehensive cycling test is carried out under the application of different unipolar and bipolar polarization waveforms in order to infer how the reliability of the realized capacitive switches, which is still limited with respect to the silicon-based devices due to the less consolidation of the III-V technology, can be improved. Under the application of unipolar waveforms, the switches show a short lifetime and a no correct deactuation for positive pulses longer than 10 ms probably due to the charging phenomena occurring in the dielectric layer underneath the moveable membrane. These charging effects are found to vanish under the application of a waveform including consecutive positive and negative voltage pulses, provided that proper durations of the positive and negative voltage pulses are used. Specifically, a correct switch deactuation and a lifetime longer than 1 million cycles, being this value limited by the duration of the used testing excitation, are achieved by applying a 1-kHz waveform with 20-μs-long positive and negative consecutive pulses.

Resonant-cavity-enhanced heterostructure metal–semiconductor–metal photodetector

We report a GaAs-based high-speed, resonant-cavity-enhanced, heterostructure metal–semiconductor–metal photodetector with Al0.24Ga0.76As/Al0.9Ga0.1As distributed Bragg reflector operating around 850 nm. The photocurrent spectrum shows a clear peak at this wavelength with full width at half maximum (FWHM) of around 30 nm. At resonance wavelength, a seven-fold increase can be achieved in quantum efficiency compared to a detector of the same absorption depth. The top reflector is a delta modulation doped Al0.24Ga0.76As that also acts as the barrier enhancement layer thus providing very low dark current values. The breakdown voltage is above 20 V. Time response measurements show rise time, fall time, and FWHM of 8.8 ps, 9 ps, and 8.1 ps, respectively, giving a 3-dB bandwidth of about 33 GHz. Combination of low dark current, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device especially suitable for short haul communications purposes.

Electron cloud effect on current injection across a Schottky contact

The electron cloud that is formed in the narrow gap material in a modulation-doped heterostructure affects the Schottky contact made to the wide gap material. It also influences absorption and collection of the optically generated carriers. Photocurrent spectra, current–voltage, and current–temperature measurements show that the increase in electron cloud density decreases dark current flow while increasing photoresponsivity. We propose that the Coulombic interaction between the confined electron cloud and the emitted electrons from metal to the wide gap material increases the barrier height. The electric field in the direction of growth due to modulation doping accounts for the increase in photoelectron collection efficiency. Implementation of this effect increases efficiency of photodetectors while, simultaneously, reducing the noise due to dark current.

Optically Modulated High-Sensitivity Heterostructure Varactor

A novel optically modulated high-sensitivity heterostructure varactor, demonstrated as a strong candidate for high-order frequency-multiplier applications, is reported. The device is a delta modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts on the top. The capacitance-voltage (C-V) measurements show a C max/Cmin ratio up to 113 and an extremely high nonlinearity during the transition from high to low capacitance with sensitivity of up to 35. These results are one of the best obtained so far among similar structure devices. In addition, optoelectronic experimental results demonstrate that the slope of the C-V relationship can be modulated by the intensity of the incident optical power. A model describing the source of the reported C-V results is proposed along with the simulation results verifying the observed C-V behavior

Photodetectors based on heterostructures for opto-electronic applications

In this paper, we present four photodetector devices that have the benefit of compatibility with established high electron-mobility transistor technology and are, thus, more conducive to monolithic integration with high-speed opto-electronic integrated circuitry. These AlGaAs-GaAs heterojunction-based planar devices all use the wide-gap material to enhance the Schottky barrier height between metal and semiconductor. We show that doping of this layer produces an internal electric field that aids in the transport and collection of photoelectrons. Addition of a resonant optical cavity by means of a distributed Bragg reflector reduces the required thickness of the absorption layer, thus achieving good responsivity and high speed, as well as wavelength selectivity. Current-voltage, current-temperature, photocurrent spectra, high-speed time response, and on-wafer frequency-domain measurements are presented, which point out that the often contradictory requirements of responsivity, noise, and speed may be addressed by proper engineering of the internal electric field and optical properties. Numerical simulations are performed to describe internal electric and optical behavior and a small-signal model based on frequency-domain data is extracted in order to facilitate photoreceiver design. The low dark current, in tens of femtoamps per square micrometer, full-width at half-maximum time responses below 10 ps, and high bandwidth in tens of gigahertz, make these devices of interest for applications ranging from optical communications to imaging systems.

A highly tunable heterostructure metal-semiconductor-metal capacitor utilizing embedded 2-dimensional charge

We report on a variable capacitor that is formed between Schottky contacts and the two dimensional electron gas (2DEG) in a planar metal-semiconductor-metal structure. Device capacitance at low bias is twice the series capacitance of anode and cathode, enhancing to a maximum value, Cmax, at a threshold voltage, before reaching a minimum, Cmin, lower than the geometric capacitance of the coplanar contacts, thus resulting in ultra high Cmax/Cmin tuning ratio. Sensitivity, the normalized change of capacitance with voltage, is also very large. The dense reservoir of the 2DEG charge maintained between contacts is shown to be responsible for this remarkable performance.

An Unconventional Hybrid Variable Capacitor With a 2-D Electron Gas

Moderation of internal quantum mechanical energies, such as exchange energy of an unconventional contact, comprised of a system of 2-D charge carriers, improves performance merits of variable capacitors, varactors, mainly in tuning ratio (TR), and sensitivity, S. Energy transfer from the unconventional contact to the dielectric increases the energy density and enhances the capacitance of the varactor. Here, we analyze the performance of an unconventional varactor based on a planar metal-semiconductor-metal (MSM) structure with an embedded layer of high-density 2-D electron gas (2DEG). Through localized field-assisted manipulation of the 2DEG density, a twice larger equilibrium capacitance and a minimum capacitance, less than the geometric capacitance of a conventional MSM, are achieved. Moreover, the maximum capacitance increases through a Batman-shaped capacitance enhancement at a threshold voltage. Therefore, giant is attained while maintaining quality factors of up to 30. Capacitance-voltage characteristics exhibit a switched-capacitor behavior with S as high as 350 that is due to localized transitions from a dense 2DEG to a complete depletion. This MSM 2-D varactor combines the unconventional features of 2DEG with superior electrical properties of MSMs.

Time Response of Two-Dimensional Gas-Based Vertical Field Metal–Semiconductor–Metal Photodetectors

We have fabricated and characterized 2-D gas-based, including 2-D electron gas (2DEG) and 2-D hole gas (2DHG), heterostructure metal-semiconductor-metal (MSM) photodetectors on GaAs. Both the high-speed measurement of time response and the simulation results show that a vertical field developed in the active absorption region due to the delta-doping layer facilitates one type of photogenerated carrier transport. In addition, the confined carriers facilitate collection of the optically generated carriers that reach them. The vertical field in the MSM structure that is created by a 2-D gas transforms a traditional lateral MSM device to a vertical one, although remaining as a planar structure, thus allowing a device design for high-speed performance without sacrificing the external quantum efficiency.

Hall effect measurements in gas sensors based on nanosized Os-doped sol-gel derived SnO/sub 2/ thin films

The influence of dopants on the electrical properties of gas sensitive layers used in semiconductor gas sensors has to be carefully understood for getting a deeper insight in the relationship between the sensor performance and its chemical composition. In this work, undoped and Os-doped SnO/sub 2/ thin films have been prepared by the sol-gel process with an Os-Sn atomic ratio of 5%. The films have been characterized by resistivity and Hall effect measurements in a temperature range from 100 K to 500 K, both in air and in vacuum. The results have been investigated according to grain boundary scattering mechanism. We found that in air, the ambient oxygen species adsorbed on the film increase the height of the grain boundary barriers and the activation energy for the electrical conductivity increases in the doped film. In vacuum, the results showed that the height of the intergranular barrier is lower than the corresponding value in air. Both in air and in vacuum, the conductivity of the Os-doped sample is higher than the value in the undoped SnO/sub 2/ sample. The same occurs for the Hall mobility and the carrier concentration. The experimental results have been used to explain the better methane sensitivity, at low temperature, of the Os-doped films as compared with the undoped ones.