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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 TR>2000 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. [ABSTRACT FROM PUBLISHER]

In this paper, shunt capacitive RF microelectromechanical systems (MEMS) switches are developed in III–V technology using tantalum nitride (TaN) and tantalum pentoxide (\Ta2 \O5) for the actuation lines and the dielectric layers, respectively. A compositional, structural, and electrical characterization of the TaN and \Ta2 \O5 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 \Omega \cdot \cm by changing the deposition parameters. On the other hand, dielectric \Ta2\O5 films show a low leakage-current density of few nanoamperes per square centimeter for E \sim 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 \sim\!-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. \hfill[2010-0236] [ABSTRACT FROM AUTHOR]

The potential of InAs quantum-dot (QD) photodetectors for room-temperature high-speed operation at wavelengths near 1.3 µm is evaluated. Specifically, planar metal-- semiconductor--metal structures on GaAs substrates containing one absorption layer of self-assembled InAs QDs embedded in Ga(In)As matrices are fabricated, characterized, and analyzed. Light absorption, optically generated carrier transport, and collection mechanisms are studied. The role of the QD embedding matrix in the lateral transport of the photogenerated carriers is also studied by comparing structures with QDs in GaAs and In0.15Ga0.85 As matrices. Devices show low dark currents in tens of nanoamperes and high light sensitivity when adjusted to QD volumes, whereas external quantum efficiency remains in the range 10-5 -- 10-4 for all fabricated samples. The time response of the fabricated devices is obtained using an excitation wavelength resonant with QD interband transitions, thus allowing the photogeneration of electron--hole pairs inside the dots. Results prove detection capability of a single layer of QDs in a common photodetector structure with a full-width half-maximum time response on the order of 10 ps. A long tail, about 100 ps, but at a small fraction of the peak response amplitude, is also observed, suggesting mechanisms for charge transport and collection. [ABSTRACT FROM AUTHOR]

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 δ-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. [ABSTRACT FROM AUTHOR]

We have previously reported an AlGaAs-GaAs-based delta modulation doped resonant cavity-enhanced, heterostructure metal-semiconductor--metal (HMSM) photodetector design with wavelength selectivity, high sensitivity, and high-speed properties. In this paper, the influences of the resonant cavity and the delta modulation-doped layer on the optical and electrical performances of HMSM photodetectors have been studied by Using the transmission line model and Ramo's theorem, respectively. The micro-cavity offers narrow spectral bandwidth detection accompanied by a drastic increase of the optical field at the resonant wavelength, solving the tradeoff between high efficiency and high speed with the thinner absorption regions. The top AlGaAs delta modulation-doped layer presents an enhanced barrier height associated with the vertically oriented two-dimensional potential and field profiles rather than the horizontally oriented ones, providing very low dark current values and an increase in device responsivity and speed of the response. Optimizing the optical and electric field simultaneously results in high-performance photodetection without further scaling down the devices. Two separate groups of photodetectors with various geometries have been fabricated, characterized, and simulated: One with a delta modulation-doped structure, the other without this doped layer. Delta-doped photodetectors show wavelength selectivity at 850 nm with about a 30-nm full width at half-maximum (FWHM), 9.2 fA/μm2 dark current, less than 30 fF capacitance, 10.6-ps FWHM, and 18.4-ps fall time. Photocurrent spectral response, dark current, time response, and capacitance-voltage measurements consistently show that the delta-doped detectors have the better optical and electrical performances over the undoped ones. [ABSTRACT FROM AUTHOR]

The spectroscopic response of CdTe detectors has been investigated with an unusual irradiation geometry, i.e., with the beam impinging at various angles through the cathode surface. In fact, the classical planar parallel field (PPF) irradiation geometry, i.e., with photons impinging perpendicularly to the cathode, limits the spectroscopic performance at high energies (E > 60 keV), if thick detectors are used, owing to the large spread in the paths of the charge carriers. These paths for the holes can be greatly reduced by using larger impinging angles, thus improving the energy resolution without losses in detection efficiency. Several sets of measurements have been carried out with a narrow photon beam, coming from collimated 57Co, 109Cd and 241Am sources; measurement angles ranged from 0° (orthogonal to the cathode) to 87° (beam grazing the cathode). The spectroscopic responses of the detectors for 14, 22, 60 and 122 keV photon energies have been analyzed. Detectors equipped with both full anodes and microstrip anodes were used. In this last case the rectangular collimated spot of the beam was used with the long side both parallel and orthogonal to the strip direction. The experimental results show that an actual improvement in the spectroscopic performance is obtained, mostly for the high-energy X-rays, by this tilting procedure. Further, this technique allows the evaluation of the detector dead layer thickness. The obtained performance and possible medical and space applications are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Deals with a study of the spectroscopic performance of a CdTe detector, equipped with a "strip anode" placed between 2 parallel focusing strips. Experimental setup; Experimental results and discussion; Numerical simulations; Conclusion.

Presents a study on the charge collection properties of Cadmium Telluride strip detectors with different contact configurations. Scope of the study; Principles of the finite difference numerical method; Results and conclusion.

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 δ modulation-doped heterostructure of AIGaAs/GaAs with two Schottky contacts on the top. The capacitance-voltage (C-V) measurements show a Cmax/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. [ABSTRACT FROM AUTHOR]

An AIGaAs-GaAs-based resonant-cavity-enhanced, heterostructure metal-semiconductor-metal photodetector with delta modulation doping operating at 850 nm is reported. Delta doping of the top AIGaAs layer produces a confined electron cloud and an associated electric field. Photocurrent spectral response shows the delta-doped photodetector has larger spectral response than the undoped one at all wavelengths. The delta-doped device also shows lower dark current and higher photo response compared to an undoped one, resulting in over an order of magnitude increase in its dynamic range. Time responses indicate that the doped devices have larger amplitudes but smaller full-width at half-maximum (FWHM) than the undoped ones. The increase in responsivity and speed of response is attributed to the vertical electric field and suitable potential profile in the direction of growth, while the decrease of the dark current is due to the confined electron cloud. [ABSTRACT FROM AUTHOR]