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The fabrication and characterization of two-dimensional electron gas (2DEG)- and two-dimensional hole gas (2DHG)-based metal-semiconductor-metal (MSM) photodetectors (PDs) is discussed. Findings reveal the facilitation of one type of photogenerated carrier transport by a vertical field developed in the active absorption region due to the gamma-doping layer.

The transmission line model and the Ramo's theorem are used to study the influences of the resonant cavity and the delta modulation-doped layer for the optical and electrical performances of HMSM photodetectors. The top AlGaAs delta modulation-doped layer presents an enhanced barrier height associated with the vertically oriented two-dimensional potential.

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 [sup.57]Co, [sup.109]Cd and [sup.241]Am sources; measurement angles ranged from 0[degrees] (orthogonal to the cathode) to 87[degrees] (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. Index Terms--CdTe detectors, CCE, spectroscopy, various irradiation angles.

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 photnreceiver design. The low dark current, in tens of femtoamps per square micrometer, full-width at hall-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. Index Terms--Distribute Bragg reflector, heterojunction, heterostructure metal-semiconductor-metal (MSM), high electron mobility, photodetector, resonant cavity enhanced (RCE), Schottky contact, two-dimensional electron gas (2-DEG).

A study of the spectroscopic performance of a CdTe detector, equipped with a 'strip anode' placed between two parallel focusing strips, was carded out. The focusing strips were kept at an intermediate bias voltage with respect to anode and cathode. This configuration increases the intensity and focuses the weighing field on the region surrounding the strip anode, thus improving the uniformity of signals due to the collection of electrons. The detector was irradiated with a narrow photon beam emitted by a [sup.57]Co source. When the photon beam is impinging perpendicularly to the cathode plane, the spectroscopic response of the strip anode detector looks better than that obtained by the usual 'full anode' CdTe detectors, especially with regard to the photopeak in the 122-keV energy region. Previous measurements carried out on detectors having the same crystal size, but equipped with a full anode, showed a considerable improvement in spectroscopic performance at 122 keV, when the photon beam was impinging on the detector at almost grazing angle with respect to the cathode surface [1]. In this paper, a similar set of measurements at variable angle of the photon beam is carried out on detectors having strip anode configuration in order to investigate their behavior under the same experimental conditions. Index Terms--CdTe spectrometer, microstrip detectors, semiconductor detectors, various irradiation angles, X- and Gamma-ray spectroscopy.

CdTe is an attractive material for X-ray and gamma-ray detectors, but the poor transport properties of holes affect the performance by introducing a low energy tailing in the observed spectra. A possible solution to this problem is to optimize the electrode geometry, for example, by reducing the dimension of the anode with respect to the cathode. In this way, the charge signal in the external circuit is mostly due to the electrons moving toward the anode that is where the weighing field becomes localized. The optimization of the electrode geometry can be approached by a numerical analysis as the problem of charge collection is strictly connected to trapping and detrapping processes, which are difficult to be analytically treated in nonuniform electric fields. In this paper, we present a numerical simulator based on a finite difference numerical method (which follows the weighing field approach) and on a Monte-Carlo procedure, which is able to analyze, in two dimensions, the effect of different electrode configurations: single strip, multiple strip, a single strip with a lateral extended cathode, and, for comparison, the uniform geometry. The results are analyzed in terms of the maps of the local charge collection efficiency and their histograms, equivalent to the spectra due to high-energy X-rays.