The delta-doped hybrid advanced detector is a developmental integrated electronic imaging and detecting circuit designed to be sensitive to (1) charged particles with kinetic energies ranging from hundreds through millions of electron volts and (2) photons ranging from visible light to x rays. Heretofore, the lower kinetic-energy limit of detectability of charged particles has been about 10 keV. To extend the limit downward to hundreds of electron volts, the design of the delta-doped hybrid advanced detector calls for a combination of features from several recent lines of development of detector and readout circuits.

The detector would be a hybrid of (1) a square array of reverse-biased positive/intrinsic/negative (PIN) diodes with (2) a correspondingly patterned complementary metal oxide/semiconductor (CMOS) integrated readout circuit fabricated by a standard process. The array of PIN diodes would be electrically and mechanically connected to the readout circuit through indium bump bonds (see figure).

Detector Diodes Would be Joined With CMOS Readout Circuitry by use of indium bump bonds. Pixels could be raster-scanned or read out nonsequentially. The column signal-processing circuitry would enable the incorporation of correlated double sampling or other advanced features.

In a reverse-biased PIN diode, the applied electric field depletes the intrinsic region of charge carriers. When an energetic charged particle or photon of sufficient energy enters the depletion region, some or all of its energy is dissipated in the generation of electron/ hole pairs, which are then swept out by the electric field and detected. However, low-energy particles typically do not penetrate to the intrinsic region in a conventional PIN diode. Instead, they tend to dissipate their energies in a "dead" layer near the surface through which they enter. The dead layer contains an undepleted, highly p- or n-doped, diode contact sublayer where electrons and holes recombine before they can be detected. The dead layer also contains a surface depletion sublayer with an electric field that (a) tends to confine some charge carriers until they recombine at the surface and (b) drives other charge carriers into the undepleted region, where they recombine before detection.

The delta-doping aspect of the delta-doped hybrid advanced detector would extend the low-energy detection limit by reducing the effect of the dead layer. Delta doping is so named because its density-vs.-depth characteristic is reminiscent of the Dirac d function (impulse function). The dopant is concentrated in a very thin layer (preferably, a single atomic layer). In the delta-doped hybrid advanced detector, the delta-doped layer would be placed within 5 or 10 Å of the detector surface. The delta-doped layer would provide a very thin undepleted region and surface depletion region, with a resultant dead-layer thickness of only 15 to 20Å, making possible the collection of electrons and holes generated by low-energy charged particles and photons that can penetrate at least 20 Å or so.

Heretofore, delta doping has been used on charge-coupled devices (CCDs). Although CCDs could be used as detectors, they are relatively highly doped; it would be necessary to thin them to enable their charge-collecting depletion regions to extend through the thickness. Thinning can be accomplished only by a difficult fabrication process step, and the remaining thickness is not sufficient for measuring the energies of x rays and high-energy particles that deposit charge carriers at great depths. Accordingly, the array of PIN diodes in the delta-doped hybrid advanced detector are made from a wafer of high-resistivity silicon [nominally intrinsic, though actually very lightly p-doped (to a concentration of 1012cm¯3)] like that used in silicon strip detectors. The use of this high-resistivity silicon makes thinning unnecessary, thereby making it possible to retain sufficient thickness for detection of highly energetic charged particles and photons.

The CMOS readout circuit would be of the active-pixel sensor (APS) type, offering enough sensitivity to enable resolution of the small signals generated by low-energy charged particles. This circuit would consume only milliwatts of power, in contradistinction to a CCD, which typically consumes watts. Moreover, whereas a CCD must be read out sequentially by rows and columns, the APS circuit would be capable of nonsequential readout of pixels, making it possible to use various advanced readout schemes. Like a CCD, this readout circuit could preserve the low capacitance of the detector diodes, with resultant readout noise of 10 electrons or less (vs. hundreds to thousands of electrons for a conventional strip detector).

A pixel-guarding technique would be used to preserve low effective input capacitances for the APS readout circuit, even in the presence of relatively large indium bump pads on the input nodes, thereby preserving the high conversion gain needed for high sensitivity and a high signal-to-noise ratio. In this technique, each bump bond would be connected to a source follower serving as a unity-gain buffer, the output of which would be fed to a metal guard electrode underneath the bump bond and separated from the bump bond by a thin insulating layer.

This work was done by Eric Fossum, Thomas Cunningham, Shouleh Nikzad, George Soli, and Bedabrata Pain of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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Refer to NPO-20111

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Delta-doped hybrid advanced detector

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Photonics Tech Briefs Magazine

This article first appeared in the October, 1998 issue of Photonics Tech Briefs Magazine.

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