An avalanche photodiode including a substrate, a first semiconductor region of a first conductivity type having a relatively large band gap, a second semiconductor region of a second conductivity type having a relatively large band gap, and a third semiconductor region of the first conductivity type having a band gap smaller than the band gap of the first and second semiconductor regions, is disclosed in which, in order to suppress an increase in dark current and to cause the avalanche photodiode to operate on a low voltage, a fourth semiconductor region equal in conductivity type to and larger in impurity concentration than the first semiconductor region is formed in the first semiconductor region at a position below a central portion of a pn junction formed between the first semiconductor region and the second semiconductor region. The avalanche photodiode formed with this structure has low-noise and low operation voltage characteristics.
The present invention provides photodiodes exhibiting photoconductive gain. It is shown that photodiodes may exhibit photoconductive gain under certain conditions, and traditional photoconductive gain theory has been extended to describe these cases. Particularly, there is introduced the basic principles of photoconductive gain in p-i-n diodes, and there is described several approaches to designing photodiodes with photoconductive gain. In one approach, photogenerated carrier delay is used to obtain photoconductive gain in a photodiode. Delay structures inserted into the intrinsic region preferentially impede the flow of one of the carriers relative to the other to obtain the gain. Another method of obtaining photoconductive gain in a photodiode is to increase the rate at which electron-hole pairs are generated in the p-region or n-region, so as to decrease the times .tau..sub.p or .tau..sub.n. One way of decreasing .tau..sub.p or .tau..sub.n is to include a region of "ultra-fast" high B-coefficient material in or near either the p-region or n-region, which has a much lower value of .tau..sub.p or .tau..sub.n for a given doping level.
Relates to constructions of a backplane which comprises an array of addressable active elements 52 on a semiconductor substrate 51 for selectively energizing respective first electrodes 65 of the array, for example in a liquid crystal matrix cell. To reduce photo-induced degradation of images produced thereby (a) at least part of the region beneath a first electrode is adapted to act as a capacitor, for example a depletion layer 66 acting as a reverse biased diode, and/or (b) substantially the whole of each active element is covered by a metallic conductor (59, 60--coupled to row and column conductors). In a variant of (b) the array of active elements may covered by an insulating layer, and each active element is connected to a metal electrode on the insulating layer, the array of said metal electrodes thus formed covering more than 65% of the area of said array.
A new reach-through APD structure which includes a five-layer, double-drift-region, double-junction device, having a p.sup.+ -p-n-p.sup.- -n.sup.+ structure. The middle three layers of the new APD constitute most of the thickness of the device and are fully depleted when the device is biased to its normal operating voltage. In the present invention, only primary carriers generated in the relatively narrow first drift region are subject to the full avalanche gain, resulting in dark current and noise levels much lower than conventional reach-through APD's. The new structure can be used to fabricate integrated arrays of APD's. These arrays are more durable and easily fabricated than are prior art APD's. Additionally, the new array structure does not require segmentation or isolation of the multiplying regions of the different elements of the array, allowing arrays to be made with little or no dead space between elements.
The disclosed invention as directed to a semiconductor material avalanche photodiode of a separate multiplication and absorption region heterostructure design (SAM-APD). The improved SAM-APD of this invention is characterized by a plurality of floating guard rings which are separated about a central region and doped in the opposite high concentration from that of the multiplication region in which they are positioned. These rings float in the sense that they have no contact with the metalized p-contact of the photodiode; and, therefore, no direct contact with the current source. This structure results in an enhanced avalanche effect in the central region with limited edge breakdown undesirable consequences. In addition to this structure, an alternative embodiment suggest the use of both a floating ring and thin slab below the central region, of a dimension slightly smaller than the smaller region and concentric with it to achieve an optimized central avalanche breakdown with reducd edge breakdown of the electric fields formed during reversed biasing of the APD (avalanche photodiode).
A photosensitive device includes a semiconductor body (1) having a first region (2) of one conductivity type adjacent a given surface (3) of the body with a second region (4) of the opposite conductivity type surrounding the first region (2) so as to form with the first region a main pn junction (5) terminating at the given surface (3), the main pn junction (5) being reverse-biassed in operation of the device. One or more further regions (6) of the one conductivity type surround the main pn junction (5) adjacent the given surface (3) so that each further region (6) forms a photosensitive pn junction (17) with the second region (4), the further region(s) (6) lying within the spread of the depletion region of the main pn junction (5) when the main pn junction (5) is reverse-biassed in operation of the device so as to increase the breakdown voltage of the main pn junction (5). Means are provided for transmitting light into the semiconductor body via the given surface to enable photogeneration of charge carriers in the vicinity of the photosensitive pn junction(s) (17). The further region(s) may include one or more annular floating regions (6") spaced apart from the main pn junction(s) and each other.
