GB1288056A - - Google Patents

Abstract

1288056 Semi-conductor devices; image pick-up tubes PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 24 Nov 1969 [27 Nov 1968] 57343/69 Headings H1D and H1K The identical radiation-sensitive elements of a mosaic formed in a semi-conductor body are mutually isolated by the provision of a doping profile in part of the substrate around each active junction or barrier such that minority carriers formed by incident radiation are swept only to the respective junction or barrier. An individual detecting element may be in the form of a junction photo-diode, a junction phototransistor, a PNPN device, or a Schottky barrier diode. The embodiments described are junction photo-diodes or photo-transistors and are provided as vidicon targets but variants may have an electrode array for use in scanning. Positive ion beams may replace electron beams in scanned vacuum tubes. Fig. 9 shows a phototransistor target with the base-collector junctions 6 acting as detectors; in a photo-diode variant (Fig. 1, not shown) the emitter zones 15 are omitted. The lightly doped N-type zones 4 are formed by epitaxial deposition in pockets in a highly doped N-type substrate 3. (The heavy doping is sufficient to prevent inversion under the oxide coating 12). The structure may be given a gradual doping gradient in the deposited zones 4 by allowing diffusion from the substrate 3 during or subsequent to the epitaxial growth. If the image and the scanning beam are both to be projected on to the upper surface a uniform electrode is applied to the lower surface, but if image projection is to be on to the substrate the substrate is thinned and provided with an annular electrode. A somewhat similar device is made as shown in Fig. 10. Here the P-type zones 16, 17 extend above the level of the initial substrate 3. In this case impurities for the substrates are diffused into the epitaxial layer 4 to give a doping gradient for a drift field and the substrate and that part of the epitaxial layer below line 21 (which separates a doping level of 10<SP>17</SP> atoms cm.<SP>-1</SP>) are removed by a selective electrolytic etching step to leave a body in which the N-type region does not extend in thickness further than a diffusion length from the junction so that nearly all absorbed radiation is effective. In a further manufacture (Figs. 12 and 13, not shown) a photodiode array is produced from a uniformly doped N-type starting wafer by providing an apertured oxide mask and out-diffusing phosphorus through the apertures to leave graded regions in which P-type regions are then formed by indiffusion. Process details are given for all embodiments. The devices described are of silicon but germanium and A<SP>III</SP>B<SP>V</SP> semi-conductors may be used and heterojunctions may be employed.