Patent GB-1273464-A (GB)
Title: Photosensitive semiconductor device
Inventor: MIYASHIRO SHOICHI
IPC: H01J29/39
Filed: 1970-06-24
Abstract: 1,273,464. Semi-conductor devices; image pick-up tubes; photo-electric detectors. TOKYO SHIBAURA ELECTRIC CO. Ltd. 24 June, 1970 [24 June, 1969; 24 July, 1969], No. 30633/70. Headings H1D and H1K. A device sensitive to energetic particles (e.g. electrons, light photons, X-rays or γ-rays) has a junction at one surface and a layer of high resistivity material at the opposite surface on which the particles are incident so that a charge corresponding to the amount of the particles is built up on the high resistivity layer. A camera tube comprises a photocathode (11), the photo-electrons emitted by which are focused on to one face of a target (1) the opposite face of which is scanned by an electron beam, Fig. 1 (not shown). The target, Fig. 2, comprises an N-type Si substrate 2 one face of which is masked with a layer 4 of SiO 2  having openings through which B is diffused to form a mosaic of P-type regions 3. A thin film 5 of high resistivity material, e.g. Sb 2 Se 3  is applied over the oxide layer, P is diffused into the opposite face of the substrate 2 to form an N<SP>+</SP> type layer 6, and a mixture of Y 2 O 3  and TiO 2  is vacuum deposited to form a high resistivity layer 8. In operation the photo-electrons 12 incident on the layer 8 charge it negatively due to its secondary electron emission efficiency being less than unity. This produces an electric field which tends to prevent the minority carries (holes) diffusing to the P-type regions 3. The result is a target characteristic in which the conversion gain decreases as the illumination increases. The time constant of the high resistivity layer 8 is arranged to be of the order of one frame scanning period. The high resistivity layer may also be of CdTe, As 2 S 3 , or Sb 2 S 3 . In modifications, the high resistivity layer is formed as a pattern of islands or as a mesh, Fig. 4 (not shown), or is covered with a thin film of material providing the desired secondary emission efficiency, e.g. porous ZnS, porous As 2 S 3  or porous Sb 2 S 3  or a film of material of high lateral resistivity and low light transmission coefficient, e.g. C to suppress stray light, Fig. 5 (not shown). The high resistivity layer may also be covered with a metal layer in a pattern of dots, Fig. 6 (not shown), or be covered with a coating of an oxide, nitride, sulphide or selenide to prevent contamination by Cs during formation of the photocathode. The high resistivity layer may be covered with a layer of Si or Ge which has a lower lateral resistivity than the high resistivity layer, Fig. 7 (not shown). This results in the negative charge leaking laterally to reduce the gain in adjacent areas of the target so that a dark fringe is produced round the edge of bright objects to increase the apparent contrast. A grid may be placed parallel to the high resistivity layer and biased so that secondary electrons are reflected back to the target, Fig. 8 (not shown), to produce a charge spreading effect similar to that of the previous embodiment. The invention may be applied to a photomultiplier, Fig. 9, in which a single P-type region 3 is formed in an N-type substrate, the opposite face being provided with a high resistivity layer of MgF 2  covered with a thin Al film 7 which is biased to provide the desired target characteristic. The semi-conductor substrate may also be of Ge, GaAs, CdS or CdSe, the active region may have an NPN transistor or NIP structure, Fig. 10 (not shown), and the charging of the high resistivity layer may be achieved by the electron bombardment induced conductivity effect, by photo-electron emission or by photoconductivity phenomena. Scanning may be effected by a solid state system instead of by an electron beam.