Patent FR-1326520-A (FR)
IPC: H01L29/00
Filed: 1962-03-28
Granted: 1963-05-10
Abstract: 1,026,524. Semi-conductor devices. SOC. EUROPEENNE DES SEMICONDUCTEURS. March 28, 1963 [March 28, 1962], No. 12343/63. Heading H1K. A semi-conductor device having operating characteristics intermediate those of field transistors and junction transistors comprises, in one embodiment, a silicon wafer 13 (Fig. 5) of high N-type conductivity welded to a metal plate 20 (forming the cathode) and carrying on its upper surface an epitaxially deposited layer 14 of silicon of lower N-conductivity between which and an electrode 19 (forming the anode) consisting of antimony doped gold a control electrode 15 is formed the latter having a mesh form the average diameter of the openings of which is of the order of one micron and the width of the bars of the mesh of the order of 0À1 micron. The control electrode is made of aluminium and some of this alloys with and/or diffuses into the layer 14 thus producing areas of P-type conductivity in the regions 17 immediately underlying the mesh elements the upper surfaces 18 of which are oxidized to provide an insulating alumina coating between these surfaces and electrode 19. Because the small size of the mesh openings, i.e. about one micron, is of the same order as the depth of the depletion zone arising near the interface between the opposite conductivity regions 14, 17 in the absence of a potential difference between these regions, no appreciable current flows between the (anode) electrode 19 and the (cathode) electrode 20 (unless, of course, the voltage applied between the latter electrodes exceeds the breakdown voltage at the P-N- junction). One method of producing the fine mesh which the invention requires comprises screening-off a marginal area 31 (Figs. 7 to 10, not shown) of layer 14, evaporating on to the latter a one micron silver layer 30, heating to 500‹ C. (in vacuo) to produce a multiplicity of silver spherules 32 (2 or 3 microns in diameter), evaporating aluminium over the entire surface and then removing the silver spherules by nitric acid. Heat treatment is then employed to alloy the resulting aluminium grid 33 with the underlying silicon. In another method, a marginal area is masked-off, as before, and over the remaining surface of layer 14 a layer of particles 35 (Figs. 11 and 12, not shown) of silver containing 1% antimony (of about one micron diameter) is deposited from a liquid dispersion to form a micro-screen. Microparticles of gold containing 2% gallium are then sedimented over this screen and settle in part at 36 and in part at 38 and by heating to a temperature above the melting point of the gold-gallium-silicon eutectic the particles 38 are alloyed to the silicon layer 14 to form a foraminous grid structure 39. The anode of the device is then formed by a gold or goldplated metal strip 41 attached by thermocompression at about 300‹ C. and the connection to the control electrode constituted by the grid 39 is effected through the marginal portion 37. The theory of the device is discussed with reference to Fig. 4 (not shown) and a complete encapsulated device is described with reference to Fig. 6 (not shown).