Patent GB-1431209-A (GB)
Title: Method and apparatus for sensing radiation and providing electri cal readout
IPC: H01L27/146
Filed: 1973-06-20
Abstract: 1431209 Light imaging arrays GENERAL ELECTRIC CO 20 June 1973 [21 June 1972 (2)] 29288/73 Heading H1K [Also in Division H4] In operation of a photosensitive MIS device in an imaging array a voltage is first applied between metal and semi-conductor to create beneath the insulation a local depletion region in which photogenerated minority carriers collect to form an inversion layer at the surface. When the voltage is subsequently reduced the accumulated carriers are injected into the substrate where they recombine, the resulting balancing current flowing into the semi-conductor, which is a function of integrated illumination intensity, being measured directly or charging up a capacitor, possibly the total capacitance of the other elements of the array, to produce an output voltage. The cycle is restarted by taking the capacitor out of circuit and restoring the original voltages. The capacitance of the dielectric beneath an electrode is about 10 times that of the associated depletion layer and good signal/noise ratio is achieved if more carriers are generated by the light than thermally. In a modification comprising pairs of closely spaced electrodes with coupled depletion regions the photogenerated carriers are collected under one electrode and then transferred to beneath the other by suitable application of voltages, from where they are injected into the semiconductor. In one array of this type (Figs. 4 and 5) the electrode pairs are located on the thin portions 87 of insulation, one of each pair being formed integrally with a column conductor Y and the other connected to a row conductor X as shown in detail at 94 in Fig. 5. The semiconductor is 4 ohm. cm. N-type silicon and in manufacture the thin oxide is thermally grown in holes formed in thicker previously formed oxide. After forming the metal plates and column conductors by pattern etching of vapour deposited molybdenum, borosilicate glass is deposited overall and heated to form P + layers by diffusion through the thin oxide exposed between pairs of plates to link their depletion regions, and is then apertured as at 99 (Fig. 5) for connection of plates to the row conductor pattern formed subsequently from a deposited aluminium layer. At the periphery of the back face of the target is electrode 98. The diffusion step may be dispensed with if electrodes of a pair are sufficiently closely spaced or if they overlap as in the construction of Figs. 13 and 14 (not shown). Read-out is achieved by row by row scanning. To read-out a row the voltage on the selected row conductor is altered so that stored charge is transferred to beneath the column connected plates of all devices in the row. The voltage on each of the column conductors in turn is changed to a voltage causing injection of stored charge, the original voltage being restored after the resulting output voltage has been read and the injected carriers recombined. When the whole row has been scanned the original row conductor voltage is restored. Various modies of operation are described using different combinations of row and column conductor voltages to facilitate charge transfer between electrode pairs, prevent injection during this transfer and give increased storage capacity. Some of the circuitry providing the scanning voltages may be integrated into the wafer containing the array (Fig. 18, not shown). Alternative materials are Ge, InAs, InSb in place of Si and silicon nitride or oxynitride or alumina in place of silicon oxide.