Patent GB-1444544-A (GB)
Title: Semiconductor photocathode
IPC: H01L27/144
Filed: 1972-09-22
Abstract: 1444544 Photocathodes MULLARD Ltd 16 Aug 1973 [22 Sept 1972 (3)] 43959/72 Heading H1D A semi-conductor photocathode comprises a field effect transistor structure; an injecting junction with P-type semi-conductor material in series with the source or drain of the transistor, a free surface of the P-type s.c. being not more than a diffusion length for electrons from the junction, the transistor having a gate for producing a depletion region in the channel of the structure situated so that incident e.m. radiation can generate charge carriers in the depletion region or within a diffusion length of the region. The cathode comprises a substrate 2; P- and N-type Si layers 1, 3; arrays of annular P<SP>+</SP> regions 4 and N<SP>+</SP> regions 5 formed by diffusion; insulating layers 6, 9; annular An or Al contacts 7, 12; and a thin P-type layer 10 with a coating of Cs-O to reduce the work function. Contacts 7, 12 are interconnected by conductors 8, 13 on layers 6, 11 respectively. A positive setting pulse is applied to contacts 7 to forward bias the junctions 4, 3 and charge the capacitors formed by the contacts layer 6 and regions 4. At the end of this pulse contacts 7 return to the potential of layer 3 carrying regions 4 negative, thus reverse biasing junctions 4, 3 and producing a depletion region below each which extends to layer 1. This cuts off the path between terminal 14 and region 5 and the structures act as F.E.T.s with a common source terminal 14, gates being regions 4 and drain electrodes being regions 5. Incident light which generates free charge carriers in layer 3 causes the depletion regions to contract in accordance with the local intensity. When a subsequent positive pulse is applied to electrode 12 relative to layer 3 a current whose magnitude is dependent on the radiation absorbed since the end of the setting pulse flows to regions 5, electrons are injected by P-N junction 5, 10 into layer 10 and are emitted therefrom. The conductors 8 may be coupled to gates 4 by rectifying junctions and contacts 7 form Schottky barriers with regions 4. In another embodiment comprising an array of JFET structures and N<SP>+</SP> grid region 27 is formed on the lower surface of the N-type layer 21 to provide a source for all JFET's. A Pt layer 28 provides a connection to the region 27 and forms a Shottky junction with the remainder of the layer 21. Contacts 7, 12 are replaced by annular contacts 31 within which the surface is coated with a Cs or Cs-O layer 33. Depletion regions are shown by dotted lines. Radiation may be incident on either side of the device. In a three terminal arrangement the N<SP>+</SP> grid is formed on the top surface of layer 21 and connected to layer 28. The P-type layer 29 is restricted to the areas bounded by electrodes 31 and these are surrounded by C-shaped eletrodes above regions 25 and connected to the third terminal. The devices are used on image converter and intensifier tubes. A single device is used to convert a modulated laser beam into a modulated electron beam. Specifications 1,444,541 and 1,444,542 are referred to.