Patent GB-1147883-A (GB)
Title: Improvements in and relating to cathodes for electron tubes
IPC: H01J1/30
Filed: 1966-04-20
Abstract: 1,147,883. Semi-conductor cathodes. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 20 April, 1966 [22 April, 1965], No. 17320/66. Headings H1D and H1K. A cathode for an electron tube comprises a P-type semi-conductor region coated with a material having a work function equal to or less than the distance between the Firmi level and bottom of the conduction band in the P region, and having an injection connection so that forward biasing results in electrons emerging through the coated surface; the thickness of the P-type region should be less than a diffusion length. Fig. 5 shows an electron tube (e.g. klystron, cathode-ray tube or pentode) with a cold cathode comprising P- region 14 coated with work function reducing material 13; electrons are injected utilizing electrode 15 and the injecting connection 16, 17 of N-type material and contact. Input signals may be applied to the cathode from source 26. To verify that the work function requirements are being satisfied the work function of materials such as alkali metals or alkaline earth metals coated on semi-conductor materials may be measured by ascertaining the limiting long wavelength of photo-electric emission of electrons. The cathode may be produced by diffusing Zn into the surface of an N-type (10<SP>17</SP> donors) GaAs disc to form a 10Á P-type layer; a 1Á layer of Zn is then evaporated on the P-type layer and portions of the Zn and P-type layer removed to leave a grid of P-type material 34 covered by Zn 36 (Fig. 8). The semi-conductor body is then gold-tin soldered to molybdenum plate 37 and placed in the envelope of the electron tube being manufactured. After evacuation, the body is heated inductively or by electron bombardment and the Zn coating 36 diffuses into the exposed N-type regions to form a thin (less than 1Á) P-type layer 45 (Fig. 11) with an acceptor surface concentration greater than 10<SP>19</SP>. Caesium is evaporated to form the work function reducing layer 46. Electrode 41 to the P-type layer, comprises a gold plated molybdenum wire. Several modifications to the process are described; the zinc may be diffused in separately (instead of from the source layer 36) utilizing Zn 3 As 2 . The body may consist of an N-type region of GaP covered with a P-type region of GaAs produced by epitaxial deposition, or by heating in an atmosphere of ZnAs produced by epitaxial deposition, or by heating in an atmosphere of Zn and As whereby P is given off leaving a P-type layer of GaAs. GaP may be used as the P-type material in which case Ba is suitable coating material and Cu may be used instead of Zn as the acceptor. Cu provides deep level acceptors which facilitates the injection of electrons. During manufacture the P-type surface may be cleaned by spattering. An additional (e.g. capacitative) control electrode may be provided on the semi-conductor body, and the injection contact may comprise a point contact or Schottky metal semi-conductor contact. Cd or Mn may be used in place of Zn, and the semi-conductor may also consist of a mixed crystal of GaAs and GaP or of SiC or a II, VI compound.