Semiconductor devices having epitaxial films and methods of making them

摘要

998,415. Semi-conductor devices. WESTERN ELECTRIC COMPANY Inc. Sept. 14, 1961 [Sept. 20, 1960], No. 33047/61. Heading H1K. In a semi-conductor device the net impurity concentration on at least one side of a PN junction formed within a layer epitaxially deposited on a semi-conductor substrate decreases with increasing distance from the junction. A voltage variable capacitance diode of this type is made from a wafer of silicon, 0À013 inch thick along the 110 crystallographic axis, containing a uniform N-type doping of 1020 atoms/c.c. of arsenic by the following steps. One face of the wafer is first exposed for 1 hour at 1250‹ C. to a gaseous mixture of silicon dioxide with 0À01- 0À001% by volume of boron trioxide to produce an N-type surface layer with a surface concentration of 1018 atoms/c.c. of boron. After grinding the treated surface flat, etching in a hydrofluoric-nitric acid mixture, and washing first in dilute hydrochloric acid and then in water, the wafer is heated for 30 minutes at 1200‹ C. in pure dry hydrogen. It is then maintained at 1200‹ C. in a gas consisting of silicon tetrachloride and hydrogen in the volume ratio 0À02: 1, flowing at a rate of 1 litre/minute for 1 minute to produce an epitaxial layer 0À00025 cm. thick over the boron doped layer. Other thicknesses may be produced by deposition over a period ranging from minutes to hours at temperatures of 850-1400‹ C. During deposition or in a subsequent heating step the arsenic diffuses into the epitaxial layer and the boron more deeply to produce a PN junction in the layer on the remote side of which the uncompensated boron concentration decreases rapidly with distance. After provision of an ohmic contact over the back face of the wafer and a series of spaced alloyed aluminium ohmic contacts on the epitaxial layer unwanted parts of the layer are removed and the wafer subdivided into mesa diodes. In these diodes as a result of the heavy doping of the wafer the space charge expands mainly into the retrogradely doped P- type layer to give a capacity roughly inversely proportional to voltage. The method may also be performed using an arsenic or antimony doped silicon wafer with indium or boron as diffusing impurity, using gallium or indium doped P-type germanium with phosphorous or arsenic as diffusing impurity, or using a silicon doped N-type gallium arsenide wafer with zinc or cadmium as diffusing impurity. In all cases the epitaxially deposited layer may be a different semi-conductor from the wafer, e.g. germanium, deposited from its chloride or iodide on silicon. Specifications 692,250 and 972,511 are referred to.