Patent GB-861196-A (GB)
Title: Electrically asymmetrically conductive systems with at least one p-n junction and methods of producing such systems
IPC: H01L29/00
Filed: 1957-06-07
Abstract: 861,196. Semi-conductor devices. LICENTIA PATENT.VERWALTUNGS-G.m.b.H. June 7, 1957 [June 7, 1956], No. 18185/57. Drawings to Specification. Class 37. An electrical asymmetrically conductive system comprises a single-crystal semi-conductor body which consists of germanium, silicon, or of a semi-conductive intermetallic compound (i.e. a compound of two elements, one being boron, aluminium, gallium or indium, and the other being nitrogen, phosphorus, arsenic or antimony), and one part of which has P-type and its other part N-type conductivity, both parts forming, where they merge together, a PN junction, and the gradient of the concentration of the impurity centres in a direction perpendicular to the PN junction being flatter in that zone of the junction which is adjacent the line of intersection between the PN junction, and the external surface of the semi-conductor body than in the central part of the junction. The radial extent of the outer zone of the junction with flatter gradient can be small compared with the radial extent of the central part of the junction. In one method a semiconductor body with a PN junction which has a flat and over its whole extent substantially equal gradient of concentration of the impurity centres in a direction perpendicular to the PN junction is subjected to a treatment which increases the gradient in the central region of the PN junction. The treatment can be carried out by alloying on a substance, e.g. indium or antimony in the case of a germanium body or gold (with 5% antimony) or aluminium in the case of a silicon body, which contains or consists of impurity centres which do not cause any change in the conductivity type in the part of the semi-conductor body which extends between the portion of the surface of the semiconductor body to which the substance is applied and the PN junction, and driving the alloying process so deep into the semi-conductor body that it reduces the depth of the PN junction in the central region. The semi-conductor body having a flat gradient before treatment can be obtained by drawing from the melt by means of a seed crystal, and adding to the melt substance producing impurity centres of both types, e.g. gallium and arsenic or indium, and antimony or arsenic and antimony in the case of germanium, the substances differing from one another as regards their coefficients of precipitation in such a manner that there is a rate of growth at which impurity centres forming substances of both types are incorporated in the resulting crystal in equally effective amounts, whereas, on deviation from this rate of growth, one or the other type of impurity centre is preferentially incorporated, according to the sense of the deviation. Alternatively the semi-conductor body having a flat gradient before treatment is obtained by incorporating impurity centres producing substances which change the conductivity type by means of diffusion into a semiconductor body which at first has uniform conductivity type. In the case of a germanium body lithium, and in the case of a silicon body lithium or boron (from a boric iodide atmosphere) can be used with after-etching. Two different impurity centres forming substances can be used which produce the same conductivity type but have a slight difference in the rates of diffusion, or two different impurity centres producing substances which have at least substantially the same rate of diffusion but produce a different conductivity type are incorporated in such proportions that the amount of the substance which alters the conductivity type of the semi-conductor body is greater than the amount of the substance which has the same conductivity type as the semi-conductor body. In another method of preparing the system a semi-conductor body with a PN junction which has a steep, and over its whole extent substantially equal gradient of concentration of the impurity centres in a direction perpendicular to the PN junction is subjected to a treatment which flattens the gradient in that zone of the junction which is adjacent the line of intersection between the PN junction and the external surface of the body. The treatment can be carried out by applying an impurity centres forming substance, e.g. thallium or antimony, to the semi-conductor body, e.g. germanium-indium, adjacent to the line of intersection between the PN junction, and the external surface of the body, and causing the substance to diffuse into the body so as to increase the depth of the junction in its marginal zone. The semi-conductor body with an even steep gradient before treatment can be obtained by alloying a substance on to a semi-conductor body, e.g. germanium, which at first has uniform conductivity, the substance containing impurity centres or consisting of impurity centres, e.g. indium, which alter the conductivity type of the body.