Patent GB-795675-A (GB)
Abstract: 795,675. Germanium alloys. RUTTER, J. W., and TILLER, W. A. June 24, 1955 [June 30, 1954], No. 18324/55. Class 82(1) [Also in Group XXXVI] A semiconductor body having regions of P and N type conductivities is prepared by progressively freezing, in conditions which avoid convective mixing, a part of a melt of semiconductor containing an initial concentration C 0 (1) of a first impurity and a greater concentration C 0 (2) of a second impurity characteristic of the conductivity type opposite to that of the first impurity, the segregation coefficient K 1  of the first impurity being less than that of the second K 2  and the product K 1 C 0 (1) being greater than K 2 C 0 (2), so that the concentration of the first impurity in the solidified material is first greater and then less than that of the second. The theoretical aspect is discussed and it is shown that when solidification proceeds along a melt containing two such solutes, there are three phases which can be utilized to produce PN junctions. The first utilizes the initial transient during which the concentration in the solid builds up from the initial value KC 0 , to the steady state value C 0 , which is produced when the concentration in the melt immediately adjacent the interface, has built up to its steady state value of C 0 /K. The solute distribution from the interface into the main body of the melt (where the concentration is still C 0 ) is calculated, and shown to depend on the rate of solidification and the diffusion constant of the solute in the melt. The second phase utilizes the transient produced by a change in rate of solidification, which necessitates a change in the steady state concentration of the melt region immediately adjacent the interface. The third phase utilizes the change in concentration which occurs as the advancing solid-liquid interface approaches the end of the melt. Assuming impurity (2) is a donor, the melt will originally consist of N-type material. The portion to solidify will therefore first consist of P-type and then N-type as the initial transient passes. The thickness of the P-layer is dependent on the rate of solidification and on the ratio of the solutes C 0 1/C 0 2. It is shown that the P-layer is thinnest if K 2  equals ¢ and K 1  is as high as possible (i.e. greater than unity). If solidification is stopped, the melt stirred (to destroy the non-standard concentration at the interface) and solidification recommenced, the "initial transient" process occurs again and a PNPN body is produced. The junction produced during the transient conditions is graded, while that due to interrupting the solidification is of the stepped type. Fig. 6 shows suitable apparatus in which a graphite or silica boat C movable by rod H, is situated in a silica tube B. Heating is provided by coil A1, and a temperature gradient by heating coil A2 at one end and cooling jacket A3 at the other. Water cooled caps G permit the operation to be performed in an argon atmosphere. A charge E of germanium with the correct impurities is put in boat C together with a single crystal D of germanium. The charge is melted with a small portion of crystal D, stirrer F ensuring uniform composition. The temperature is then slowly lowered to cause solidification at the desired rate e.g. 3 mm. per minute. The process may be interrupted and restarted to get PNPN bodies. The thickness of the P layer on the final body may be determined by utilizing themo-electric effect (which for P is opposite to that for N) provided by a hot copper probe. The proportions of impurities required in some semiconductor alloys may be so low that a direct preparation is not practicable. In one example utilizing germanium with antimony and gallium a master alloy of germanium and antimony was first prepared, which by measuring its Hall coefficient, was found to contain 3.2 X 10<SP>16</SP> Sb atoms per gram, and a second master alloy was also prepared and found to contain appropriate 5.4 X 10<SP>16</SP> Ga atoms per gram. By combining quantity of gallium alloy, Sb alloy, and pure Ge (i.e. resistivity greater than 35 ohm. cm.) an alloy containing 2.7X 10-<SP>5</SP> atomic per cent Ga, 1.3 atomic per cent Sb, the balance being Ge, can be prepared, giving a C 0 (Ga)/C 0 (Sb) concentration ratio of 0.2. In a second Example, 10 grams of pure Ge was melted in the presence of 10 mg. of arsenic for 1 hour and then cooled. A second master alloy was prepared by melting 10 gm. of Ge with 7 mg. of boron for 24 hours in vacuo, heating being continued at a higher temperature (1150‹C) by induction heating for 1 hour, and the sample then allowed to cool. Measurement of Hall coefficient showed the impurity content of the B and As alloys, and these were combined with pure Ge in calculated quantities to provide a final alloy containing 6.3 X 10<SP>15</SP> As atoms per cc, and 1.2 x 10<SP>15</SP> B atoms per cc. Silicon may be used in place of germanium, and the semiconductor bodies may be used for transistors or PN rectifiers. Reference has been directed by the Comptroller to Specification 752,457.