US2809135A - Method of forming p-n junctions in semiconductor material and apparatus therefor - Google Patents

Description

Oct. 8, 1957 F. KOURY 2,809,135

METHOD OF FORMING P-N JUNCTIONS IN SEMICONDUCTOR MATERIAL AND APPARATUS THEREFOR Filed July 22, 1952 l I p I ..l.

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I NVENTOR mmm/c K0012) United States Patent METHOD OF FORMING P-N JUN CTIONS IN SEMI- CONDUCTOR MATERIAL AND APPARATUS THEREFOR Frederic Koury, Lexington, Mass, assiguor to Sylvania Electric Products Inc., a corporation of Massachusetts Application July 22, 1952, Serial No. 300,171

9 Claims. (Cl. 1481.5)

The present invention relates to semi-conductors, especially silicon and germanium, and in particular to the formation of area junctions therein. Such junctions exist between portions of the semi-conductor which are of opposite conductivity types, adjoining each other at a barrier and are effective for rectification, photodetection, etc.

The formation of a single crystal of germanium, for example, with uniform conductivity of either P-type or N-type has been accomplished by drawing a crystal out of a supercooled melt of germanium containing the proper amount of the desired type of impurity. In order to produce a P-N junction in a crystal thus grown, one practice suggested is to add to a melt of one type of germanium a sufficient amount of impurity to convert the melt to the opposite type during the crystal growing process. This has the effect of growing, as a single integral crystal, an electrical device having N-type and P-type conductivity portions at opposite sides of a barrier. This method presents the difficulty that each time the melt is to be changed in conductivity type, the amount of impurity in the molten material increases, thereby affecting the electrical properties of the crystals pulled from the melt subsequently. Semiconductors are known to be greatly affected by amounts of impurity in parts per million of the pure semiconductor material, and each time the predominating type of impurity is reversed by adding impurities, the total amount of impurities increases.

An object of the present invention is to devise novel methods of growing crystals having P-N junction-s therein without the necessity of contaminating the melt, either once or repeatedly in order to effect repeated reversals of conductivity type in a grown crystal having one or more barriers therein. A further object is to devise a noval method for producing either single reversals or plural reversals of conductivity type in a single bodyof germanium.

In the illustrative embodiment of the invention described in detail below it is seen that two separate cavities are provided in a crucible where one cavity contains germanium or other semiconductor such as a semiconductor of one conductivity type and another cavity contains the semiconductor of opposite conductivity type; and a crystal growing arrangement is adapted to shift either a seed crystal or a partly grown crystal from either melt to the other, and back if desired. When this is done, a germanium crystal having opposite conductivity types adjoining at a barrier is produced. Such structure has particular value where the barrier is rectifying in character; and in order to effect a conversion of the barrier from one which may be predominantly of high resistivity as may occur, the crystal is subjected to a prolonged annealing treatment after which the efficient rectifying property of the barriers at the P-N junctions in the crystal is assured. The illustrative embodiments of invention, in different aspects, are shown in the accompanying drawings, wherein:

Figure 1 is a diagrammatic cross-section of apparatus ice illustrating the preparation of a crystal having multiple conductivity types therein;

Figure 2 is a diagrammatic illustration, greatly enlarged, of a crystal having multiple junctions therein; and

Figure 3 is a diagrammatic illustration, greatly enlarged, of a modification of the crystal illustrated in Figure 2.

Referring first to Figure 1 there is shown apparatus suitable for growing crystals having multiple conductivity types therein. This apparatus includes a furnace having a wall 10 which is adapted to be evacuated and is suitably formed of quartz so as to be transparent; and there is further a crucible 12 as of highly pure graphite surrounded by coil 14 for heating the crucible either by induction or by heat developed in the coil as a resistance element.

The apparatus includes a vacuum seal 16 such as an O-ring assembly, through which a rod 18 may be drawn axially and indexed. Shaft 18 has a plate 20 that may be indexed when removed from its fiat-walled guide passage 22 so that an offset seed-crystal holder 24 may be moved either to cavity 12a as shown or cavity 1212 as indicated in the dotted position. The furnace advantageously may be operated evacuated, but it may contain a gas inert in respect to the semiconductor type produced.

Germanium of a very high degree of purity (strainfree and of high resistivity when cool) is charged into the two cavities of the crucible, one charge predominantly containing an acceptor impurity and the other predominantly containing a donor impurity, as designated in the drawing by P and N. Crystal holder 24 is lowered to bring crystal C into engagement with the N-type melt, and a cable 26 connected to shaft 18 is gradually pulled to raise the crystal holder. This causes an N-type crystal to grow gradually at the end of crystal C. By abruptly raising the crystal, as by lifting pulley 28 over which cable 26 extends, holder 24 can be indexed to bring crystal C with its N-type lower end into vertical alignment with the P-type melt, as indicated by the dotted lines. Then the crystal is lowered into contact with that melt by lowering pulley 28 while plate 20 arrests holder 24 against swiveling away from the P-type melt. After contact of the N-type crystal end with the P-type melt is established, growing of the crystal is commenced, with a P-type portion growing, on an N-type crystal as a single integral crystal. Before actual pulling of the P-type part is com menced, it is desirable to pause until the N-type surface is brought up to the temperature of melt P, to minimize the tendency of developing a frozen high-resistance joint. Some slight amount of the previously pulled N-type crystal may actually melt into the crucible at this time, but this is of only slight concern when the volume of the melt in relation to this remelted portion of the crystal is appreciated.

The process is advantageously repeated many times, to produce many barriers, either unevenly distributed as shown in Figure 2 or evenly as in Figure 3. Where only a thin N-type layer is formed between two large P-type portions, a series of multiple barrier units 36 can be formed by cuts along the broken lines shown in Figure 2, appropriate terminal leads 30, 32, and 34 being formed to the thin central N layer and to the larger P-type ends thereon. Naturally a thin P-type region can alternatively be formed if desired between two larger N-type regions. Where large P-type portions of the crystal alternate with large N-type portions, a large number of area-rectifiers 38 may be produced by transverse diamond-wheel cuts as represented by the broken lines in Figure 3.

The furnace is one well adapted to controlled reproduction of area-junctions or P-N barriers; but because it is a time-consuming procedure in setting up each crystalgrowing operation, the foregoing is especially advantageous not only because of the comparative immunity r 3 of the melts from contamination, but also because of the number of units that can be formed during each furnace set-up. The number of units in any one crystalpulling sequence .is limited by the height of the furnace, the content of the crucibles, and the dimensions of the P and N portions of the crystal grown.

Before the crystals in Figures 2 and 3 are subdivided it is quite desirable to subject them to a prolonged heat treatment, as for example 24 hours at approximately SSW-608 C. for germanium, for the purpose of improving the barriers, in respect to increased rectification efficiency and reduced resistance of the barrier in the forward on low-resistance direction of the current.

Germanium with appropriate acceptor and donor impurities is readily and effectively processed as described; but other semiconductors as silicon are contemplated, and the successive melts are described as of the same material but in broad concept the melts may be of different semiconductor materials as of silicon and germanium that are of the same diamond-cubic type of crystal structure.

The foregoing represents an eminently effective method of producing semiconductor devices having P-N barriers, and devices having multiple barriers at any desired spacings along single crystal, such as junction type crystal amplifiers. Variations in detail and varied application will occur to those skilled in the art, and accordingly the appended claims should be accorded broad interpretation, consistent with the spirit and scope of the invention.

This disclosure includes concepts more broadly claimed in copending application Serial No. 237,001 filed July 15, 1951, by Edward N. Clarke, which application is assigned to the assignee hereof.

What I claim is:

l. The method of froming a P-N junction in a crystalline semiconductor, which includes the steps of providing in a furnace chamber a melt of a semiconductor containing predominantly an impurity effective to impart one conductivity type to the semiconductor, providing in said chamber another melt of a semiconductor containing an impurity effective to impart the opposite type of conductivity, pulling a crystal from one of said melts, transferring instantly Within the existing furnace atmosphere the then pulled crystal to the other melt, and pulling an additional amount of crystal extending integrally from said first-pulled crystal.

2. The method in accordance with claim 1 including the further step of returning directly the crystal with the second-pulled crystal portion to the first melt after only a thin layer has been added in the second-mentioned melt, and pulling an additional crystal portion from the firstmentioned melt whereby to produce multiple closely spaced junctions.

3. The method of forming a P-N junction in a semiconductor in accordance with claim 1, including additionally the step of heat-treating the multi-part crystal grown at an elevated temperature and for a prolonged time, thereby to improve rectification efficiency and reduce the forward resistance of the PN junction.

4. The method of making electrical devices each having a rectifying junction Within a crystal body, including the steps of maintaining a P-type melt and an N-type melt in a common furnace enclosure, alternately transferring instantly within the existing enclosure atmosphere a crystal from the P-type semiconductor melt to the N-type semiconductor melt and reversely, holding the crystal end in contact with each melt sufficiently long to establish continuity of growth of a single crystal and after such contacts pulling the crystal to add alternating portions of N-type semiconductor and of P-type semiconductor.

5. The method of making electrical devices in accordance with claim 4 wherein the pulled crystal is thereafter subdivided into units each of which has portions of different conductivity types and providing an ohmic contact to each such portion.

6. The method of producing electrical devices each having a rectifying junction therein, which includes the steps of maintaining multiple melts of different conductivity types in a common furnace chamber, pulling additions to a semiconductor crystal of alternating conductivity types by transferring the crystal to said melts instantly and successively within the existing furnace atmosphere, growing a crystal portion at each melt, and adding ohmic connections to the different crystal portions.

7. The method according to claim 6 including the steps of alternately and repeatedly shifting the crystal directly from a melt of one semiconductor type to a melt of the opposite semiconductor type, and subdividing the resulting crystal into units each embodying portions of different semiconductor types.

8. The method according to claim 6 wherein the melts are of germanium predominantly containing, respectively, acceptor and donor impurities.

9. Apparatus for forming a P-N junction in a crystalline semiconductor which includes multiple crucible chambers for containing semiconductor melts each of a different conductivity type, means for mantaining the melts molten, a crystal holder, all the foregoing devices being contained in a furnace chamber, means for gradually raising the crystal holder, and means for successively positioning the crystal holder in alignment with each of said crucible chambers.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,356 Sparks et a1 Mar. 17, 1953 2,727,839 Sparks Dec. 20, 1955 2,750,262 Pfann June 12, 1956 2,753,280 Moore July 3, 1956

Claims (1)

1. THE METHOD OF FORMING A P-N JUNCTION IN A CRYSTALLINE SEMICONDUCTOR, WHICH INCLUDES THE STEPS OF PROVIDING IN A FURNACE CHAMBER A MELT OF A SEMICONDUCTOR CONTAINING PREDOMINANTLY AN IMPURITY EFFECTIVE TO IMPART ONE CONDUCTIVITY TYPE TO THE SEMICONDUCTOR, PROVIDING IN SAID CHAMBER ANOTHER MELT OF A SEMICONDUCTOR CONTAINING AN IMPURITY EFFECTIVE TO IMPART THE OPPOSITE TYPE OF CONDUC-

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