US 3349299 A
Description (OCR text may contain errors)
United States Patent Filed Sept. 13, 1963, Ser. No. 308,830 Claims priority, application Germany, Sept. 15, 1962, S 81 478 5 Claims. ci. 317-235 My invention rel-ates to semiconductor rectifiers of the four-layer or npnp type and has for its general object to secure a low voltage drop in the forward direction together with a relatively high and temperature-stable forward breakover voltage as well as a high reverse blocking voltage, to an optimal extent beyond that heretofore attained in semiconductor devices of this kind.
Semiconductor circuit components for the control of power currents are supposed to pass a relatively high current at a smallest feasible voltage drop in the forward direction, and they must also be capable of blocking a relatively high voltage in the reverse direction at a highest feasible operating temperature. The power loss resulting from the voltage drop in the forward direction in conjunction with the permissible operating temperature on the one hand, and the blocking ability in the reverse direction on the other hand, determine the rated current and the rated voltage and thus the amount of electric power controllable by the semiconductor component.
However, the requirements for low voltage drop in the forward direction and for high reverse blocking ability are not always compatible with each other. On the contrary, they are often contradictory to some extent and require observing special expedients to achieve a compromise. In this respect, pnpn rectifiers, consisting of a monocrystalline semiconductor body which has four sequential layers of alternately different conductance type, involve the following difliculty:
If pnpn rectifiers are so produced as to exhibit a low voltage drop in the forward direction, it is generally found that the blocking voltage in the same forward direction--the so-called forward breakover voltager-apidly decreases with increasing temperature so that the permissible operating temperature is relatively low and the permissible current-carrying capacity and power limit are likewise low. Conversely, a good temperature stability of the forward breakover voltage is obtained if one puts up with relatively poor forward conductance properties, particularly a higher voltage drop in the forward direction.
It is therefore a more specific object of my invention to devise a pnpn rectifier which simultaneously exhibits a temperature-stable forward breakover voltage as well as a low forward voltage drop.
To this end, and in accordance with a fundamental concept of the invention, the semiconductor crystal of the rectifier is provided with distributed recombination centers or traps having an energy level about in the middle of the forbidden band and having greatly different capture cross sections (action cross section) for electrons and holes (defect electrons) respectively.
More specifically, according to the invention a pnpn rectifier for power current, having respectively different dopant concentrations in the two inner p-type and n-type layers, has at least the more highly doped one of the two inner layers additionally enriched with recombination centers or traps consisting of impurity atoms whose energy lever is closer to the middle than to the edge of the forbidden band and which have a larger capture cross section for the minority charge carriers of that inner 3,349,299 Patented Oct. 24, 1967 layer than for the majority carriers. The recombination centers or traps consist generally of transistion metals and are preferably taken from the elements copper, gold, manganese, iron, zinc or sulphur. A suitable concentration for these recombination centers is 10 to 10 atoms per cm.
For example, if in the four-layer rectifier the p-type base layer has a higher dopant concentration than the n-type base, this being normally the case with pnpn rectifiers made of originally n-type silicon, then the capture cross section of the introduced recombination centers is to be considerably greater for electrons than for holes. This is because in this particular case the more highly doped base region, constituting one of the inner layers of the semiconductor device, has p-type conductance so that in this layer the electrons constitute the minority charge carriers. The embodiment particularly described in the following relates to an example of the kind just mentioned, i.e. to a semiconductor controlled rectifier whose p-type is more highly doped than the n-type base. However, it will be understood that the invention is analogously applicable to other four-layer devices including those in which the n-type base region is more highly doped than the p-base. In devices of the latter kind the capture cross section of the added recombination centers must be considerably greater for holes (defect electrons) than for electrons.
The invention will be further explained with reference to an embodiment of a silicon controlled rectifier illustrated by way of example on the accompanying drawing in which:
FIG. 1 shows schematically and on greatly enlarged scale a cross section of the rectifier together with a schematic circuit diagram.
FIG. 2 is explanatory and shows schematic-ally the layer sequence of the same rectifier.
FIG. 3 is an explanatory graph relating to the dopant concentrations in the four layers plotted over the thickness of these layers.
FIG. 4 is a coordinate diagram showing typical current-voltage characteristics of pnpn rectifiers for exhibiting the improvement obtained by virtue of the invention.
FIGS. 5a and 5b are explanatory graphs showing the curves of amplifying gain in dependence upon current loading.
The semiconductor rectifier illustrated in FIG. 1 comprises a monocrystalline silicon body having the shape of a flat circular disc. The body is provided with a circular central base or gate electrode S and a surrounding ring-shaped emitter electrode K both located on one side of the silicon body. The opposite side is occupied almost entirely by a ring-shaped emitter electrode K In this embodiment the base region 4 adjacent to the base electrode S has p-type conductance and is adjacent to an n-type base reg-ion 1. The emitter region 2 adja cent to the emitter electrode K has p-type conductance; and the emitter region 3 adjacent to the emitter contact K has n-type conductance. The npnp layer sequence is more clearly apparent from the schematic showing in FIG. 2.
One way of producing a silicon controlled rectifier as shown in FIG. 1 is the following. Employed as starting body is a monocrystalline circular disc of weakly doped silicon having a donor concentration of 10- cm. or less, for exampie a donor concentration of approximately 10 cm. A portion of this original disc is left unaffected by the subsequent manufacturing process and forms the n-type base layer 1 in the completed component illustrated. For the purpose of illustration, the drawing shows the dimensions on distorted scales, particularly with respect to the relation of thickness to diameter of the layers. The actual dimensions of a disc employed in production described were 18 mm. diameter and 0.3 mm. thickness. The weakly n-type disc was doped on its entire surface by diffusing gallium from the gaseous phase into the surface until the disc was coated with a p-type layer. Other acceptor dopants may be employed instead of gallium, for example aluminum or boron. A particularly good result was obtained by applying a two-stage diffusion process, using gallium as dopant in the first stage, and aluminum in the second stage.
Thereafter a circular foil of aluminum was placed on one side (the right-hand side in FIG. 1) of the silicon disc for producing the p-emitter region 2 and the emitter contact K this foil covering substantially the entire surface area on this side of the disc. On the opposite side (at the left in FIG. 1), a small circular wafer of aluminum, punched out of aluminum foil, was placed upon the disc for producing the base or gate contact S, and a ring-shaped gold foil containing an alloy content of about 0.5% antimony was placed about the circular contact S for producing the n-type emitter region 3 and the emitter contact K The three metal components were alloyed into the silicon surfaces simultaneously by a single alloying operation. The resulting two emitters 2, 3 consist of recrystallization layers having correspondingly high dopant concentrations, and the electrodes or contacts K K consist of alloys of the contact metals with silicon in the eutectic ratio. By suitable choice of the foil thicknesses for a given alloying temperature, a ratio of the penetrating depth of the alloy with respect to the thickness of the silicon disc can be obtained that qualitatively corresponds to the illustration in FIG. 1. That is, the p-conducting diffusion layer (p-emitter 2) on the bottom side is completely subjected to the alloying process, whereas on the top side the diffusion or alloying layer extends over only a portion of the surface while the remaining surface portion remains unaffected in order to for the p-base 4.
To prevent the p-base 4 from being in low-ohmic connection with the p-emitter 2, the diffusion layer is interrupted by a ring-shaped groove G. This groove is produced prior to or after completion of the alloying process by mechanical or chemical processing. The semiconductor device can thereafter be provided in known manwith suitable surface protection means, for example an oxide coating or varnish coating, and can also be encapsulated in a housing under vacuum or protective gas.
For electrically operating the semiconductor device, a load circuit is to be connected to the two emitters 3, 2 by means of the appertaining electrode contacts K and K As shown in FIG. 1 by way of example, the load circuit comprises a voltage source A and a load V. The appertaining control or gate circuit contains a source of control voltage B, here exemplified by a battery, and a suitable control component represented by a switching member H. The control circuit is connected between contacts S and K and hence between the p-base 4 and the next adjacent emitter 3. The control circuit is poled in the forward direction of the p-n junction between the p-base 4 and the n-emitter 3. Designated as the forward direction of the semiconductor device as a whole is the direction from the p-emitter 2 to the n-emitter 3, i.e., from contact K to contact K When the auxiliary switching member H is controlled in synchronism with the alternating voltage from source A, so that in each positive half wave of the voltage a control pulse is applied through contact S, the load circuit is traversed by direct current. By varying the time position of the gate pulses within the half-wave period, the median value of the direct voltage in the load circuit can be controlled or regulated.
The schematical representation of the four different layers with contacts K K S in FIG. 2 may be looked upon as constituting a portion of the device according to FIG. 1 located between the lines II-II, disregarding the thickness distortions employed in FIG. 2 for the purpose of representing the layer boundaries in conjunction with the diagram of FIG. 3.
In FIG. 3 the vertical reference axis denotes in atoms per cm. the dopant concentrations in the individual regions corresponding to the four different layers 1 to 4. According to the diagram, the acceptor excess A2 produced by the above-described alloying process in the pemitter 2 has a value of 3-10 cm." which is nearly constant over the entire layer thickness. As previously mention, the donor thickness D1 in the region of the n-base 1, that remained unaffected by the alloying process, is approximately 10 cm? and is likewise constant over the entire thickness of this layer. In contrast thereto, the acceptor concentration A4 is the p-base 4 has become non-uniformly distributed over the layer thickness due to the effect of the above-described diffusion process, this being apparent from the corresponding curve A4 which at the right boundary of the region commences with a value of 10 cm.- and attains at the left boundary a value of 10 cm.-
According to the illustrated curve A4, the average value of the dopant concentration in the p-base layer 4 is somewhat higher than 3-10 Cm." The required median dopant concentration of this range, of course, can also be obtained by means of an alloying process. For example a gold foil containing a slight amount of boron may be alloyed into the left side (FIG. 1) of the semiconductor body. This results in the formation of a recrystallization layer having p-type conductance. The alloy layer above the recrystallization layer, consisting of a gold-silicon alloy with a content of boron, can be subsequently eliminated by etching after the alloying process is terminated. The recrystallization layer produced in this manner than constitutes the p-base 4 and exhibits over its entire thickness a substantially uniform dopant concentration, for example of about 10 Cm. this being indicated by a broken line A4 in FIG. 3. Upon the p-base thus obtained by alloying, the contact S and the n-emitter with contact K can be produced in the same manner as described above with reference to a p-base obtained by diffusion.
The n-emitter 3 according to FIG. 3 has over its entire thickness a substantially constant donor density D3 between 10 and 10 cmf The diagram presented in FIG. 4 manifests the improvement achieved according to the invention by the introduction of recombination centers or traps for the minority charge carriers in the more highly doped base region, by a comparison of different current-voltage characteristics of a pnpn rectifier. The portion of the blocking characteristic located in the third quadrant, i.e. the left lower quadrant, has been assumed to be identical for the two rectifiers compared with each other, namely a rectifier with added recombination centers and an otherwise identical rectifier without the added centers. The diagram shows the reverse leakage current I per unit area of the blocking p-n junction (in ma./cm. plotted downwardly from the origin along the ordinate, versus the reverse blocking voltage U (in volt) which is plotted from the origin toward the left on the abscissa, this reverse blocking voltage being impressed across the two outer p-n junctions and particularly upon the junction between p-emitter and n-base. In the first (right upper) quadrant there are shown the characteristics, collectively denoted by I in the forward direction of the pnpn rectifier, this direction being identical with the blocking direction of the middle p-n junction between layers 1 and 4. These characteristics I are shown up to the forward breakover voltage. The breakover voltage U, (in volts) is plotted on the abscissa from the origin toward the right. Further shown in the diagram are two curves, 1 and 1 representing the forward-current density or load-current density. Applicable to these is a considerably reduced current-measuring scale (in amps/cm?) which is entered on the right-hand r side of the ordinate axis. The curves I and 1 indicate the dependance of this current density upon the forward voltage drop U the latter being indicated (in volts) on a greatly increased scale upon a line parallel to the abscissa on the upper margin of the diagram.
The previously known pnpn rectifiers of silicon often exhibit at normal room temperature of about 20 C. a forward breakover voltage U above 1000 volts. However, when the temperature of the rectifier increases during operation, a large reduction in forward breakover voltage U results, as is shown by the curves I for 90, 100, 120 and 150 C. for example. Such reduction in forward breakover voltage with increasing temperature is exhibited particularly by those pnpn rectifiers whose forward voltage drop U at full rated operating current is relatively low, particularly not appreciably above 1 volt. These known rectifiers must not be continuously subjected to such a high current that the maximum temperature of about 150 C., although permissible as such, remains maintained, because then the forward breakover voltage would decline to an excessively low value so that ignition takes place at each beginning of a positive half wave of operating voltage, regardless of the desired and adjusted ignition moment, and the rectifier loses its controllability. Consequently such pnpn rectifiers cannot be fully utilized in operation. Other known rectifier types that exhibit a better temperature stability and in which the forward breakover voltage does not or only little decline with increasing temperature up to 120 or 150 C., have the disadvantage that their forward current corresponds to a characteristic similar to the curve I in the diagram of FIG. 4. That is, the forward voltage drop U when operating at relatively high current densities, assumes high values and considerably increases beyond 1 volt. For that reason the power losses with such rectifiers are considerably higher and the permissible maximum temperature is reached at a much lower current. Hence the latter rectifiers can likewise not be fully utilized in operation.
While thus in known pnpn rectifiers an advantage is always accompanied by a deficiency, the addition of recombination centers or traps for the minority carriers in the more highly doped base region according to the invention affords combining the advantages of both known types of pnpn rectifiers but avoiding their deficiencies. That is, the novel pnpn rectifiers retain full controllability even at the highest permissible temperature because their forward breakover voltage is not appreciably lower than at normal room temperature, and they also have relatively low losses because their forward voltage drop at the highest permissible current does not or not appreciably exceed 1 volt. This improvement can be explained as follows:
Impurity atoms and other imperfections whose energy level is approximately in the middle of the forbidden band do not have an appreciable effect upon the doping conditions in a semiconductor. They form recombination centers or traps at which a recombination and generation of electron-hole pairs takes place, thus determining the lifetime 7- of the charge carriers and thus their diffusion length in a semiconductor, particularly in the two base regions.
By the introduction of disturbance localities or impurity atoms having the above-mentioned recombinationcenter or trap properties, the lifetime of the charge carriers in the low-ohmic base, in the above-described specific example therefore in the p-base, becomes greatly dependent upon the current intensity in the sense that at low currents the lifetime is very short and becomes very large only at relatively high current densities of more than 10 ma./cm. This has the consequence that the current amplification is this p-base at low current is at first very low and rapidly increases when current densities exceed the value of about 10 ma./crn. where the amplification approaches the unity value. With sufliciently high doping, and hence a sufficiently high acceptor excess in the p-base, this characteristic behavior of the current amplification factor (gain) remains substantially preserved without change up to relatively very high operating temperatures, such as 150 C. and more.
Recombination centers of the described kind, producing a current-dependent lifetime in one of the base regions, have the elfect, if they are also present in the second base region of the reverse type of conductance, of producing in the second region a virtually current-independent lifetime and thus a largely current-independent value of current amplification. Consequently, by virtue of the invention the current amplification factors m and a in the two base regions assume a characteristic as typified by the curves shown in FIG. 5a, at least to the extent these amplifying factors are determined by the volume lifetime. This current-gain characteristic can be additionally modified by surface effects, for exampleparticularly at very low currents-by augmented recombination in the space-charge regions between emitter and base. Such additional infiuences do not affect the applicability of the foregoing considerationspThe absolute value of the current amplification factors at high current values can be adjusted by the number or concentration of the recombination centers, preferably 10 to 10 atoms per cm. diffused into the semiconductor and by the selected thickness of the base regions.
On the basis of the current-amplifying characteristics exemplified in FIG. 5a, the achieved combination of low forward voltage drop and high temperature stability of the forward breakover voltage can be explained as follows. As is known, pnpn rectifiers have a blocking action in the forward direction only as long as the sum of the current amplification factors, illustrated in FIG. 5b, in the n-base and in the p-base has a total value smaller than unity. That is, such a rectifier will trigger if this sum reaches unity value. By the described enrichment of the p-base with recombination centers according to the invention, the current amplification factor (r in the p-base at low currents is limited to a very low value. As long as the current in the forward direction does not amount to at least 10 ma./cm. or more, so that the current amplification factor in the p-base becomes large, the rectifier cannot be triggered, assuming that simultaneously the current amplification factor a in the n-base has an acceptable value not too close to the unity value. The latter condition, is achieved, for example, by entering recombination centers according to the invention also into the n-base and by suitably dimensioning the thickness of the n-base within the range of 0.05 to 0.15 mm. Normally, the mentioned high leakage currents of more than 10 ma./cm. are reached by the pnpn rectifier in the blocked condition only at the maximal blocking voltage (peak reverse voltage) determined by punch-through or breakdown phenomena. Only at very high temperatures can such high leakage currents occur already at lower voltages. Since, as already mentioned, the stated characteristic of the lifetime 1- remains largely independent of temperature if the p-base is sufficiently highly doped, for example with an average acceptor concentration of 10 cm. and more, the forward breakover voltage also remains temperature stable; that is, the latter voltage is equal to the breakdown or punch-through voltage up to very high temperatures of C. or more. Thus the invention achieves the objective of securing a high and temperature-stable forward breakover voltage. Simultaneously, the invention also secures the desired good forward-conductance properties. For good forward conductance the diffusion lengths in both base regions in the range of high forward currents must be sufficiently large in comparison with the width of these base regions. This requirement can also be formulated as follows:
The sum of the current amplification factors in the two base regions in the range of high forward currents must be appreciably larger than the unity value. This is likewise achieved by virtue of the invention. Since the current amplification factor in the p-region is very small up to relatively high blocking currents, the current-amplification factor in the n-region, as already mentioned, can be made relatively high, for example 0.5 to 0.8. Since the current amplification factor in the p-base at relatively high currents becomes very large, i.e. approaches the unity value, the sum of both current amplification factors in the range of high forward currents is definitely above the unity value. Consequently, the second objective, calling for a low forward voltage drop, is likewise achieved.
The foregoing is analogously applicable to pnpn rectifiers in which the n-base is more highly doped than the p-base. In this case, an approximately current-independent current amplification is attained in the p-base of smaller dopant concentration, whereas in the more highly doped n-base the current amplification becomes dependent upon the current intensity in the above-described manner. This requires that suitable impurity atoms, such as one of the above-mentioned transition metals, be diffused into the more highly doped n-base, these atoms having a larger capture cross section for holes (defect electrons) than for electrons.
As explained above, the introduction of recombination centers of given properties into the more highly doped base region has the fundamental effect of producing in this one region a current-dependent carrier lifetime which is small at low current intensities but larger at high current intensities. For achieving this aim it is sufficient to introduce the recombination centers only into the more highly doped base region. However, it is generally simpler and preferable to distribute the described recombination centers approximately uniformly throughout the entire semiconductor crystal of the rectifier. Obtaining such a uniform distribution is facilitated by the transition-metal properties of such recombination centers. That is, recom bination centers whose energy level is located approximately in the middle of the forbidden band of silicon, generally exhibit a relatively high diffusion constant with respect to silicon. This applies particularly to recombination centers or traps consisting of copper, gold, manganese or iron.
According to a preferred production method of the invention, advantage is taken of this phenomenon. The selected substance is entered into a raw-crystal, for example a discor plate-shaped monocrystal, by diffusion or alloying. As a result, the recombination center atoms become uniformly distributed throughout the silicon crystal. Such recombination centers can also be introduced into a silicon rod during performance of a zone-melting process. By virtue of the very low distribution coefficient of the additional substances in silicon, such introduction and a resulting uniform distribution are obtainable in a relatively simple manner. Both methods also permit controlling or adjusting the absolute content of such impurity atoms in the semiconductor material.
The above-mentioned substances Cu, Fe, Mn, Au are known as recombination centers in silicon. With respect to the capture cross section of these substances for electrons and holes, recombination centers with acceptor character generally have a larger capture cross section for holes, whereas substances with donor characteristic normally have a larger capture cross section for electrons. Accordingly, it is preferable to employ manganese as recombination-center substance in pnpn rectifiers of silicon whose phase is more highly doped than the n-base, whereas copper is preferably employed for silicon rectifiers having a more highly doped n-base. Iron and gold are akin to donors as well as to acceptors and are applicable in both cases.
While the invention has been described and explained above with reference to the example of a semiconductor component made of silicon, the invention is analogously applicable .to other semiconductor substances employed for similar purposes as silicon in electronic components, particularly for semiconductor substances that crystallize in the diamond latice. Thus the invention is applicable in the above-described manner with germanium and silicon carbide. It is likewise applicable to intermetallic semiconductor compounds of respective elements from the third and fifth groups of the periodic system (A B compounds) such as gallium arsenide or indium antimonide, as well as to semiconductor compounds of respective elements from the second and sixth groups of the periodic system (A D compounds) such as zinc selenide, for example. The above-mentioned transition metals are also applicable as recombination centers or traps for these other semiconductor elemental and compound substances. Other transition metals, particularly those of the iron group (Ni, Co), are also applicable, for example with germanium. Generally the preferred addition substances to serve as recombination centers or traps are best selected for each particular semiconductor material on the basis of its particular energy-band configuration.
As is apparent from the foregoing, the methods suitable for introducing the recombination center or trap substances into the semiconductor material are in principle the same as the methods conventionally employed for doping semiconductor materials with donor and acceptor substances, particularly the above-mentioned diffusion method and the alloying method. Described in the following by way of example are three different modes of entering the recombination centers during the abovedescribed production of the pnpn rectifier shown in FIG. 1, into the semiconductor body of the rectifier or into the more highly doped base layer of the body.
Example 1 The addition substance such as Mn or Cu that is to form the recombination centers is introduced into an elongated silicon rod to be subsequently sliced into monocrystalline discs for use as starting body in the abovedescribed production of the device according to FIG. 1. The entering of the addition substance is effected by subjecting the silicon rod to crucible-free zone melting. The silicon rod employed is monocrystalline, having a donor concentration between 10 and 10 atoms per cm. A pellet consisting of an alloy of silicon with the desired quantity of the addition substance is placed into the interior of the monocrystalline rod at any location of the length to be subjected to zone melting. There are two ways of doing this. One way is to drill a a lateral bore into the rod and to place the pellet into the bore. The second way is to produce a molten zone in the rod and then immerse the pellet into the molten zone. By then passing the molten zone lengthwise through the rod, the recombination-center substance becomes distributed over the entire rod volume being processed. For securing a substantially uniform distribution of the transitionmetal atoms in the silicon, the rod is subsequently subjected to zone levelling. That is, the floating zone is passed repeatedly and alternately in opposed directions lengthwise through the rod, each time increasing the degree of uniformity in distribution. The same result is obtained by subjecting the rod to tempering at glowing temperature for at least one full day, preferably several days. Thereafter the semiconductor discs to be used as starting body in the production described above with reference to FIG. 1 can be cut off the rod.
Example 2 After preparing starting bodies of silicon having the circular shape described above with reference to FIG. 1 and being 18 mm. in diameter and 0.3 mm. in thickness, these discs are placed into a quartz ampule together with a small quantity of the selected transition metal that is to be introduced into the plates to form recombination centers. Thereafter the ampule is evacuated and fused off. Now the sealed ampule is placed into an annealing surface and heated to a temperature sufficiently high to evaporate the addition substance at least partially, the
temperature being below the melting point of silicon. As a result, the addition substance diffuses from the gaseous phase into the silicon discs where it becomes uniformly distributed throughout the entire volume of the discs on account of the high diffusion constant of the transition metal, such as Cu or Mn in silicon. The discs are thereafter removed from the ampule and are then further fabricated to produce pnpn rectifiers, for example by the method described above with reference to FIG. 1.
Example 3 Used as semiconductor starting bodies in the process described above with reference to FIG. 1 are silicon discs which do not yet contain the addition substance that is to form recombination centers. The discs are subjected to the diffusion process mentioned in conjunction with FIG. 1, by means of which the conductance type of a surface layer is converted to the opposite conductance type. As mentioned, the process is then followed by an alloying process with the result of joining alloy foils with the silicon body and producing doped regions in the silicon body by formation of a silicon alloy with dopant atoms contained in the foils. For the purpose of the present invention, and in accordance with the mode of performance presently described, care is taken that the foil employed for the formation of the n-type emitter also contains the addition substance that is to form recombination centers in the silicon body. This is already the case in the example described because the foil that forms the n-emitter consists mainly of gold. Heretofore, however, the alloying process has been conducted from the viewpoint of preventing a reduction in lifetime of the charge carriers, in the same manner as when producing non-controllable highly-blocking p-s-n rectifiers. For that reason, the alloying process has been performed at a temperature up to about 700 C. and in any event as much as feasible below 750 C. In contrast thereto, when producing a pnpn rectifier according to FIG. 1 in accordance with the present invention, it is necessary to enter gold atoms at least down into the p-base. Accordingly, a higher temperature is applied during the alloying process, namely a temperature of 800 to 900 C. Furthermore, this temperature must be maintained during a longer period of time, for example five to thirty minutes, so that during this period a portion of the gold contained in the liquid alloy will diffuse into the adjacent solid layer of silicon. A penetrating depth of gold in the range from 20 to 100 microns is thus obtainable and desired with respect to a rectifier as described with reference to FIG. 1.
To those skilled in the art it will be apparent from a study of this disclosure that my invention is not limited to the embodiments of the method and resulting products illustrated and described herein but can be modified in various respects and can analogously be applied to semiconductor materials and recombination-center substances other than particularly set forth herein, without departing from the essential features of my invention and Within the scope of the claims annexed hereto.
1. A semiconductor power rectifier, comprising a substantially monocrystalline semiconductor body containing in succession a first, a second, a third and a fourth layer, which layers have alternate respective types of conductivity, of which the first, second and third layers form a first transistor element and the second, third and fourth layers form a second transistor element, the second layer having a doping concentration of substantially 10 cm. in a region adjacent to the first layer and of several powers of 10 lower in a region adjacent to the third layer, and the third layer having the lowest doping concentration of the four layers, and wherein the current gain of the first transistor element is substantially zero in the load current density range below 10 HIEL/CIILZ and begins to rise steeply in a load current density range between 10 and ma./cm. and wherein the current gain of the second transistor element has a value below unity and is substantially independent of load current in the load current density range between 10 and 100 ma./cm.
2. The rectifier of claim 1, wherein in the load current density range between 10 and 100 ma./cm. the current gain of the second transistor element is so small, that the sum of the two current gains exceeds unity only at a load current density value at which the current gain of the first transistor element exhibits substantially its maximum steepness value.
3. The rectifier of claim 2, wherein the second layer contains an addition of recombination centers consisting of impurities the energy level of which lies closer to the center than to the nearer edge of the forbidden band of the semiconductor material and which have for the minority carriers of the second layer a capture cross section substantially larger than that for the majority carriers of said second layer.
4. The rectifier of claim 3, wherein the first, third and fourth layers each contains an addition of recombination centers.
5. The rectifier of claim 3, wherein said recombination centers are formed of metal selected from the group gold, copper, iron and manganese.
References Cited UNITED STATES PATENTS 2,959,504 11/1960 Ross et al. 317-235 2,980,832 4/1961 Stein et al 317235 2,993,154 7/1961 Goldey et al 3 l7235 2,997,604 8/ 1961 Shockley 317-235 3,036,226 5/ 1962 Miller 317-235 3,064,132 11/1962 Strull 3 l7235 3,124,703 3/1964 Sylvan 317-235 3,210,560 10/1965 Stehnex 3l7234 3,260,624 7/1966 Wiesner 3 l7235 OTHER REFERENCES Effect of Surface Recombination and Channel on PN Junction and Transistor Characteristics, by Chih-Tangsah, published in IRE Transaction of Electron Devices, January 1962, pages 94 to 108.
JOHN W. HUCKERT, Primary Examiner.
A. J. JAMES, Examiner.