US2980810A - Two-terminal semiconductive switch having five successive zones - Google Patents

Description

April 18, 1961 J M GQLDEY 2,980,810

TWO-TERMINAL SE'ZMICEONDUCTIVE SWITCH HAVING FIVE SUCCESSIVE ZONES Filed Dec. :50, 1957 Q R I F/G. Y

CON 7' ROL SOURCE a 1 F/G.5

ACCESS SWITCHES AND lNEO/PMAT/ON CCI'S.

INVENTOR J. M. GOLDEY ATTORNEY United States atent TWO-TERMINAL SEMICONDUCTIVE SWITCH HAVING FIVE SUCCESSIVE ZONES Filed Dec. 30, 1957, 581. No. 706,185 3 Claims. (Cl. 307--88.-5)

This invention relates to semiconductive switches and, more particularly, to switches in which the semiconductive element is capable of two extremes of impedance characteristics for voltages applied in either direction so that it may be operated as a bidirectional switch.

This invention is related in certain aspects to the switching arrangement disclosed in the application of W. Shockley Serial No. 548,330, filed November 22, 1955, now Patent No. 2,855,524, issued October 7, 1958, which includes a diode switching element comprising a semi-conductive body having four zones arranged in succession, contiguous zones being of opposite conductivity type, and including electrical connections to only the two end zones, the two intermediate zones being allowed to fioat. In addition to the advantageous characteristics attained by the foregoing-noted arrangement disclosed by Shockley, there is a need, in the operation of a variety of electrical systems, for a transmission path link which has two stable states of impedance level for either direction of applied voltage.

In certain magnetic memory systems, for example, there is utilized a plurality of access switches to control the characteristics of transmission paths between a numher of information input sources and a number of information storage devices. 'In particular, in systems of the type disclosed, for example, in the application of A. H. Bobeck and J. H. Felker, Serial No. 555,889, filed December 28, 1955, now Patent No. 2,939,114, issued May 31, 1960, it is desirable to -provide as an element of the access switch a bidirectional multi-impedance level device to enable performance of several information input functions with a single driving source.

Therefore, one object of this invention is to improve switching systems by providing a switch capable of responding equally well to signals of either polarity.

A more specific object is to provide a bidirectional or symmetrical semiconductive switch which is rugged and reliable, yet easy to fabricate and convenient to interconnect into a switching system.

A further object of this invention is to provide a reliable two-directional semiconductive switch which is a simple two-terminal device.

For the accomplishment of these and other objects, a feature of the invention is a two-terminal switching element comprising a semiconductive body having five zones arranged in succession, contiguous zones being of opposite conductivity type, and including electrical connections to only the two end zones, the three intermediate zones being allowed to float. Moreover, to attain the, desired switching action the intermediate zones must be constructed in accordance with specific requirements to be discussed in more detail hereinafter.

For operation as a switch, there is applied between the two-terminal connections to the five-region body a voltage of either polarity so as to alternately forward and reverse bias the four junctions. That is, for a PNPNP arrangement, a positive voltage applied to either of the outer P-type regions biases the junctions, taken Patented Apr. 18, 1961 successively from the positive end, in the forward, reverse, forward and reverse directions, respectively. Under such conditions, before being triggered to the low imedance condition, the element exhibits between its two terminals an impedance which is essentially the sum of p the impedances of the four junctions and, because two are reverse biased this total impedance generally is very high. In order to trigger the device to the low impedance :state, the voltage applied across its two terminals is temporarily increased beyond a predetermined switching value. down the reverse-biased junctions enabling a sharp decrease in the impedance. The device then switches to a low impedance state at a voltage defined as the sustain voltage for the low impedance condition. For an applied voltage of the opposite polarity, the direction of bias on the four junctions, respectively, is'reversed. However, the mechanism for triggering the device from the high impedance to the low impedance st-ate remains the same. Expressed in somewhat different terms, the five-region semiconductive body is designed so that the triggering action employed results in a change in the effective alpha of the body from a value which is less than unity to a value which becomes unity, where the effective alpha of the body is defined as the sum of the inherent alphas of two of the three intermediate zones including the middle one and the inherent alpha of each intermediate zone is defined as the ratio of the current change across the collecting junction of the zone to the current change across the emitting junction of the zone if the potential across the collecting junction were held constant.

The values of the inherent alphas of the intermediate zones are made to depend on the level. of the carrier density in the various zones of the body. Since an increase in carrier lifetime with an increase in current density in any region will increase the inherent alpha for the proximate intermediate zone, :an increase in lifetime with increasing carrier density will result in an increase in effective alpha of the body with increasing current density. Before breakdown of the reverse-biased junctions, the carrier density is low, and so the inherent alpha of each intermediate zone is low and the elfective alpha of the body is such as to result in a high impedance state. After breakdown, the carrier density in the body is high, the inherent alphas increase until the effective alpha reaches unity and the body switches to a low impedance state. In the device of this inventionwhich exhibits an increase in inherent alpha with increasing carrier density, it is important that the intermediate zones be relatively thin so as to enable substantially complete carrier collection at higher current densities.

With respect to thisbidirectional switching behavior, the five-region device of this invention differs materially, from the four-region device disclosed by Shockley in the aforementioned application for the reason that reversal of the polarity of the voltage applied to the two terminals of the four-region switching element of Shockley results in biasing the middle junction in the forward direction and the two outer junctions in the reverse direction. In this condition, the element will not trigger to a low impedance state. With only the middle junction biased in the forward direction, the efifective alpha, as a practical matter, can never reach unity be cause the two three-region portions of the device have the same emitter.

It will be appreciated from the aforementioned disclosure of Shockley that behavior of the five-region device of this invention in the manner described is attributable to the change in the inherent alpha of the body, which is given by the sum of the inherent alphas of two of the three intermediate zones, one of which is the cen- This increased voltage serves initially to break tral zone. Thus, in certain aspects, the basis of this behavior is similar to thatfor the four-region device. However, because of the relationship of carrier lifetime, carrier density and inherent alpha whereby prior to breakdown of the reverse-biased rectifying junctions all'three factors are relatively low and after breakdown all three become relatively higher in value, it is important that the three intermediate regions be of relatively'small thickness.

It is a further feature of this invention that the fiveregion switching element includes three intermediate conductivity type regions, each having a thickness of the order of several ten-thousandths of an inch.

In one illustrative embodiment of the invention, a PNPNP monocrystalline silicon body having electrical connections to only its two end zones serves as the semiconductive switching element. Associated with this element is an external circuit which may include a voltage supply for biasing in reverse two of the. four'junctions as previously described and for aiding in sustaining the low impedance condition. Switching action is achieved by varying under the control of signal information from a control voltage source the current density in the body.

A better understanding of the invention and its other objects and features will be had from the following detailed description taken in conjunction with the drawing in which:

Fig. 1 shows a circuit arrangement incorporating a switch of the kind in accordance with the invention;

Fig. 2 is a schematic depiction of the four-region semiconductive diode of the prior art; a

Fig. 3 is a plot of the current-voltage relationship of the prior art deviceof Fig. 2;

Fig. 4 is a plot of the current-voltage relationship of the switch shown in Fig. 1; and

Fig. 5 shows schematically a portion of a magnetic memory array employing switches in accordance with the invention as access switches. V

In the circuit arrangement of Fig. l, the five-region semiconductive element 11 is connected in series with a utilization device represented by the resistor 13 and a source of control voltage 12. The serniconductive element comprises five zones 16, 17, 18, 19 and 20 in succession, contiguous zones being of opposite conductivity type thereby defining the PN junctions 21, 22, 23 and 24. Low resistance ohmic contacts 14 and 15 are made to the P-type end zones and 16, respectively. The intermediate zones 17, 18 and '19 are devoid of external electrical connections. As noted hereinbefore, it is important to the achievement of switching action that these intermediate zones be thin.

In the normal quiescent condition of the element 11 prior to breakdown, there will be substantially no current flow. In order to induce breakdown of the device either a positive or negative voltage pulse is applied from the control source 12. This pulse must be of suificient magnitude to break down the two reverse-biased rectifying junctions. For example, if a positive pulse is emitted by the control source 12 and applied to the electrode 15, the junctions 24 and 22 will be forward biased and the junctions 23 and 21 will be reverse biased. Referring also to the curve of Fig. 4, when the applied voltage exceeds the breakdown voltage V, of junctions 21 and 23in series, current begins to flow. As the current density increases the inherent alphas across regions 17 and 18 increase with a resultant drop in voltage represented by the negative impedance-portion 41 of the curve of Fig. 4. When the sum of the inherent alphas of regions 17 and 18 reaches unity, the device attains its low impedance state and the switch, in effect, is closed. The voltage .level of the low impedance state is sub stantially the breakdown voltage of junction 21 plus a small contribution due to the forward biased junctions 22, 23 and 24. As is known to workers in the art,

the breakdown voltage of the various junctions may be adjusted to a desired value by appropriate design.

Thus, in the arrangement of Fig. 1, the semiconductive switch will close for the duration of the voltage pulse and will return to a zero'bias condition at the end of the pulse. From the foregoing it will be understood that, although not shown, a direct current bias source of appropriate polarity and sufiicient magnitude may be included in circuit therewith to maintain the switch in the closed condition after removal of the trigger voltage.

If, on the other hand, a negative pulse is applied from the control source, the five-region device by virtue of its symmetrical structural arrangement will respond in exactly the same manner as described heretofore, with the difference, however, that polarities are reversed. Thus, the characteristic will be as shown in the left-hand portion of the graph of Fig. 4 with the breakdown of voltage represented by the value and the sustain voltage by ---V representing the low impedance state and rising to a large value of current of opposite polarity. Typical values obtained for V 1, are 82 volts and 10* amperes. For the point V 1,, representing the entrance upon the low impedance condition representative values are 11 volts and 2 X10 amperes. The characteristic is substantially symmetric for the first and third quadrants in the example shown'.-

By proper design and for particular applications it is possible to make a device in whichthe absolute values of the breakdown and sustain voltages and turn-on current I, are different in each quadrant.

By way of comparison, Figs. 2 and 3 depict in simplified form the four-region semiconductive switching element of Shockley shown schematically by the device 30 of Fig. 2. As is set forth in the disclosure by Shockley, a positive bias is applied to the left-hand P-type end region 34 of the device, a positive breakdown voltage will produce the characteristic shown in the first quadrant of the graph of Fig. 3. It will be noted that this characteristic resembles the similar portion of the curve of Fig. 4, with important dilierences. Because of the additional reverse-biased junction, the sustain voltage is higher in the case of the five-region device and is designable. A more significant dilierence, however, is illustrated by a comparison of the response when a voltage of opposite polarity is applied. In the device of Shockley, a reversal in the polarity of the voltage applied to the ends of the device 30 produces a characteristic as shown in the third quadrant of the graph of Fig. 3. Upon the application of a sufficiently high voltage both reverse-biased junctions break down and the device conducts much in the manner of a single junction avalanche diode. In other words, because only the intermediate junction 32 is forward biased and the junctions 31 and 33 are reverse biased, switching action will occur only to an insignificant extent and there will be only a very slight negative resistance portion in, the characteristic curve of the third quadnant of Fig. 3. This is in marked contrast to the switching response exhibited for both the first and third quadrants by the five-region device depicted in Fig. 4.

Turning to Fig. 5, a simplified schematic arrangement of a portion of a magnetic memory device is shown using the switching arrangement in accordance with this invention. As is well-known in the art in systems such as described in the above-noted application of Bobeck and Felker, magnetic core devices may be used for the storage of information, which information may be readily extracted by electrical means. Generally, such magnetic devices in one system may be caused to exist in two different states of magnetization as induced by the application thereto of positive and negative pulses. Heretofore, it has been necessary generally to utilize separate driving sources to apply pulses oi opposite polarity through switches, generally termed access switches. However.

with the circuit arrangement of this invention a source 50 of positive or negative pulses may be connected to an array of five-region semiconductive switching elements 51-52, each of which applies a positive or negative pulse to a corresponding row of magnetic core devices 5354. Thus, there is enabled a considerable simplification in the driving circuit arrangement of such a magnetic memory array.

The five-region semiconductive element, as shown in Fig. 1, may be readily made from a monocrystalline silicon ingot produced by means of any one of several single crystal growing processes well-known in the art. One

such process is described in the foregoing noted appli-' cation of Shockley. Likewise, in similar fashion to that described by Shockley, the ingot is divided into slices from which a single wafer 100 mils square and four mils thick is prepared. For the device of Fig. 1, the wafer is of P-type with a specific resistivity of one-half ohm centimeter. It will be understood that other semiconductive materials, such as germanium and alloys of germanium and silicon, may be used.

In accordance with the general principles of the opentube technique, as disclosed in United States Patent 2,804,405, issued August 27, 1957, to L. Derick and C. J. Frosch, the wafer is then placed in a quartz tube in a flow of oxygen along with some phosphorus pentoX- ide. The tube is kept heated to a temperature of 1300 degrees centigrade for about four hours. The tube is then cooled, the wafer removed from the furnace, and a solution of boron trioxide (B 0 in ethylene glycol monomethyl ether is painted on the opposite faces of the wafer. The wafer is again placed in a flow of oxygen and heated to a temperature of 1300 degrees centigrade for a period of six hours, after which it is removed and cooled.

As a result of the heating operations, the wafer is converted to N-type conductivity to a depth from each face sutficient to leave a central P-type layer having a thickness of .0005 inch. The diffusion of boron converts the outer faces of the wafer again to P-type conductivity so as to leave'N-type layers adjacent the central P-type layer, likewise having a thickness of .0005 inch. The wafer is then subjected to an etching operation to remove any dilfused layers on the periphery and cleaned. For making electrical connection to the P-type terminal zones, a double nickel plating was applied in accordance with the technique disclosed in R. L. Johnston-R. L. Rulison Patent 2,793,420 issued May 28, 1957, followed by the application of a soldered lead thereto. As is already known in the art, it is desirable to maintain the series resistance of a semiconductive diode element as low as possible by reducing the ohmic resistance of the terminal electrodes thereto. To this end, it is advantageous to produce heavily doped end regions by techniques well-known.in the art for facilitating such low resistance electrodes.

It is also feasible to employ light or other means capable of introducing carrn'ers for use as the switching information to provide the triggering action. Carriers may also be introduced into the region of the intermediate zones by appropriate connections thereto.

It will be understood that the foregoing embodiments are but illustrative and that other arrangements within the scope and spirit of the invention may be devised by those skilled in the art.

What is claimed is:

1. A switch comprising a' monocrystalline body including five zones arranged in succession, contiguous zones being of opposite conductivity type, and electrical connections to only the two terminal zones of the body, the three intermediate zones being floating, each of said intermediate zones having a thickness of the order of ten-tho'usandths inches and a concentration of recombination centers related to said zone thickness whereby the effective alpha of the body is substantially below unity for a current density in the body below a predetermined switching level and reaches unity for a current density in the body above the predetermined switching level for applied potentials of either polarity.

2. A switch according to claim 1 wherein the semiconductive body is silicon.

3. A switching arrangement including a switch in accordance with claim 1 in further combination with switching control means for applying potentials of either polarity.

References Cited in the file of this patent UNITED STATES PATENTS 2,569,347 Shockley Sept. 25, 1951 2,805,397 Ross Sept. 3, 1957 2,852,677 Shockley Sept. 16, 1958 2,855,524 Shockley- Oct. 7, 1958 FOREIGN PATENTS 165,635 Australia Oct. 17, 1955

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