Superconducting transistor

Abstract: A superconducting transistor comprises a superconducting collector layer (1) a first barrier layer (2) provided on the superconducting collector layer (1), a superconducting base layer (3) provided on the first barrier layer (2), a second barrier layer (4) provided on the superconducting base layer (3), and a superconducting emitter layer (5) provided on the second barrier layer (4). All of the layers (1-5) are formed of substances having the same component elements but the composition differs from one layer to the next. The barrier layers (2, 4) have a low carrier concentration and do not have superconducting properties whilst the collector (1), base (3) and emitter (5) layers have a high carrier concentration and exhibit superconducting properties. The superconducting collector layer (1) draws out quasi-particles from the superconducting base layer (3) whilst its carrier concentration is maintained at a level lower than that in metal and at a high level where a superconducting property, is exhibited.

Description: The present invention relates to a superconducting transistor, and more particularly, to a superconducting transistor in which each emitter, base and collector layer are formed by the same high temperature superconductor. Recently, an oxide high temperature superconductor having a critical temperature (Tc) higher than the boiling point of nitrogen i.e. 77 [K], has been found and will be utilized in the field of electronics. Although the well known Josephson junction used in the field of the electronics has the advantages of a high speed and low electric power consumption, this Josephson junction element is a two terminal element and thus can not be realized merely by circuitry. Therefore, if a three terminal element using a superconducting phenomena; i.e., an element wherein a transistor operation is carried out, can be obtained, various functions can be preferably realized. If the three terminal element can be realized while using a high temperature superconductor, technologies in a number of fields would be advanced. Figure 1 shows a main portion cross-sectional side view of a conventional superconducting base transistor previously developed by the present inventors. In Fig. 1, an n⁺ type InGaAs collector layer 22 having a thickness of 50 nm and an impurity concentration of 2 x 10¹⁹/cm³ is provided on a semi-insulating InP substrate 21. Further, provided on the collector layer 22, in the following order, are an n type InGaAs collector layer 23 having a thickness of 50 nm and an impurity concentration of 5 x 10¹⁷/cm³; an In(Al 0.14 Ga 0.86 )As collector layer 24 having a thickness of 100 nm; an n type InGaAs collector 25 having a thickness of 10 nm and an impurity concentration of 5 x 10¹⁷/cm³; an n⁺ type InGaAs collector layer 26 having a thickness of 30 nm and an impurity concentration of 2 x 10¹⁹/cm³; an Nb superconducting base layer 27 having a thickness of 100 nm and formed by a magnetron sputtering process; an aluminum oxide (AlOx) tunneling barrier layer 28 having a thickness of 30 Å; and, an Nb emitter layer 29 having a thickness of 100 nm and formed by a magnetron sputtering process. Between the layers 27 and 28, an aluminum layer having a thickness of 30Å is usually formed. Further, an Nb collector contact layer 30 is provided on the collector layer 22, and an Nb emitter electrode 32, Nb base electrode 33 and Nb collector electrode are connected to the Nb emitter layer 29, the superconducting base layer 27 and the Nb collector contact layer 30 respectively. Numeral 31 represents an insulating film of a silicon dioxide. Further, non-doped InGaAs spacer layers are provided between the collector layers 23 and 24 and between the collector layers 24 and 25, although not shown in Fig. 1. As shown in Fig. 1, in the superconducting base transistor, the collector is formed by a heterojunction of InGaAs/InAlGaAs. When a voltage is supplied to cause a flow of current between the emitter layer 29 and the base layer 27, quasi-particles are injected into the base layer 27. A barrier having a height of the energy gap of a superconductor exists between the base layer 27 and the collector, and the injected quasi-particles pass through the barrier and flow into a collector. Superconducting leakage current passing through the collector can be stopped at the barrier. Namely, the collector current is controlled by a current passing from the emitter to the base. In the above-explained prior superconducting base transistor, a very small barrier must be reproducibly formed, this barrier having the same level as a superconducting gap existing between a collector consisting of a plurality of semiconductor layers and a superconducting base layer. A similar superconductivity transistor having a semiconducting base layer is known from JP-A-62-163379. Generally, the interface between a semiconductor and a superconductor is unstable and an electrical barrier larger than the superconducting gap is easily formed. Consequently, due to the provision that the number of quasi-particles penetrating to the collector becomes small, a current gain cannot be obtained. Further, it is difficult to establish contact between a superconducting layer and a semiconductor at a barrier height of an energy gap of a usual superconductor, e.g., about 1.5 meV. According to this invention a superconducting transistor comprises: an oxide superconducting collector layer formed of predetermined component elements and having a predetermined composition, a first barrier layer provided on the superconducting collector layer, formed of a substance having the same component elements as that of the collector layer but a different composition, said first barrier layer having a low carrier concentration and not exhibiting a superconducting property, a superconducting base layer provided on the first barrier layer, formed of a substance having the same component elements as that of said superconducting collector layer, having a different composition from that of the first barrier layer a high carrier concentration and exhibiting a superconducting property; a second barrier layer provided on the superconducting base layer formed of a substance having the same component elements as that of the superconducting collector layer but having a different composition from that of the superconducting collector layer a low carrier concentration and not exhibiting a superconducting property, and, a superconducting emitter layer provided on the second barrier layer, formed of a substance having the same component elements as that of said superconducting collector but having a different composition from that of the first and second barrier layers, having a high carrier concentration and exhibiting a superconducting property, the superconducting collector layer for drawing out quasi-particles from the superconducting base layer while its carrier concentration is maintained at a level lower than in metal and at a high level where the superconducting property is exhibited. With the present invention an undesired barrier is not generated between the base and collector and the current transfer ratio is improved. Also the reproducibility of the element properties is improved. The superconducting collector, base and emitter layers preferably have a carrier concentration of 2x10²¹ to 2x10²²/cm³; the first barrier layer preferably has a carrier concentration of less than 2x10²¹/cm³; and the second barrier layer preferably has a carrier concentration of less than 2x10²¹/cm³. Particular examples of transistors in accordance with this invention will now be described and contrasted with the prior art with reference to the accompanying drawings, in which:- Figure 1 is a main portion cross-sectional side view of a conventional superconducting base transistor; Figure 2 is a schematic cross-sectional view of a main portion of a superconducting transistor for explaining the principle of the present invention; Fig. 3 is a potential diagram against a carrier in a superconducting transistor shown in Fig. 2; and, Fig. 4 is a main portion cross-sectional view of an example of the present invention. The preferred embodiments of the invention will now be described. Figure 2 is a schematic cross-sectional view of a main portion of a superconducting transistor, for explaining the principle of the present invention. In Fig. 2, an oxide barrier layer 2, an oxide superconducting base layer 3, an oxide barrier layer 4, and an oxide superconducting emitter layer 5 are provided in that order on an oxide superconducting collector layer 1, and a collector electrode 6, a base electrode 7 and an emitter electrode 8 are respectively connected to the oxide superconducting collector layer 1, the oxide superconducting base layer 3 and the oxide superconducting emitter layer 5. The collector layer 1, the base layer 3 and the emitter layer 5 of the superconducting transistor shown in Fig. 2 exhibit a superconducting property at a use-temperature. Although barrier layers 2 and 4 consist of the same component oxide as that of the collector layer 1, base layer 3 and emitter layer 5, since the component ratios thereof are different, the concentrations of carriers which carry the superconducting current are different, and thus the barrier layers 2 and 4 do not exhibit the superconducting quality. The oxide layers are ideally formed of a single crystal or have a similar crystal structure, and thus they can be regarded as the same component substance. As explained above, in the superconducting transistor, all oxides are essentially formed by similar crystal structure substances and an electric barrier is not generated between the base layer 3 and the collector layer 1. Therefore, the current transfer ratio is high and the reproducibility of the element property is good. Figure 3 shows a potential diagram against a carrier in a superconducting transistor shown in Fig. 2. The reference numerals of Fig. 3 show the same portions or have the same meanings as in Fig. 2. As can be seen from Fig. 3, a potential change from the emitter layer 5 to the collector 1 is shown. Since a carrier concentration is high in the emitter layer 5, base layer 3 and the collector layer 1, compared to that of the barrier layers 4 and 2, the potential is maintained at a low level in the emitter layer 5, the base layer 3 and the collector layer 1. Further, since a carrier concentration is low in the regions between the emitter layer 5 and the base layer 3, and between the base layer 3 and the collector layer 1, the potential against the carrier is maintained at a high level. Since the high potential region acts as a barrier to carriers these regions are denoted as the barrier layers 4 and 2. The barrier height of the barriers 4 and 2 can be determined by the difference in the superconducting carrier concentrations thereof. On the other hand, the barrier height in a conventional superconducting transistor can be mainly determined by a heterointerface between a semiconductor or a insulator and a superconducting metal. Therefore, the present invention has a feature superior to that of the prior art in that, to control the barrier height, a value of a carrier concentration can be changed. Thus the controllability of the barrier height in the present invention is superior to that of the prior art. Furthermore, since oxide superconductors having a larger superconducting energy gap are used in the present invention, a higher barrier height can be advantageously used between the base layer 3 and the collector layer 1 than where a conventional low temperature superconductor is used. This is because it is easier to control high barrier than to control low barrier. Figure 4 is a main portion cross-sectional view of an example of the present invention. In Fig. 4 the same reference numerals denote the same portions and have the same meanings as in Figs. 2 and 3. As shown in Fig. 4, an insulating layer 9 of MgO, a collector layer 1 of (La 0.925 Sr 0.075 )₂CuO₄ having a thickness of 400 nm, a barrier layer 2 of (La 0.96 Sr 0.04 )₂CuO₄ having a thickness of 200 nm, a base layer 3 of (La 0.925 Sr 0.075 )₂CuO₄ having a thickness of 100 nm, a barrier layer 4 of La₂CuO₄ having a thickness 10 nm, and an emitter layer 5 of (La 0.025 Sr 0.075 )₂CuO₄ having a thickness of 200 nm are provided. Even though Sr is contained in the barrier layer 4, the barrier layer acts as a barrier. The small content of Sr is preferable. In a case of (La 1-x Sr x )₂CuO₄, it exhibits a superconducting property at x≧0.05, and a semiconducting property at x<0.05. A collector electrode 6, a base electrode 7 and an emitter electrode 8 are also provided and are formed of Au and have a thickness of 1 µm. In this example, the composition of (La 0.96 Sr 0.004 )₂CuO₄ forming the collector layer 1, the base layer 3, and the emitter layer 5 exhibit a superconducting property at an operating temperature. On the other hand the composition of (La 0.96 Sr 0.04 )₂CuO₄ forming the barrier layer 2 and of La₂CuO₄ forming the barrier layer 4 do not exhibit a superconducting property at an operating temperature. Whether or not a (La 1-x Sr x )₂CuO₄ exhibits the superconducting property at an operating temperature is determined by carrier concentrations due to the compositions of La 1-x Sr x (x is changeable from 0 to 1). If a Ba₂YCu₃O x is used as an oxide superconductor the carrier concentration thereof can be changed merely by changing the amount x of oxygen. Namely, when about 6.7 to 6.8 of the values x of the compound of Ba₂YCu₃O x is selected, the compounds of Ba₂YCuO x exhibit a superconducting property, and when about 6.4 or less of the value x thereof is selected they exhibit a semiconducting (not superconducting) property. Further, a compound of BiSrCaCuO having each atom ratio of 2:2:0:1:6 has a critical temperature (Tc) of about 10 [K]. In an atom ratio of 2:2:1:2:8, the critical temperature Tc thereof is about 80 [K], and in the ratio of 2:2:2:3:10 the critical temperature Tc thereof is about 110 [K]. Therefore, when, for example, a compound of Bi₂Sr₂Ca₂Cu₃O₁₀ having a carrier concentration of 3x10²¹/cm³ is used at a temperature of, for example, 120 K as a collector, a base and an emitter, a compound of Bi₂Sr₂CaCu₂O₈ having a carrier concentration of 3x10²¹/cm³ can be used as a first and a second barrier layer. When a metal Nb having a carrier concentration of 1x10²³/cm³ is used as a superconductor the critical temperature Tc is about 9.2 [K]. Metal usually has a carrier concentration of about 1x10²³/cm³. In the superconducting transistor according to the present invention, an emitter layer, a base layer and a collector layer, and barrier layers therebetween, are formed of substantially the same substance, and therefore, the technical problems which arise when a heterojunction is formed can be ignored. Further, since an electrical barrier is not generated between each layer, particularly the base and a collector layers, a superconducting transistor having improved properties such as a large current transmitting rate can be reproducibly obtained.

Claims: A superconducting transistor comprising: an oxide superconducting collector layer (1) formed of predetermined component elements and having a predetermined composition, a first barrier layer (2) provided on the superconducting collector layer (1), formed of a substance having the same component elements as that of the collector layer (1) but a different composition, said first barrier layer having a low carrier concentration and not exhibiting a superconducting property, a superconducting base layer (3) provided on the first barrier layer (2), formed of a substance having the same component elements as that of said superconducting collector layer (1), having a different composition from that of the first barrier layer (2) a high carrier concentration and exhibiting a superconducting property; a second barrier layer (4) provided on the superconducting base layer (3) formed of a substance having the same component elements as that of the superconducting collector layer (1) but having a different composition from that of the superconducting collector layer (1) a low carrier concentration and not exhibiting a superconducting property, and, a superconducting emitter layer (5) provided on the second barrier layer (4), formed of a substance having the same component elements as that of said superconducting collector (1) but having a different composition from that of the first (2) and second (4) barrier layers, having a high carrier concentration and exhibiting a superconducting property, the superconducting collector layer (1) for drawing out quasi-particles from the superconducting base layer (3) while its carrier concentration is maintained at a level lower than in metal and at a high level where the superconducting property is exhibited. A superconducting transistor according to claim 1, wherein said superconducting collector layer (1) has a carrier concentration of 2x10²¹ to 2x10²²/cm³. A superconducting transistor according claim 1 or 2, wherein said superconducting base layer (3) has a carrier concentration of 2x10²¹ to 2x10²²/cm³. A superconducting transistor according to any one of the preceding claims, wherein said superconducting emitter layer (5) has a carrier concentration of 2x10²¹ to 2x10²²/cm³. A superconducting transistor according to any one of the preceding claims, wherein said first barrier layer (2) has a carrier concentration of less than 2x10²¹/cm³. A superconducting transistor according to any one of the preceding claims, wherein said second barrier layer (4) has a carrier concentration of less than 2x10²¹/cm³. A superconducting transistor according to any one of the preceding claims, wherein said superconducting collector (1), base (3), and emitter (5) layers are (La 0.925 Sr 0.075 )₂CuO₄ and said first (2) and second (4) barrier layers are (La 1-x Sr x )₂CuO₄ where x<0.05. A superconducting transistor according to any one of claims 1 to 6, wherein said superconducting collector (1), base (3) and emitter (5) layers are Ba₂YCu₃O 6.7 to 6.8 and said first (2) and second (4) barrier layers are Ba₂YCu₃O 6.0 to 6.4 .

Inventors: TAMURA HIROTAKA, TAMURA, HIROTAKA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H01L39
IPC codes: H01L39, B21B37