JP2687434B2 - Superconducting base transistor and its operating method - Google Patents

Description

The present invention relates to a super high frequency transistor, and more particularly to a transistor using a superconductor thin film as a base.

(Prior Art) In recent years, various transistors using compound semiconductors such as GaAs have been actively developed and commercialized, and 100-200 GHz class characteristics close to the performance limit of these transistors have been reported. However, it is certain that the characteristics required for the device will further increase in the future, and 1000 GHz class transistor development has also started. A superconducting base transistor using a superconductor thin film as a base layer is considered to be a promising device for achieving performance in the 1000 GHz class.

Such a superconducting base transistor is reported, for example, in Technical Report SCE85-41 of the Institute of Electronics, Information and Communication Engineers, entitled "Superconducting Base Hot Electron Transistor Using Nb-Based Compound Thin Film." FIG. 3 shows a sectional structure (a) and a band structure (b) of this conventional superconducting base transistor. In this conventional example, the emitter layer 24
Is composed of n ⁺ GaAs, a superconducting base layer 23 made of Nb is provided on the emitter layer, and InSb is further formed on the superconducting base layer 23.
A collector burr 22 consisting of is provided. A collector electrode 21 made of Au is provided on the collector barrier. In addition, Au is formed on the n + GaAs layer that constitutes the emitter layer.
An emitter electrode 25 made of Ge-Ni is provided. An energy band diagram of the one-dot chain line portion of this structure is shown in FIG. 3 (b). The barrier between the base layer 23 and the emitter layer 24 of the superconducting thin film is higher than the barrier between the base layer 23 and the collector barrier 22 of the same superconducting thin film. For this reason, the electrons injected from the emitter fly ballistically in the base layer 23 if the base layer 23 is as thin as 100 Å or less,
It jumps over the collector barrier and is absorbed by the collector electrode 21.

(Problems to be Solved by the Invention) In the conventional superconducting base transistor, the number of ballistic electrons injected from the emitter to the base is controlled by changing the potential between the emitter layer and the superconducting base layer. doing. In order to inject electrons, it is necessary to forward-bias the emitter / base junction, and as a result, the depletion layer capacitance C (V) becomes large as represented by the equation (1).

In (1), Vbi is the built-in voltage, q is the electronic charge, N _D is the donor density in the emitter layer, R is the Boltzmann constant, and V is the bias voltage. The large capacitance means that it takes much time to change the emitter-base voltage. That is, the charging time becomes long. Therefore, even if electrons travel ballistically in the base and the base travels, and the base travel time becomes extremely short, it was not so high due to the influence of the emitter charging time at the cutoff frequency.

An object of the present invention is to provide a superconducting base transistor in which the deterioration of cutoff frequency due to charging time is extremely small.

(Means for Solving the Problems) In order to achieve the above object, in the superconducting base transistor of the present invention, the superconducting thin film serving as the base layer has a positive electron barrier for the superconducting thin film. The first n-type semiconductor layer serving as an emitter layer and the p-type semiconductor layer serving as a ballistic electron control layer are sandwiched between the p-type semiconductor layer and the second n-type semiconductor layer serving as a collector layer. There are

A negative bias voltage is applied to the first n-type semiconductor layer with respect to the superconductor thin film, and a positive bias voltage is applied to the second n-type semiconductor layer with respect to the superconductor thin film. In addition, a negative bias voltage is applied to the p-type semiconductor layer with respect to the superconductor thin film, and a signal voltage is applied between the p-type semiconductor layer and the superconductor thin film.

(Operation) In the present invention, a constant forward bias voltage is always applied to the emitter-base junction, and it functions as a ballistic electron injector. Therefore, it is not affected by the charging time. In addition, ballistic electrons traveling in the base are controlled by changes in the conduction band bottom of the reverse biased p-type semiconductor layer. The charging time for changing the electric potential of the electronic control layer is also short. The reason is that the base / p-type semiconductor layer is reverse biased and the depletion layer is widened.

Therefore, the cutoff frequency of the transistor can be dramatically improved.

(Example) Next, the Example of this invention is described with reference to drawings.

FIG. 1 shows a cross-sectional structure of a superconducting base transistor which is an embodiment of the present invention. In the figure, the thickness to be the collector layer 5 on the semi-insulating (S ₁ I ₁ ) GaAs substrate 6
2500 Å, n-GaAs layer with a doping concentration of 3 × 10 ¹⁶ cm ^-3 , thickness of the ballistic electron control layer 4 600 Å p-InSb layer with a doping concentration of 5 × 10 ¹⁶ cm ^-3 , and the thickness of the base layer 3 An Nb layer having a thickness of 80 Å and an n ⁺ -GaAs layer having a doping concentration of 5 × 10 ¹⁸ cm ^-3 to be the emitter layer 2 are provided. AuGe-Ni which becomes the emitter electrode 1 is formed on the n ⁺ -GaAs layer.
Is provided on the p-InSb layer to serve as the control electrode 7.
-Ni is provided, and AuGe-Ni serving as the collector electrode 8 is provided on the n-GaAs. A positive bias voltage with respect to the base layer 3 is applied to the collector electrode 8, a negative bias voltage with respect to the base layer 3 is applied to the emitter electrode 1, and a negative bias voltage with respect to the base layer 3 is applied to the emitter electrode 1. Is applied to the control unit 4, and a negative bias voltage is applied to the control unit 4 as compared with the base layer 3, and a signal voltage is applied by the high frequency signal input power supply 9.

FIG. 2 is a band diagram taken along the section line 10 of the embodiment shown in FIG. 1, where (a) shows the ON state and (b) shows the OFF state. In the same figure (a), the electrons injected from the emitter to the base ballistically pass through the base layer 12 made of an Nb ultrathin film, which is a superconductor, with a kinetic energy of ΔE, and then the ballistic electron control layer.
In 13 the doping concentration of p-InSb which constitutes this is 5 ×
Since it is relatively low at 10 ¹⁶ cm ^-3 , the electrons are not scattered so much, so that the electrons pass ballistically and reach the collector layer 14. On the other hand, in FIG. 2 (b), the negative voltage applied to the control layer 13 is deeper than in the case of (a). Therefore the control layer
The conduction band bottom 14 of 13 rises and reflects the electrons 15 that have ballistically passed through the base. The reverse speed between the base and control electrodes causes the depletion layer capacitance to be small and the charging time to be short, so the rapidly reflected electrons form a superconducting electron pair after the relaxation time and form a base. Recovered through layers.

With the superconducting base transistor described in the above examples, the maximum oscillation frequency (f _max ) of 1000 GHz or higher was obtained with good reproducibility. This characteristic is that even if the thickness of the base layer is 50 to 100Å, the thickness of the emitter layer and collector layer is 10
Even if it was 00-5000Å, it was obtained without problems.

(Advantages of the Invention) In the superconducting base transistor of the present invention, the control of the electrons traveling in a ballistic manner is performed by using the newly provided control electrode having a high response speed (short charging time). It is possible to operate at a frequency about twice that of a superconducting base transistor, and f _{max of} 1000 GHz or more
Is obtained. Although the relaxation time for the electrons reflected by the control layer to form a superconducting electron pair is relatively long, the intrinsic operating region of the transistor is determined by the base, control layer, and collector, and the circuit between the emitter and base is a so-called idling circuit. It does not affect the high speed operation.

Although Nb is used as the superconducting material in the present embodiment, it goes without saying that the material is not limited to this and an oxide high temperature superconducting material may be used. The semiconductor used for the control layer is not limited to InSb, and the emitter is not limited to GaAs.
Any material that has a positive barrier to the superconducting base layer can be used. Further, the collector layer is not limited to GaAs and may be any n-type semiconductor.

[Brief description of the drawings]

FIG. 1 is a sectional structural view for explaining an embodiment of a superconducting base transistor of the present invention, and FIG. 2 is its energy band diagram. 3 (a) and 3 (b) are diagrams for explaining a conventional example. In the figure, 1,25 …… emitter electrode, 2,11,24 …… emitter layer, 3,12,
23 …… base layer, 4, 13 …… ballistic electronic control layer,
5,14 ...... Collector layer, 6 ...... Substrate, 7 ...... Control electrode, 8,
21 …… Collector electrode, 9 …… High frequency signal input power supply, 10 ・ ・ ・
… Cut line, 22 …… Collector barrier.

Claims (2)

(57) [Claims] 1. In a superconducting base transistor having a superconducting thin film as a base layer, the superconducting thin film as a base layer has a first n-type semiconductor layer as an emitter layer and a p-type as a ballistic electron control layer. And the first n-type semiconductor layer and the p-type semiconductor layer have a positive electron barrier with respect to the superconductor thin film, and the p-type semiconductor layer serves as a collector layer. A superconducting base transistor including a structure in contact with the n-type semiconductor layer. 2. The superconducting base transistor according to claim 1, wherein a negative bias voltage is applied to the first n-type semiconductor layer with respect to the superconductor thin film, and the second n-type semiconductor layer is applied. A positive bias voltage is applied to the superconductor thin film, a negative bias voltage is applied to the p-type semiconductor layer to the superconductor thin film, and a negative bias voltage is applied to the p-type semiconductor layer between the p-type semiconductor layer and the superconductor thin film. Is a method of operating a superconducting base transistor characterized by applying a signal voltage.