JPS63177573A - Superconducting transistor - Google Patents

Abstract

PURPOSE:To reduce the noise and realize a high speed operation and a high gain by a method wherein a device is operated while it is maintained at a temperature not higher than 77K and an electromagnetic wave such as a light, an ultraviolet ray, an infrared radiation, and an X-ray is applied to control the characteristics. CONSTITUTION:A device is maintained at a cryogenic condition (4.2K) in liquid helium and a light signal 8 with a wavelength of 632.8 nm is applied through an optical fiber 11. By this light application, an exciting phenomenon is induced in an electron supplying layer 3 and the surface electron concentration and the electron mobility of secondary electron gas are increased. Therefore, superconducting electron pairs penetrate into a semiconductor channel layer 2 from source and drain electrodes 5 and 6 which are superconductors and a superconducting current flows between the source and drain electrodes. On the other hand, the resistances between the source and a gate and between the gate and the drain are reduced and the cut-off frequency of the transistor is increased and the noise level is declined. Moreover, the switching speed, expressed by a time constant, is also increased. The increased electrons are maintained even if the application of the electromagnetic wave is discontinued as long as the temperature is maintained at as low as 77K or lower.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a superconducting transistor that operates at extremely low temperatures;
In particular, the present invention relates to a superconducting transistor suitable for adjusting transistor characteristics by irradiating electromagnetic waves.

[Conventional technology]

High electron mobility transistors that have been developed in the conventional semiconductor field are disclosed in Japanese Patent Application Laid-open No. 56-94779 and Japanese Patent Application Laid-open No. 57-
It is described in Japanese Patent No. 7165 and has a structure as shown in FIG. This transistor consists of a semiconductor layer (with high electron affinity) grown on a semi-insulating semiconductor substrate 1.
An electron storage layer (two layers) formed on the semiconductor layer 3 side near one heterojunction formed by a channel layer) 2 and a semiconductor layer (fl! child supply layer) 3 with a small electron affinity grown thereon. The electron concentration of the dimensional electron gas) 4 is controlled by the voltage applied to the control electrode 7, and the conductivity is formed by the electron storage layer 4 between the source electrode 5 and the drain electrode 6, which are provided with the control electrode 7 in between. This controls the impedance of the path. The source and drain electrodes 5 and 6 are usually made of a material such as gold/germanium/gold.

It is formed by vapor deposition, etc., but after formation, it is heat-treated at about 450°C to penetrate through the electron supply layer 3 and alloy up to the upper part of the channel layer 2 to form an alloyed layer IO, and through this, the source and drain electrodes are formed. 5 and 6 and the channel layer 2 are connected.

On the other hand, regarding superconducting transistors that combine superconductors and semiconductors, T.D. Crank (T, D,
Journal of Applied Physics, Volume 51, Page 2736 (1980)
(Journal of Applied Phys.
ics). This transistor controls the value of superconducting current flowing between two superconducting electrodes provided in contact with a semiconductor layer by changing the superconducting proximity effect using a voltage applied to a control electrode provided between the two electrodes. It is something.

[Problem that the invention seeks to solve]

In the conventional high electron mobility transistor, the electron concentration of the stored electron group (two-dimensional electron gas) increases when a voltage is applied to the control electrode 7, but
It is affected by impurity scattering due to ionized impurities present near the Peter junction. In order to increase electron mobility by reducing the influence of scattering due to impurities contained in the semiconductor layer with low electron affinity, that is, the electron supply layer 3, the electron areal concentration of the electron storage layer 4 is increased, that is, the electron supply layer 3 The impurity concentration of layer 3 may be increased.

However, when the impurity concentration of the electron supply layer 3 is increased, the impurity concentration diffused into the channel layer 2 due to the high temperature heat treatment process etc. during the formation of the alloyed layer 10 also increases, resulting in the influence of impurity scattering in the channel layer 2. becomes larger,
Electron mobility decreases.

Therefore, the impurity concentration in the electron supply layer 3 cannot be increased indefinitely, and the time constant (1/CR) becomes small.
The drawback was that the cutoff frequency was lower and it was more susceptible to noise. Therefore, there has been a problem in that it is difficult to increase the switching speed and increase the gain of the device.

On the other hand, when fabricating a superconducting transistor using the superconducting proximity effect, a pair of superconducting electrodes facing each other with a certain distance Q apart is provided on a semiconductor substrate, and a control electrode is provided between the opposing parts. ing. In such an element structure, the distance circle needs to be selected to be approximately 0.3- or less, which is 5 to 10 times the coherence length ξ in the semiconductor, and therefore, such an element has problems in manufacturing. Ta. Furthermore, the capacitance of the control electrode becomes large, making it difficult to achieve high speed and high gain.

An object of the present invention is to solve the above problems and provide a superconducting transistor having high switching speed and high gain characteristics.

[Means for solving problems]

The above object includes a first semiconductor layer formed on a semi-insulating semiconductor substrate, and a second semiconductor layer formed on the first semiconductor layer with a predetermined width and having a smaller electron affinity than the first semiconductor layer. - A semiconductor layer comprising a semiconductor layer and a superconductor film formed on the first semiconductor layer exposed on both sides of the second semiconductor layer.
77. A superconducting element comprising a source electrode, a drain electrode, and a gate electrode made of a superconductor film formed on the second semiconductor layer between the source electrode and the drain electrode. The gate electrode is operated while being maintained at a temperature below, and the gate electrode is exposed to light, ultraviolet rays, and infrared rays.

This is achieved by controlling the characteristics by irradiating electromagnetic waves such as X-rays.

[Effect]

Since a potential barrier is generated at the interface between layers made of two types of semiconductors having different electron affinities, electrons stay near the interface and generate an electron storage layer (two-dimensional electron gas). The electron mobility of this electron storage layer exhibits a large value at low temperatures where the impurity scattering effect becomes small. Cooling from room temperature to liquid helium temperature (4.2 K) shows an improvement of more than 100 times. This situation is shown in Figure 4. From the same figure.

When cooling from 300 to 77 to 5K (liquid helium temperature), the electron mobility increases at the same electron concentration. On the other hand, the electron supply layer has a wavelength of 637. When irradiated with amm light, an excitation phenomenon occurs in the electron supply layer due to the irradiation energy, promoting an increase in the electron surface concentration in the electron storage layer. Even after the electromagnetic wave irradiation is stopped, the increased electrons are retained as long as the temperature remains below 77°C. This situation is shown in FIG. 3. As the electron concentration increases, the resistance of the conductive path decreases, and as a result, the time constant of the circuit can be decreased, the cut-off frequency can be increased, and the noise can be reduced.

〔Example〕

Hereinafter, the present invention will be explained in detail based on Examples.

FIG. 1 is a sectional view of a superconducting transistor which is a first embodiment of the present invention. In the figure, 1 is a substrate made of semi-insulating gallium arsenide (GaAs) containing chromium (Cr), etc., and 2 is a substrate made of semi-insulating gallium arsenide (GaAs), which is formed on this by crystal lattice matching, and substantially eliminates impurities that function at extremely low temperatures. (Impurity concentration l This is a single crystal layer made of aluminum gallium arsenide (An GaAs) and constitutes an electron supply layer. In this example, the n-type impurity concentration of the electron supply layer 3 is 2.
X 10"am-", and the thicknesses of the channel layer 2 and electron supply layer 3 are 600 nm and 1100 nm, respectively.

In this crystal profile, an electron storage layer (two-dimensional electron gas) 4 with an electron surface concentration of 3XIQ11an-'' is generated near the heterojunction interface. These are input/output electrodes (source and drain electrodes) formed on Nb film, which is a superconductor, and are ohmically connected to the electron storage layer 4.

Reference numeral 7 denotes a control electrode (gate electrode) made of a Nb film provided on the electron supply layer 3 between the source and drain electrodes 5 and 6, and functions as a Schottky barrier gate. The control electrode 7 must have a thickness sufficient to transmit the optical signal 8 sent through the optical fiber 11, and in the case of the Nb film of this embodiment, the thickness is 1100 nm or less.

The device with the above structure was placed in liquid helium at an extremely low temperature (4
, 2K), and using the optical fiber 11 as the control electrode 7, the wavelength 632. An optical signal 8 of amm is input.

This light irradiation causes an excitation phenomenon in the electron supply layer 3, increasing the electron surface concentration and electron mobility of the two-dimensional electron gas. Therefore, superconducting electron pairs leak out from the source and drain electrodes 5 and 6 which are superconductors into the semiconductor channel layer 2, and a superconducting current flows between the source and drain electrodes 5 and 6. In other words, as the electron surface concentration and electron mobility increase, the probability of superconducting electrons existing in the semiconductor layer increases. On the other hand, the resistance between source and gate and between gate and drain decreases, increasing the cut-off frequency of this transistor and lowering the noise level. Also, it works with a time constant,
Switching speed is also faster. This effect is maintained for a sufficiently long time as long as the temperature is kept low, below about 77°C.

FIG. 2 is a sectional view of a superconducting transistor which is a second embodiment of the present invention. In this embodiment, an insulating film 9 is provided at the interface where the electron supply layer 3 and the superconducting electrodes 5 and 6 are in contact with each other in the device shown in FIG. This insulating film 9 is a self-oxidized film of the electron supply layer 3, and its thickness is 20 nm.

In a superconducting element having such a structure, a current flowing between the source 5 and the drain 6 via the semiconductor layer flows through the channel M.
Pass only through 2. Therefore, efficiency is increased and gain is also improved.

Next, the method for manufacturing the superconducting transistor of the present invention will be explained using semi-insulating G a A doped with eCr etc.
Ga as a channel layer 2 having an impurity concentration lXl017■-3 or lower carrier concentration on a single crystal substrate 1
An As layer is formed to a thickness of about 60 On-.

1100n thick Al2 with n-type impurities on it
An electron supply layer 3 made of an XGa1-xAs (x = 0.3) layer is formed. The above steps can be formed by a molecular beam epitaxial (MBE) growth method. Then, leaving a predetermined width of the electron supply layer 3, both sides thereof are removed by plasma etching until the channel layer 2 is reached. In the case of the device having the structure shown in the second embodiment of the present invention shown in FIG.
An insulating film 9 with a thickness of 0 nm is formed. Next, Nb with a thickness of 1100 nm is etched on the electron supply layer that was removed in the previous etching process.
Electrodes 5 and 6 made of films are formed by a vapor deposition method, are ohmically connected to the channel layer 2, and are connected to the source and drain electrodes 5.
, 6. Finally, a control electrode 7 made of Nb with a thickness of 1100 nm is placed on the electron supply layer 3 via a Schottky junction.
was formed. In this manner, the transistor of the present invention was manufactured.

In the above embodiments, a light emitting diode was used as the light source, but a semiconductor laser may be used instead. In addition, although Nb was used as the superconducting material, NbN, Nb3Ge
, Nb, Sn, Nb compounds such as Nb5AQ, BaLaC
Superconductors such as uO or pb-Au, Pb-In-
Similar effects can be obtained even when an organic superconductor containing a pb gold alloy organic material such as Au or Pb-B1 is used.

〔Effect of the invention〕

According to the present invention, the electron surface concentration of the electron storage layer can be sufficiently increased, and the electron mobility increases without increasing the influence of impurity scattering, so superconducting current flows effectively and only a small amount of electromagnetic wave irradiation is required. This makes it possible to provide a superconducting transistor that can control this current, and also to reduce the resistance of the conductive path, which increases the cutoff frequency of this transistor, reduces noise, and increases speed and gain. .

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views of embodiments of the superconducting transistor of the present invention, and Figure 3 shows the electron concentration versus light irradiation amount when an optical signal is irradiated to the electron supply layer of the superconducting transistor of the present invention. FIG. 4 is a graph showing the relationship between electron mobility and electron surface concentration using temperature as a parameter in the case of light irradiation and in the case of no irradiation in the superconducting transistor of the present invention. FIG. FIG. 2 is a cross-sectional view of a high electron mobility transistor. In the figure, 1... Semi-insulating semiconductor substrate 2... Channel layer 3.
...Electron supply layer 4...Electron storage layer 5...
Source electrode 6...Drain electrode 7...Control electrode 8...Optical signal 9...Insulating film
10... Alloyed layer 11... Optical fiber agent Junnosuke Nakamura 1 Figure 3 Nowata (cm-2)

Claims (1)

[Claims] 1. A first semiconductor layer formed on a semi-insulating semiconductor substrate; and a first semiconductor layer formed on the first semiconductor layer with a predetermined width;
a second semiconductor layer having a smaller electron affinity than the semiconductor layer; and a source electrode and a drain electrode made of a superconductor film formed on the first semiconductor layer exposed on both sides of the second semiconductor layer. a gate electrode made of a superconductor film formed on the second semiconductor layer between the source electrode and the drain electrode;
A superconducting transistor characterized in that the gate electrode can be irradiated with electromagnetic waves while being maintained at a temperature of 7K or less. 2. In the superconducting transistor according to claim 1, the source electrode and the drain electrode are connected to the second
A superconducting transistor characterized by being in contact with a semiconductor layer via an insulating film. 3. A superconducting transistor according to claim 1 or 2, wherein the gate electrode is transparent to the electromagnetic waves.