WO1990004266A1 - Transistor devices - Google Patents
Transistor devices Download PDFInfo
- Publication number
- WO1990004266A1 WO1990004266A1 PCT/GB1989/001178 GB8901178W WO9004266A1 WO 1990004266 A1 WO1990004266 A1 WO 1990004266A1 GB 8901178 W GB8901178 W GB 8901178W WO 9004266 A1 WO9004266 A1 WO 9004266A1
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- WO
- WIPO (PCT)
- Prior art keywords
- superconducting
- layers
- layer
- transistor
- current gain
- Prior art date
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Classifications
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N60/00—Superconducting devices
- H10N60/10—Junction-based devices
- H10N60/128—Junction-based devices having three or more electrodes, e.g. transistor-like structures
Definitions
- This invention relates to transistor devices, and particularly to transistors formed of superconducting materials.
- Figure 1 of the accompanying drawings shows, schematically, the configuration of such a transistor. It comprises three layers 1, 2 and 3 of aluminium with layers 4 and 5 of aluminium oxide between the layers 1 and 2, and 2 and 3, respectively.
- the layers 1, 2 and 3 will be called emitter, base and collector electrodes, by analogy with junction transistors.
- FIG. 1 A problem arises with the known transistors of the Gray type. As the operating temperature of the device increases, the current gain decreases sharply.
- Figure 2 of the drawings illustrates an example of a device exhibiting this phenomenon.
- the curves A, B, C and D show current gain against temperature (in ), each curve being plotted for a respective collector bias voltage which is different from that of the other curves. It will be seen that for each curve the value of current gain decreases with increase in temperature throughout the whole operating range of 0.625 to 1.0K.
- a superconducting transistor comprising first and second superconducting layers having the same or substantially the same bandgap; a first electrically insulating layer sandwiched between said first and second superconducting layers; and a third superconducting layer separated from said second superconducting layer by a second electrically insulating layer; wherein said third superconducting layer has a larger bandgap than said first and second superconducting layers; wherein each of said first and second electrically insulating layers is sufficiently thin for tunnelling to take place therethrough; and wherein electron flow in the device takes place through said first, second and third superconducting layers in that order.
- said first and second superconducting layers are formed of aluminium, and said third superconducting layer is formed of a material selected from niobium, lead, tin and niobium nitride.
- Figure 1 is a schematic cross section of a known Gray-type superconducting transistor, as described above,
- Figure 2 illustrates the decrease in current gain with increase in temperature of the known Gray-type transistor for various collector bias voltages, as described above,
- Figure 3 is a schematic curve of collector current against collector voltage for explaining the operation of the present device
- Figure 4 shows curves of current gain against collector bias voltage for a device in accordance with the present invention, the curves being taken at various operating temperatures
- Figure 5 shows curves of current gain against operating temperature of the present device, the curves being taken at various collector bias voltages.
- this compensation is effected by forming the collector layer 3 ( Figure 1) of a superconducting material wh i ch has a higher energy bandgap than the emitter and base l ayers 1 and 2 , respectively.
- the layers 1 and 2 are formed of aluminium as in the conventional Gray transistor, niobium is a suitable material for the col lector layer 3 in the present invention.
- Th is fundamental change in the Gray transistor structure brings about a marked improvement in the current gain/temperature characteristic, whilst not detracting from the general level of current gain attainabl e, whi ch is largely determined by the dimensions and material of the base l ayer 2.
- the layers of the present device may be of simil ar dimensions to the conventional Gray transistor, i.e. the layers 1 , 2 and 3 may be of the order of 30QA thick , and the insul ating layers 4 and 5 must be suffi ciently thi n, for example 20-50.1, to al l ow quantum tunnel l ing thereth rough. If aluminium is used for the layers 1, 2 and 3 , aluminium oxide is a convenient material for the insulating layers 4 and 5. However, other insul ating materials might alternatively be used.
- Figure 3 is a general ised curve showing variation in col lector current against col lector bias at a constant temperature. It will be seen that the current rises with increasing bias voltage up to a peak 6 at a bias voltage V d . In a region 7 the current falls and then levels out. At a voltage V s there is an abrupt rise in current and thereafter the current increases substantial ly l inearly.
- the voltage V ⁇ at which the current peak occurs is equal to the difference between the bandgap of the col lector layer and the bandgap of the base layer.
- the voltage V s is equal to the sum of those bandgaps.
- the bandgap difference will therefore increase, and the peak 6 will move towards the selected bias point 8, or, in effect, the operating point will move up the slope of the current curve towards the peak.
- the current (and the current gain) will therefore rise, and this will counteract the drop which would otherwise be experienced.
- Figure 4 shows curves of current gain against bias voltage values within the range between V ⁇ and V s of Figure 3, the five curves being plotted at respective different temperatures. It will be seen that, in this example, if a temperature range between 0.625K and 0.8K is considered (.i.e. ignoring the curves taken at 0.90K and l.OK), the curves substantially coincide at a collector bias voltage of about 1.28mV, and this therefore appears to be an optimum operating value.
- Figure 5 shows how the resulting current gain of a device in accordance with the invention changes with temperature, the curves being plotted at different collector bias voltage settings. It will be seen that each of the curves has a region of substantially constant current gain. If these curves are compared with the curves of Figure 2, obtained for a conventional Gray transistor, the very great improvement in current gain stability afforded by the present invention will be apparent.
- collector bias voltage is dependent upon the particular temperature range for which gain stabilisation is required. If the required temperature range is reduced, the optimum bias voltage decreases and the operation will move to a higher curve in Figure 5. The current gain therefore increases. Conversely, if a higher gain is required, a narrower temperature range for stabilisation must be accepted.
- a transistor comprising emitter and base layers 1 and 2 of aluminium and a collector layer 3 of niobium is described above, alternatively a collector layer of lead, tin or niobium nitride might be used. Furthermore, other materials might be used for any of the layers, provided that the bandgap of the collector layer is greater than that of the base layer. If other materials are used, the collector bias voltage must be selected accordingly.
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- Superconductor Devices And Manufacturing Methods Thereof (AREA)
- Bipolar Transistors (AREA)
Abstract
A superconducting transistor of the Gray type comprises first and second superconducting layers (1, 2) which form the emitter and base electrodes, respectively. Those layers have substantially the same bandgap. A first insulating layer (4) is sandwiched between the first and second superconducting layers. A third superconducting layer (3), forming the collector electrode, is separated from the second superconducting layer by a second insulation layer (5). The insulating layers are sufficiently thin for tunnelling to take place therethrough. In a conventional Gray transistor the bandgap of the collector layer is the same as that of the emitter and base layers, and it is found that in such a transistor the current gain decreases sharply as the operating temperature of the device increases. In the present invention the collector layer is formed of a material having a larger bandgap than the emitter and base layers. A set of current gain/temperature characteristics having substantially flat regions is thereby obtained.
Description
Transistor Devices
This invention relates to transistor devices, and particularly to transistors formed of superconducting materials.
One of the first three-terminal superconducting devices to be devised was the Gray transistor. Figure 1 of the accompanying drawings shows, schematically, the configuration of such a transistor. It comprises three layers 1, 2 and 3 of aluminium with layers 4 and 5 of aluminium oxide between the layers 1 and 2, and 2 and 3, respectively. The layers 1, 2 and 3 will be called emitter, base and collector electrodes, by analogy with junction transistors.
A problem arises with the known transistors of the Gray type. As the operating temperature of the device increases, the current gain decreases sharply. Figure 2 of the drawings illustrates an example of a device exhibiting this phenomenon. The curves A, B, C and D show current gain against temperature (in ), each curve being plotted for a respective collector bias voltage which is different from that of the other curves. It will be seen that for each curve the value of current gain decreases with increase in temperature throughout the whole operating range of 0.625 to 1.0K.
It is an object of the present invention to provide a Gray-type superconducting transistor configuration in which the current gain remains at least approximately constant over a substantial part of the operating temperature range.
According to the invention there is provided a superconducting transistor, comprising first and second superconducting layers having the same or substantially the same bandgap; a first electrically insulating layer sandwiched between said first and second superconducting layers; and a third superconducting layer separated from said second superconducting layer by a second electrically insulating layer; wherein said third superconducting layer has a larger bandgap than said first and second superconducting layers; wherein each of said first and second electrically insulating layers is sufficiently thin for tunnelling to take place therethrough; and wherein electron flow in the device takes place through said first, second and third superconducting layers in that order.
Preferably, said first and second superconducting layers are formed of aluminium, and said third superconducting layer is formed of a material selected from niobium, lead, tin and niobium nitride.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which
Figure 1 is a schematic cross section of a known Gray-type superconducting transistor, as described above,
Figure 2 illustrates the decrease in current gain with increase in temperature of the known Gray-type transistor for various collector bias voltages, as described above,
Figure 3 is a schematic curve of collector current against collector voltage for explaining the operation of the present device,
Figure 4 shows curves of current gain against collector bias voltage for a device in accordance with the present invention, the curves being taken at various operating temperatures, and
Figure 5 shows curves of current gain against operating temperature of the present device, the curves being taken at various collector bias voltages.
In order to overcome the reduction in current gain with increasing temperature inherent in the conventional Gray transistor, it is necessary to introduce into the transistor structure some mechanism which will exhibit a compensating rise in current gain.
In a device in accordance with the invention this compensation is effected by forming the collector layer 3 (Figure 1)
of a superconducting material wh i ch has a higher energy bandgap than the emitter and base l ayers 1 and 2 , respectively. Assuming that the layers 1 and 2 are formed of aluminium as in the conventional Gray transistor, niobium is a suitable material for the col lector layer 3 in the present invention.
Th is fundamental change in the Gray transistor structure brings about a marked improvement in the current gain/temperature characteristic, whilst not detracting from the general level of current gain attainabl e, whi ch is largely determined by the dimensions and material of the base l ayer 2.
The layers of the present device may be of simil ar dimensions to the conventional Gray transistor, i.e. the layers 1 , 2 and 3 may be of the order of 30QA thick , and the insul ating layers 4 and 5 must be suffi ciently thi n, for example 20-50.1, to al l ow quantum tunnel l ing thereth rough. If aluminium is used for the layers 1, 2 and 3 , aluminium oxide is a convenient material for the insulating layers 4 and 5. However, other insul ating materials might alternatively be used.
Figure 3 is a general ised curve showing variation in col lector current against col lector bias at a constant temperature. It will be seen that the current rises with increasing bias voltage up to a peak 6 at a bias voltage Vd. In a region 7 the current falls and then levels out. At a voltage Vs there is an abrupt rise in current and thereafter the current increases substantial ly l inearly. The voltage V^ at which the current peak occurs is equal to the difference between the bandgap of the col lector layer and the bandgap of the base layer. The voltage Vs is equal to the sum of those bandgaps. It is found that if the teπperature is increased, the superconducting bandgaps of the materials change such that the voltage Vd at which the current peak occurs moves towards Vs. If the col lector bias voltage is set at a point 8 just above the peak position, the current will remain substantially constant despite the increase in temperature. The mechanism is as fol l ows. If the temperature increases, the current gain would drop due to a decrease in the value of the quasiparticle recombination lifetime. The rise in temperature will l ower the bandgap in the base layer 2 more than that in the col lector l ayer 3, because the latter is the larger.
The bandgap difference will therefore increase, and the peak 6 will move towards the selected bias point 8, or, in effect, the operating point will move up the slope of the current curve towards the peak. The current (and the current gain) will therefore rise, and this will counteract the drop which would otherwise be experienced.
Figure 4 shows curves of current gain against bias voltage values within the range between V^ and Vs of Figure 3, the five curves being plotted at respective different temperatures. It will be seen that, in this example, if a temperature range between 0.625K and 0.8K is considered (.i.e. ignoring the curves taken at 0.90K and l.OK), the curves substantially coincide at a collector bias voltage of about 1.28mV, and this therefore appears to be an optimum operating value.
Figure 5 shows how the resulting current gain of a device in accordance with the invention changes with temperature, the curves being plotted at different collector bias voltage settings. It will be seen that each of the curves has a region of substantially constant current gain. If these curves are compared with the curves of Figure 2, obtained for a conventional Gray transistor, the very great improvement in current gain stability afforded by the present invention will be apparent.
The choice of collector bias voltage is dependent upon the particular temperature range for which gain stabilisation is required. If the required temperature range is reduced, the optimum bias voltage decreases and the operation will move to a higher curve in Figure 5. The current gain therefore increases. Conversely, if a higher gain is required, a narrower temperature range for stabilisation must be accepted.
Although a transistor comprising emitter and base layers 1 and 2 of aluminium and a collector layer 3 of niobium is described above, alternatively a collector layer of lead, tin or niobium nitride might be used. Furthermore, other materials might be used for any of the layers, provided that the bandgap of the collector layer is greater than that of the base layer. If other materials are used, the collector bias voltage must be selected accordingly.
Claims
1. A superconducting transistor, conpπ'sing first and second superconducting layers (1,2) having the same or substantially the same bandgap; a fi rst electrical ly insul ating layer (4) sandwiched between said first and second superconducting layers; and a third superconducting l ayer (3) separated from said second superconducting layer by a second electrically insul ating layer (5) ; wherein said th i rd superconducting l ayer has a larger bandgap than said first and second superconducting layers ; wherein each of said first and second electrical ly insulating layers is sufficiently thin for tunnelling to take pl ace thereth rough ; and wherein electron fl ow in the devi ce takes place th rough said fi rst, second and third superconducting l ayers in that order.
2. A transistor as cl aimed in Claim 1, wherein said first and second superconducting l ayers (1 ,2) are formed of aluminium and said thi rd superconducting layer (3) is formed of niobium.
3. A transistor as cl aimed in Claim 1, wherein said fi rst and second superconducting layers (1,2) are formed of aluminium and said third superconducting layer (3) is formed of a material selected from lead, tin and ni obium nitride.
4. A transistor as cl aimed in Claim 1 or Cl aim 2 , wherein said electrical ly i nsulating l ayers (4,5) are formed of aluminium oxide.
5. A transistor as cl aimed in any preceding claim, wherein a characteristic of current gain against bias voltage for said third superconducting layer (3) for a given tenperature exhibits a current gain peak followed by a region of l ower current gain; and wherein in operation the bias voltage is selected to be sl ightly higher than that at which said peak occurs.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8823399A GB2227388A (en) | 1988-10-05 | 1988-10-05 | Superconducting transistors |
GB8823399.4 | 1988-10-05 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1990004266A1 true WO1990004266A1 (en) | 1990-04-19 |
Family
ID=10644757
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/GB1989/001178 WO1990004266A1 (en) | 1988-10-05 | 1989-10-04 | Transistor devices |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0400110A1 (en) |
JP (1) | JPH03501911A (en) |
GB (1) | GB2227388A (en) |
WO (1) | WO1990004266A1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5318952A (en) * | 1992-12-24 | 1994-06-07 | Fujitsu Limited | A superconducting transistor wherein hot electrons are injected into and trapped from the base |
US5550389A (en) * | 1988-11-28 | 1996-08-27 | Hitachi, Ltd. | Superconducting device |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4334158A (en) * | 1980-06-06 | 1982-06-08 | International Business Machines Corporation | Superconducting switch and amplifier device |
-
1988
- 1988-10-05 GB GB8823399A patent/GB2227388A/en not_active Withdrawn
-
1989
- 1989-10-04 JP JP1510810A patent/JPH03501911A/en active Pending
- 1989-10-04 EP EP89911589A patent/EP0400110A1/en not_active Withdrawn
- 1989-10-04 WO PCT/GB1989/001178 patent/WO1990004266A1/en not_active Application Discontinuation
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4334158A (en) * | 1980-06-06 | 1982-06-08 | International Business Machines Corporation | Superconducting switch and amplifier device |
Non-Patent Citations (5)
Title |
---|
Applied Physics Letters, Vol. 20, No. 7, 1 April 1972, New York (US) R.B. LAIBOWITZ et al.: "Josephson Junctions with Nb-A1 Composite Electrodes", pages 254-256 * |
Applied Physics Letters, Vol. 32, No. 6, 15 March 1978, American Institute of Physics, New York (US) K.E. GRAY et al.: "A Superconducting Transistor" pages 392-395 * |
Applied Physics Letters, Vol. 41, No. 11, 1 December 1982, American Institute of Physics, New York (US) A. SHOJI et al.: "New Fabrication Process for Josephson Tunnel Junctions with (Niobium Nitride, Niobium) Double-Layered Electrodes", pages 1097-1099 * |
Electronic Engineering, Vol. 60, No. 741, September 1988, Woolwich, London (GB) J. EVETTS et al.: "High Tc Superconductors at Cambridge", page 25 * |
Japanese Journal of Applied Physics, Vol. 24, No. 9, Part II, September 1985, Tokyo (JP), H. TAMURA et al.: "Current Injection Effects in a Nb/A10x-A1/Nb/n-InSb Triode", pages L709-L710 * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5550389A (en) * | 1988-11-28 | 1996-08-27 | Hitachi, Ltd. | Superconducting device |
US5318952A (en) * | 1992-12-24 | 1994-06-07 | Fujitsu Limited | A superconducting transistor wherein hot electrons are injected into and trapped from the base |
Also Published As
Publication number | Publication date |
---|---|
GB8823399D0 (en) | 1988-11-09 |
GB2227388A (en) | 1990-07-25 |
JPH03501911A (en) | 1991-04-25 |
EP0400110A1 (en) | 1990-12-05 |
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