A new superconducting transistor has been investigated, both theoretically and experimentally. The device has a current gain of 10 and an estimated power-delay product of 90 aJ. It is shown that in principle, the gain of the device is limited only by its length. Possible applications include analog amplification and digital logic.
We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density jcaof the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as |δIca|/|δIi|, where a change |δIi| in the injector (SFIFS junction) current produces a change |δIca| in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.
A superconducting transistor with large current gain and bandwidth at low temperatures would have many applications. We have fabricated and tested a new three-terminal superconducting device with transistor-like properties at an operating temperature of 4.2 K. It is based on a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb and an Al quasiparticle trap which can work either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting, while large current gains of more than 50 are observed when the Al is in the normal state. The device shows a high degree of unidirectionality. The results can be explained on the basis of the recently proposed QUAsiparticle TRApping TRANsistor, which should have wide applications in detection systems operating at low temperatures.
Transistors are invariably made of semiconducting materials, but the special properties of superconducting materials make them attractive for some, highly demanding, applications Indeed, physicists have been trying to build superconducting devices with transistor-like properties – in particular devices that can amplify electrical currents – for more than 20 years. Now Giampiero Pepe of the University of Naples in Italy and colleagues led by Antonio Barone, together with Norman Booth of Oxford University in the UK, have built a superconducting device that exhibits large current amplification and other transistor-like properties (G P Pepe et al. 2000 Appl. Phys. Lett. at press). The device can also he operated to obtain negative current gain, which is not possible with semiconductor devices.
The authors have modified the design of the superconducting current injection transistor (Super-CIT) for use in a distributed amplifier. The super-CIT consists of a resistively damped long Josephson junction in which the maximum Josephson current is modulated by the magnetic field produced by current in a control line. The device is modified by DC-coupling the control line from the Josephson junction so that the Super-CIT can be embedded in a distributed amplifier configuration. The distributed amplifier design is used to raise the input impedance and the power gain of the amplifier over a wide bandwidth. The small signal current gain of the device can be increased by biasing the Josephson junction in an asymmetric configuration. A simulation using practical values results in an amplifier with an input impedance of 6.5 Omega using only four devices, which produces 15 dB of gain into a 2.5- Omega load over a bandwidth of 90 GHz with very little reverse gain.>
The superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively) are promising for various applications in superconducting electronics. Specifically, as current amplifiers, they can be used as line drivers and cell selectors in cryogenic memory. Here we report on results of a study of dependence of current gain on the SFIFS injector and SIS acceptor barrier transparency. We demonstrate that making the acceptor junction much more transparent than the injector junction results in improvement of the current gain; in particular it can exceed unity. Therefore proper device engineering allows one to efficiently control the maximum Josephson current in the SIS acceptor junction using the quasiparticle injection.
We report experimental results on characteristics of SFIFS junctions and multi-terminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS junctions serve as injectors in the SFIFSIS devices which have transistor-like properties; for this reason we call them Superconducting-Ferromagnetic Transistors (SFTs). We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic (CVC) becomes linear. Furthermore, we investigated the DC and AC characteristics of SFTs of two types: ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. In the first case, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For devices of the second type, we studied voltage amplification properties when the operating point was chosen in the sub-gap region of the acceptor CVC. By applying an AC signal (in the kHz range) while biasing the injector (SFIFS) junction with a constant DC current, we observed a voltage gain above 25 on the double acceptor. In the reverse transmission experiment, we applied DC current and an AC modulation to the acceptor junction and, within the accuracy of the experiment, observed no response on the injector junction, which implies an excellent input-output isolation in our SFIFSIS devices. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.