Investigation of Current Gain in Superconducting-Ferromagnetic Transistors

The superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure (where S, I, and F denote a superconductor (Nb), an insulator (AlO
_x
), 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.